PhD Projects

//PhD Projects
PhD Projects 2018-11-19T09:37:28+00:00

Fundamental Physics in Radio Astronomy Group at the MPIfR

Director: Prof. M. Kramer

Group website

Pulsars as tools for fundamental physics (MK01)

The Fundamental Physics in Radio Astronomy group of the MPIfR led by Prof. Michael Kramer concentrates on various aspects of fundamental physics, namely the Galactic population of neutron stars, their use for precision tests of general relativity and alternative theories of gravity, the detection of low-frequency gravitational waves and the structure and properties of super-dense matter. We are seeking a student for one of the following research areas:

a) Searching for pulsars and transient radio sources

The discovery of new pulsars frequently leads to new scientific results depending on the pulsar’s properties. With recent advancements in receivers and computing we are able to probe deeper into the Galaxy than ever before. Radio astronomy also offers a unique chance to explore the dynamic sky as many energetic process are expected to be visible in this part of the electromagnetic spectrum.

b) Pulsars as probes of gravity

Pulsars are very compact objects, exhibiting the strongest gravitational fields next to black holes. Acting as cosmic lighthouses and precise cosmic clocks, pulsars can be used to probe gravitational physics under strong-field conditions. High precision timing of pulsars provided the first, evidence for the existence of gravitational waves as predicted by Einstein’s theory of gravity (Nobel Prize 1993), and leads to some of the best constraints on alternative gravity theories. Our research in that field of experimental gravity is driven by improvements in instrumentation, new pulsar discoveries, and ongoing theoretical work.

c) Pulsar as detectors for low frequency gravitational waves

The direct detection of gravitational waves from merging black holes with LIGO has opened up a completely new window to the Universe: the gravitational wave sky. Using a Pulsar Timing Array a number of high-precision millisecond pulsars are timed to detect low frequency gravitational waves,  and complement the observations of ground-based gravitational wave detectors like LIGO and VIRGO. To make such a detection a European collaboration, the European Pulsar Timing Array, uses the largest radio telescopes in Europe, including the 100-m radio telescope in Effelsberg.

d) Pulsars as probes of super-dense matter and stellar evolution

Neutron stars represent the most extreme state of matter in the observable Universe. We can study the properties of this super-dense matter when we observe neutron stars in the form of radio pulsars, e.g. mass measurements in binary pulsars offer some of the best constraints for the “equation-of-state”. These mass measurements also offer clues for understanding the birth mass distribution of neutron stars and for the evolution of different types of binary pulsars.

The applicant’s research proposal should be related to the above research themes.

Contact: Prof. Dr. Michael Kramer (mkramer@mpifr.de), Dr. David Champion (champion@mpifr-bonn.mpg.de), Dr. Paulo Freire (pfreire@mpifr-bonn.mpg.de), Dr. Norbert Wex (wex@mpifr-bonn.mpg.de),

Radio interferometers are the instruments that provide the highest resolution for astronomical observations. For given baselines, higher frequencies correspond to higher resolutions. Under certain conditions, however, lower frequencies can be advantageous. The interstellar medium can deflect and scatter radiation and sometimes (particularly at low frequencies) produce multiple signal paths that interfer with each other. These paths can be used as huge interstellar interferometric baselines with extreme resolution.

Using this effect requires a good understanding on many levels: low-frequency interferometry on long baselines (e.g. with LOFAR), details of the scattering properties of the interstellar medium and sophisticated analysis techniques

The successful applicant will have the opportunity to work on these challenging topics and advance the field with own developments.

Contact: Dr. Olaf Wucknitz (wucknitz@mpifr-bonn.mpg.de),  Dr. Robert Main (ramain@mpifr-bonn.mpg.de), Prof. Dr. Michael Kramer (mkramer@mpifr.de)

Targeted searches for pulsars (MK03)

The study of neutron stars is motivated by fundamental open questions in physics, such as the properties of dense nuclear matter, and the behaviour of gravity under extreme conditions (see MK01). Searches for new radio pulsars play a crucial role in advancing the field: new pulsars improve our overall knowledge of neutron star properties while, in rare occasions, the discovery of extreme objects leads to a paradigm shift.
Most modern pulsar surveys are optimized to cover wide areas of the sky in as little time as possible. While this approach has multiple benefits, it often comes at the expense of sensitivity which depends strongly on integration time.
Targeted surveys provide a solution to this problem by focusing on specific sky positions that have a higher-than-average probability of coinciding with a pulsar. Those searches allow for much longer integration times and therefore improved sensitivity, accelerating the discovery rate and often leading to the discovery of exotic systems.
The advent of multi-wavelength sky surveys (such as Fermi and GAIA) and sensitive telescopes (MeerKAT, SKA, LSST) that are currently coming online, make targeted surveys more timely and urgent than ever.
The successful applicant will investigate target selection and survey optimization strategies using a number of theoretical and observational tools.
In particular, the PhD project will focus on:
• constraining the dynamical and observational properties of binary pulsars using state-of-the-art stellar evolution and population synthesis techniques.
• comparing the outcome of those simulations to real datasets from GAIA and Fermi to optimize target-selection strategies
• leveraging those results to search, discover and follow-up pulsars using the Effelsberg, Arecibo and MeerKAT telescopes.
Contact: Dr. John Antoniadis (janton@mpifr.de), Prof. Dr. Michael Kramer (mkramer@mpifr.de)

Fast radio bursts as astrophysical tools (MK04)

Fast radio bursts (FRBs) are short-duration, luminous flashes of radio waves that originate from so-far unidentified astrophysical sources in distant galaxies. Their unknown origin is one of the biggest mysteries in astronomy today. Nevertheless the physical properties of the ionized medium of the host galaxy, our own Milky Way, and importantly the diffuse medium between the galaxies are imprinted in the observed properties of each FRB. FRBs have great potential as probes of a wide range of astrophysical environments and can be used to study cosmology and the large scale structure of our Universe.

In the decade since the discovery of FRBs, progress in understanding their origins has been limited by the small number of FRB sources discovered and the inability to identify the source’s host galaxy. This will be changing in the next few years as several new radio telescopes come online that will revolutionize the field.

We will not be able to fully leverage the potential of a large population of known FRBs without the statistical tools that link FRB observables to astrophysical questions. A successful applicant will develop these tools by synthesizing results from a variety of surveys.

Contact: Dr. Laura Spitler (lspitler@mpifr.de) and Prof. Michael Kramer (mkramer@mpifr.de)

Broadband polarimetry of the COSMOS deep field (MK05)

This PhD project exploits the polarization aspect of the Jansky Very Large Array survey of the 2-deg2 COSMOS field at S band (2-4 GHz) at sub-arcsecond resolution down to a sensitivity of 2μJy/beam. Over 104 radio sources in total intensity were detected in this deep field with excellent multi-wavelength ancillary data, resulting in science covering topics in AGN population and star forming galaxies out to peak of cosmic star formation and beyond. The prospects of polarimetry of this highly sensitive 2-deg2 field is exciting – as polarized sources in this deep field are inaccessible in shallow all-sky surveys. The PhD student will perform both on-axis and off-axis instrumental polarization calibration of the data, and source finding in polarization. The science goals of this thesis are:
1. understanding the nature of polarized source population down to μJy level;
2. characterization of the broadband polarization properties of spatially resolved AGNs ;
3. polarization properties of star-forming galaxies and their redshift evolution.

Contact: Dr. Sui Ann Mao (mao@mpifr-bonn.mpg.de) and Prof. Michael Kramer (mkramer@mpifr.de); in collaboration with Prof. Vernesa Smolčić

Mapping the redshift evolution of galactic magnetic fields (MK06)

Dynamically important magnetic fields have been shown to play pivotal roles in processes that are intimately linked to galaxy evolution, such as disk-halo interaction, gas accretion and galactic-scale outflows in local galaxies. However, how galaxies and their magnetic fields have co-evolved since the early Universe remains an unsolved fundamental question in astrophysics due to the lack of galactic magnetic field measurements beyond the local Universe.
Faraday rotation towards radio-bright and polarized quasars enables one to probe magnetized gas in foreground intervening galaxies. The successful candidate will use broadband radio polarization observations of gravitational lensing systems and quasar absorption line systems to provide powerful constrains on galactic magnetic field strength and structures in and around distant galaxies. The student will (1) constrain magnetic fields in individual distant galaxies using lensing systems, and (2) will statistically infer magnetic fields associated with intervening galaxy populations. To interpret these observations, the student will (3) compare the results with  theoretical expectations.
Contact: Dr. Sui Ann Mao (mao@mpifr-bonn.mpg.de) and Prof. Michael Kramer (mkramer@mpifr.de)

Testing large scale cosmology with MeerKAT (MK07)

MeerKAT is the largest and most sensitive radio telescope array in the southern hemisphere, it consists of 64 13.5-metre diameter dish
antennae and can observe the sky at 3 different frequency bands (UHF, L, and S-Band). On July the 13th this year the array as been official opened and is now online for science operations. For the next 5 years most of the observing time of the MeerKAT array is dedicated to the LSP (“Large Survey Projects”) addressing some of the key questions in astrophysics.
One of the LSP is the MeerKAT Absorption Line Survey (MALS). The main objective of the survey is to use MeerKAT’s L- and UHF-band receivers to carry out the most sensitive dust-unbiased search of intervening neutral hydrogen (HI) and hydroxyl (OH) absorption lines up to redshifts of 2. For this goal the array will observe hundreds of individual sources at mirco-Jansky sensitivities distributes over the southern sky. At each source pointing the field of views per observations is of the order of 1-2 square degree and therefore allows us to observe several hundreds of sources in addition. These additional, measurements will enable us to test large scale cosmology.
The PhD-Project will use continuum- and polarised radio emission of extragalactic sources to investigate the cosmic radio dipole and to test cosmology.
The fundament of modern cosmology is the assumption of isotropy and homogeneity of the Universe at large scales. Whereas the isotropy is best seen in the cosmic microwave background at per cent level. However at much lower levels the CMB temperature revealed a prominent anisotropy signal, the dipole. These signal is largely caused by the motion of the Solar system through the Universe and is fully consistent with the concordance model of cosmology. However, the CMB observations is not enough to disentangle the difference between a signal introduced by motion or dipole contributions from other physical phenomena (e.g. a primordial signal). So far measurements of the radio dipole, using less sensitive radio observations, revealed a higher dipole amplitude than expected by the CMB. The MeerKAT measurements enables us to provide a better constrain of the dipole in continuum and for the first time in polarisation.

Contact: Dr. Hans-Rainer Klöckner (hkloeckner@mpifr.de) and Prof. Michael Kramer (mkramer@mpifr.de); in collaboration with Prof. N. Gupta and Prof. D. Schwarz

Very Long Baseline Interferometry and Radio Astronomy Group at the MPIfR

Director: Prof. Dr. J. Anton Zensus

Group website

Direct imaging of the extreme vicinity of cosmic black holes with mm VLBI (AZ01)

Very-long-baseline interferometry (VLBI)  is the only method for a direct imaging of  regions in the immediate vicinity of super-massive cosmic black holes. This includes the attempt to image the shadow around a black hole and its surrounding photon ring (event horizon), and thus test Einstein’s Theory of General Relativity.

Since the angular resolution of an interferometer increases with decreasing wavelength and with increasing baseline length,  millimetre VLBI and space VLBI provide the highest angular resolutions in astronomy (lower than 30 microarcseconds). Since compact radio sources become more transparent with increasing frequency (lower opacity at mm-wavelength),   mm-VLBI allows to probe deeper into the self-absorbed regions of AGN, which is not possible at the longer cm-wavelengths. Our group operates the Global Millimeter VLBI Array (GMVA), which combines up to 14 telescopes into regular 3mm/7mm VLBI observations and is a key partner at the Event Horizon Telescope (EHT), performing VLBI observations at 1.3mm wavelength.  Both 3.5mm and 1.3mm observations include the ALMA telescope at the array. One of the main goals of this effort is to probe the ‘shadow of the black hole’ in the Galactic Centre and in M 87, as well as to study the origin of jets in more distant radio-galaxies and quasars (AGN) with unprecedented resolution.

One of the goals of the PhD candidate is to image and study Active Galactic Nuclei (Quasars, BL Lac objects, Radio Galaxies, etc.) with highest possible  resolution in the mm-band, addressing questions related to AGN activity, outburst-ejection relations, the physical origin of jets, the details of the jet launching and of the primary jet acceleration  processes. For this the jet kinematics, their spectral and polarimetric properties will be studied on spatial scales which are as close as possible to the central engine (scales of a few 10 – 1000 gravitational radii).

VLBI studies of the polarised fine structure of AGN probe the orientation and nature of magnetic fields at the innermost part of their relativistic jets.  mm-VLBI has the advantage that polarimetric observations at the shortest wavelengths are only marginally affected by Faraday rotation and probe the intrinsic linearly polarised emission.  We aim to probe if and how oblique shocks in the innermost jet regions are responsible for the observed polarised emission. The detailed study of the three-dimensional, spatially bent complex structure of the jet close to its nozzle  will help us to answer one of the most fundamental questions in AGN physics, which is how Black Holes launch relativistic jets.

Bibliography:

• Boccardi, B., Krichbaum, T.P., Ros, E., Zensus, J.A.: Radio observations of active galactic nuclei with mm-VLBI, Astron Astrophys Rev (2017) 25 4, https://doi.org/10.1007/s00159-017-0105-6
• Lu, R.S., Krichbaum, T.P., Roy, A.L.: Detection of Intrinsic Source Structure at ˜3 Schwarzschild Radii with Millimeter-VLBI Observations of Sgr A*, Astroph. J. 859, 60 (2018) https://doi.org/10.3847/1538-4357/aabe2e
• Tilanus, R.P.J., Krichbaum, T.P., Zensus, J.A., et al: Future mmVLBI Research with ALMA: A European vision, arXiv:14060.4650 (2014) https://arxiv.org/abs/1406.4650
• Fish, V., Akiyama, K., Bouman, K., et al.: Observing—and Imaging—Active Galactic Nuclei with the Event Horizon Telescope, Galaxies 4, 54 (2016)

Contact:  Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de), Dr. Thomas P. Krichbaum (tkrichbaum@mpifr-bonn.mpg.de),  Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de), Prof. Dr. Antxon Alberdi (antxon@iaa.es), Dr. RuSen Lu (rslu@mpifr.de).

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI group in collaboration with the Frankfurt University in Germany, Shanghai Astronomical Observatory in China, and the IAA/CSIC in Spain.

Extragalactic jet physics: numerical simulations and observations (AZ04)

Extragalactic jets are formed in the environments of supermassive black holes (SMBH) in active galactic nuclei (AGN). They are among the most powerful and energetic astrophysical objects. Their relevance is not only due to their role as laboratories of relativistic plasmas, but they have also an important effect in their environments, namely, in the interstellar medium of the host galaxy and the intergalactic medium. Understanding their nature and physics can thus give us key information about the progenitor SMBH and its surroundings, but also about the host galaxy and its history. A combination between detailed VLBI observations and theoretical modelling via numerical simulations has proven to be a very good approach to reach the goal of this research.

At present, we are able to perform numerical simulations in supercomputers, including all the relevant physics of these objects: relativistic gas, magnetic fields, different composition.  VLBI is addressing the innermost radio structure of these objects, and gives the observational input to theoretical studies. Here we propose to continue an already started line of research, which consists in trying to relate the emitting, non-thermal population of particles, studied through observations, with the thermal gas in the jet and the magnetic fields, responsible for the macroscopic jet dynamics.

Bibliography:

Contact: Prof. Dr. Anton Zensus (azensus@mpifr-bonn.mpg.de), Prof. Dr. Eduardo Ros (ros@mpifr-bonn.mpg.de), Prof. Dr. Manel Perucho (Univ. Valencia, Spain, perucho@uv.es)

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group in collaboration with the Universitat de València, Spain and the Goethe-Universität Frankfurt.

Multi-wavelength study of the Galactic center’s supermassive black hole and its immediate vicinity (AZ06)

The accretion flow onto the Galactic center supermassive black hole (Sgr A*) generates a fluctuating source of electromagnetic radiation that has been detected at all observable wavelength domains, including radio, submillimeter, near-infrared (NIR) and X-ray. The rapid fluctuations, with significant changes on timescales < 10 minutes, indicate that the variable emission originates in the innermost regions of the accretion flow, near the event horizon. However, there is still no conclusive understanding of the statistics of these correlations or the physical processes underlying the variability. Recently new pointing models have allowed the Spitzer Telescope to observe Sgr A* in the NIR for continuous 24-hr periods with a S/N similar to that of ground-based, AO-supported 8-m telescopes. On the strength of this breakthrough, we have successfully organized a major multi- wavelength campaign in which both the Spitzer and Chandra space telescopes simultaneously followed the variability of Sgr A*, supplemented with observations from SMA, VLA, ALMA, the VLT, and Keck. In summer 2019 another two 24-hour periods of Chandra and Spitzer are scheduled.
This PhD project will focus on the radio and submm regime. The candidate has the opportunity to work on one or more aspects of this unprecedented and rich dataset: From training in data reduction of the ALMA interferometric imaging data and interpretation of maps of radio recombination lines in the vicinity of SgrA* (H30-alpha at 232.2 GHz and 13CO J=2-1 at 220.2 GHz),over insights into NIR adaptive optics imaging and light curve extraction from Spitzer data, to time series analysis and modeling of radiative processes behind the variability.
Collaboration webpage: https://www.cfa.harvard.edu/irac/gc

Bibliography

Contact:  Dr. Gunther Witzel (gwitzel@mpifr.de), Prof. Dr. J. Anton Zensus (azensus@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group

Extragalactic jet physics: the radio-gamma connection and monitoring (AZ02)

Very-long-baseline observations are the only direct evidence for relativistic outflows from supermassive black holes in active galactic nuclei (AGN).  The MOJAVE (north) and TANAMI (south) programs provide long-term, systematic monitoring of relativistic motion in AGN on parsec-scales. These observations aim to characterize the kinematics and evolution of AGN jets and to determine how these relate to other source properties.  The MOJAVE data, together with the Boston University Blazar Monitoring survey also address the polarised emission and the morphology of magnetic fields in jets. A big amount of information of the high-energy photons emitted by blazars is coming from the all-sky observations provided by the Large Area Telescope onboard the Fermi Gamma-Ray Observatory.

The proposed project will deal with the analysis of data from the monitoring surveys and to compare the radio properties determined from the VLBI observations (related to jet opacity, kinematics, magnetic field, etc.) with the properties derived at other bands of the spectrum, especially in x and gamma rays, and even beyond it, as it is the case for high-energy neutrinos.

Contact

Prof. Dr. Anton Zensus (azensus@mpifr.de), Prof. Dr. Eduardo Ros (ros@mpifr.de), Prof. Dr. Matthias Kadler (Univ. Würzburg, Germany, matthias.kadler@astro.uni-wuerzburg.de)

The MOJAVE Program: http://www.physics.purdue.edu/astro/MOJAVE/

The TANAMI Program: http://pulsar.sternwarte.uni-erlangen.de/tanami/

Magnetic fields in the vicinity of black hole event horizon (AZ05)

Prominent jets in active galactic nuclei are believed to be powered by the enormous amount of energy released in the regions surrounding the central supermassive black holes. Understanding the  physical mechanism of the generation and transport of this energy is crucial for both building a self-consistent picture of extragalactic relativistic jets and uncovering the ultimate nature of the cosmic black holes. Theoretical explanations relate the production and outward transport of energy to black hole rotation, accretion disks, relativistic plasma jets, and magnetic fields. At radio wavelengths, where VLBI observations achieve highest-resolution imaging, one can trace the strongly collimated relativistic jets down to linear scales of a few lightdays. Observations with the Event Horizon Telescope have now made it possible to image the immediate vicinity of the suspected black holes on event horizon scales. This project will address a major remaining astronomical challenge in the AGN puzzle and potentially the most crucial piece of the unequivocal proof of the very existence of black holes: measuring with sufficient precision the strength and three dimensional structure of the magnetic fields on scales smaller than 1000 gravitational radii from the central black hole in order to answer the questions how jets are launched and powered.  The project will by combining high-resolution multi-frequency polarimetric VLBI radio imaging and opacity measurements for a sample of AGN jets with advanced analytical and numerical modelling of relativistic flows. This combination has the best potential for yielding unprecedented constraints on the theoretical models for cosmic black holes and the production of extragalactic relativistic jets.

Contact

Prof. Dr. Anton Zensus (azensus@mpifr.de), Prof. Dr. Eduardo Ros (ros@mpifr.de), Dr. Andrei Lobanov (alobanov@mpifr.de)

Advanced imaging for radio interferometry with the Event Horizon Telescope and the Global Millimetre VLBI Array (AZ07)

Robust radio interferometric imaging at millimetre and submillimetre wavelengths requires focused developments of novel and advanced methods for deconvolution and imaging of visibility data. A number of new approaches are currently being considered in connection with imaging of the data obtained during several observing campaigns of the Event Horizon Telescope. These methods are aimed at improving the “canonical” CLEAN deconvolution broadly applied in VLBI data analysis and at addressing in particular the issues of time variability of emitting regions and an incomplete coverage of the Fourier domain (uv-coverage). The sparsity of uv-coverage affects substantially data from the GMVA snapshot observations and poses a particularly acute problem for the EHT data where it introduces scale-dependent noise in the image. Such scale-dependent noise can be effectively identified and suppressed by implementing wavelet deconvolution of visibility data. Our recent work has demonstrated a remarkable efficiency and power which wavelets provide for decomposing, classifying, and tracking structural details dominating different spatial scales in astronomical images. This project will build up on the success of the wavelet decomposition method and extend it to the Fourier domain, constructing an algorithm and a software for wavelet deconvolution of visibility data and enabling robust noise suppression on spatial scales affected by the uv-coverage deficiencies. The project will have immediate applications to EHT and GMVA data where it is expected to bring about an order of magnitude improvement of image dynamic range and uncover much stronger constraints on physical conditions near the event horizon and in the innermost regions of relativistic jets.

Contact

Prof. Dr. Anton Zensus (azensus@mpifr.de), Dr. Andrei Lobanov (alobanov@mpifr.de), Prof. Dr. Eduardo Ros (ros@mpifr.de).

Millimeter and Submillimeter Astronomy Group at the MPIfR

Director: Prof. Dr. Karl Menten

Group Website

The physics and chemistry of the diffuse interstelar medium (KM01)

The Heterodyne Instrument for the Far-Infrared (HIFI) abord Herschel opened a whole new era of studies of the diffuse interstellar medium (ISM). Utilizing Herschel’s superb sensitivity combined with its high spectral resolution receivers operating in a low thermal background space environment, HIFI revolutionized observations of light hydrides, many of which have their ground-state rotational transitions in the submillimeter and far infrared range. As the building blocks of larger molecules, the (mostly) di- or triatomic species detected by HIFI and (also by APEX and SOFIA) are of central astrochemical interest. In addition, they triggered new interest in the physics and chemistry of diffuse and translucent clouds, for example in their role as interfaces between the cold (and warm) neutral atomic and the denser, cold molecular phases of the ISM in our, but also in external galaxies.

Building on the success of Herschel, new opportunities arise with ground-based and airborne observations of hydrides: in the submillimeter range, hydrides such as 13CH+, OH+, and SH+ can be studied using the APEX telescope with new receivers and the exceptional atmospheric conditions on the Chajnantor Plateau in northern Chile. Access to even higher frequencies is provided by the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) and the GREAT receiver and allows observations of CH, SH and OH, to name just a few of the simplest hydrides. A thorough analysis of such observations will lead to a comprehensive characterization of hydrides in diffuse clouds throughout our Galaxy.

Possible dissertation projects comprise observational studies with, predominantly, SOFIA and APEX, but also with other telescopes as well modeling of the data and bringing them into context with a glabal view of the Milky Way galaxy’s ISM.

Contact: Prof. Dr. Karl Menten (kmenten@mpifr-bonn.mpg.de)

Effelsberg K-band fingerprints of star forming regions (KM02)

The unprecendented sensitivity and large bandwidth (8GHz) of the new Effelsberg K-band receiver, covering 18-26 GHz in frequency, offer new opportunities to study the physical and chemical conditions of star forming regions and mant other interesting sources with unbiased line surveys. These surveys give access to probes of temperature, density and chemistry that can be used to characterize the evolutionary stages of the regions. The project will be very complementary to line survey projects conducted with the IRAM 30m and APEX 12m telescopes in the millimeter and submillimeter wavelength range, respetively.

The K-band contains many interesting lines, amongst them the many transitions of ammonia (including 15NH3 and non-metastable lines), as well as lines from carbon chains like HC3N, HC5N (for both also isotopologues and vibrationally excited levels), from molecules like methanol, CCS, C3H2, C6H and several recombination lines and maser transitions (water, methanol, ammonia), make K-band line surveys very appealing for molecular fingerprints, that can be used to rapidly characterize the main charactistics and the evolutioniary stage of the regions, then leading to a new “K-band spectral classification” of star forming regions.

For such surveys, a sophisticated data reduction pipeline, capable of processing and calibrating the large amount of data will be needed. The PhD candidate will participate in setting up this pipeline to prepare for the analysis of the wealth of molecular line data. A background in software engineering and experience with data reduction of radio astronomical data would be a plus.

Contact: Dr. Friedrich Wyrowski (wyrowski@mpifr.de), Dr. Benjamin Winkel (bwinkel@mpifr.de), Dr. A. Kraus (akraus@mpifr.de) Prof. Dr. Karl Menten (kmenten@mpifr-bonn.mpg.de)

The Galactic center and the Central Molecular Zone (KM03)

Studying the physical and chemical processes occurring in the nuclear regions of galaxies is of fundamental importance for understanding the formation, evolution and dynamics of galaxies. Many galaxies, including our Milky Way, contain huge amounts of gas in their central hundred parsecs. This molecular and atomic material is strongly affected by the presence of the large potential gradients, X-rays, cosmic rays, and magnetic fields. These extreme conditions result in the well known properties of the molecular clouds in our Galactic center, such as the large velocity widths, high kinetic temperatures and the high chemical complexity observed in this region.

At ~8 kpc from us, the center of our Galaxy presents a unique laboratory to study the phenomena occurring in the heart of the galaxies with high spatial resolution. APEX and ALMA with their location in the southern hemisphere, which allows 11 h of observing time per day, and the excellent weather conditions at the Chajnantor site, are unique telescopes to study the Galactic center in the submm regime.

In this ambitious project the large scale kinematics and the unique physical and chemical conditions in the Central Molecular Zone will be studied with new muti-beam receiver arrays on the APEX telescope. ALMA will be used for a high angular resolution view of selected regions.

Contact:  Prof. Dr. Karl Menten (kmenten@mpifr-bonn.mpg.de)
Site: Bonn, Max-Planck-Institut für Radioastronomie, Millimeter and Submillimeter Astronomy Group

A comprehensive Galactic plane radio wavelength star formation survey (KM04)

Understanding the circumstances of massive star formation is one of the great challenges of modern astronomy. In the last years, our view of massive star forming regions has dramatically been changed by Galactic plane surveys covering centimeter to infrared wavelengths. These surveys enable us for the first time to study ALL evolutionary stages of massive star formation in an unbiased way. With the exciting results of the new submm/FIR surveys from the ground (ATLASGAL) and space (Hi-GAL) the massive and cold dust clumps from which massive cluster form are now detected in an unbiased way. Complementary, the EVLA will allow incredibly powerful and comprehensive radio- wavelength surveys of, both, the ionized and the molecular tracers of star formation in the Galactic plane.

In this project, the extremely wideband (4-8 GHz) new C-band receivers of the EVLA will be used for an unbiased survey to find and characterize star-forming regions in the Galaxy.  This survey of the Galactic plane, that is now ongoing, will detect tell-tale tracers of star formation: compact, ultra-, and hyper-compact Hii regions and molecular masers which trace different stages of early stellar evolution and will pinpoint the very centers of the early phase of
star-forming activity. Combined with the submm/infrared surveys it will offer a nearly complete census of the number, luminosities and masses of massive star forming clusters in a large range of evolutionary stages and provide a unique dataset with true legacy value for a global perspective on star formation in our Galaxy.

Contact: Dr. Friedrich Wyrowski (wyrowski@mpifr- bonn.mpg.de), Prof. Dr. Karl Menten (kmenten@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, Millimeter and Submillimeter Astronomy Group

A 350 micron Galactic Plane survey of the inner Milky Way (KM05)

The very successful APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) has revealed for the first time the structure of the cold interstellar medium over several 100 sq. deg. The survey was conducted with LABOCA at the APEX telescope to conduct an unbiased census of massive star forming clumps and their different evolutionary stages in the inner Galaxy.

Soon a new, much larger bolometer camera will be commissioned at the APEX telescope, the A-MKID dual color camera with ~3500 pixel at 870 and ~20000 pixel at 350 micrion. At the short wavelengths, this instrument will allow to resolve sources of 0.1 pc size within 3 kpc towards giant star forming complexes.  When combining the A-MKID data with the Herschel/Hi-GAL survey at 60 to 600 micron, it will be possible to:

• derive evolutionary stages for a representative sample of star forming clumps

• compute dust emissivity and temperature over a large star forming complex

• analyze how the physical conditions vary within one giant complex

• study the fragmentation of clumps into cores that can give birth to individual stars

Given its high angular resolution and unbiased nature, such a survey

will provide a legacy and pathfinder for years to come.

Contact: Dr. Friedrich Wyrowski (wyrowski@mpifr- bonn.mpg.de), Prof. Dr.Karl Menten (kmenten@mpifr-bonn.mpg.de)

Site: Bonn, Max-Planck-Institut für Radioastronomie, Millimeter and Submillimeter Astronomy Group

Physical and chemical conditions in giant molecular clouds (KM06)

Massive stars form in dense clumps within giant molecular clouds (GMCs), but how do these clouds form and evolve, how do the dense clumps form within them and what are the conditions for star formation within these dense clumps? To address these questions the MPIfR operates powerful heterodyne cameras for line observations, such as the upcoming new LAsMA array for observations at 345GHz and the CHAMP+ array (690 and 810 GHz), both at APEX, complemented by the upGREAT array on SOFIA for THz observations of important fine structure cooling lines. The high mapping speed of these cameras enables to map giant molecular clouds on degree scales in a variety of molecules. Such a chemical inventory provides a new view on the large scale properties of molecular clouds. Furthermore, by combining results from the different cameras the excitation and cooling of the clouds can be constrained to form a comprehensive picture of the formation and condition in GMCs.

Some of scientific questions to tackle with these observations are: What is the dynamics of the clouds and the embedded clumps? How strongly are the chemical conditions in the clouds altered by interaction processes, either from the outside of the clouds or by feedback from the ongoing star formation in the clouds? Can some of the chemical variations be used in conjunction with chemical models as chemical clocks to put different clumps in the clouds into an evolutionary sequence? How do these large-scale chemical properties of Galactic GMCs compare with observations of their extra-galactic counterparts that are now feasible with the high sensitivity and angular resolution of ALMA?

Contact: Dr. Friedrich Wyrowski (wyrowski@mpifr-bonn.mpg.de), Prof. Dr.Karl Menten (kmenten@mpifr-bonn.mpg.de)
Site: Bonn, Max-Planck-Institut für Radioastronomie, Millimeter and Submillimeter Astronomy Group

Star forming galaxies in the early universe (KM07)

In the last two decades, millimeter and submillimeter surveys have transformed our understanding of galaxy formation and evolution by revealing that luminous, dusty galaxies were a thousand times more abundant in the early universe than they are at the present day and form an important population of star forming galaxies in the high redshift universe. Large scale surveys, such as the LABOCA Submillimeter Survey of the Extended Chandra Deep Field South (LESS) and South Pole Telescope survey (SPT) have revealed hundreds of high redshift DSFGs. In order to determine the formation mechanisms and the physical conditions in these actively star- forming galaxies this project aims on conducting detailed follow-up observations using ALMA, ATCA and APEX. In conjunction with archival Herschel these observations can be used to derive the redshifts, the dust properties as well as the molecular excitation via observations of multiple CO, the CI fine structure lines and observations of ionised carbon. The aim of this project is to combine all the available information, potential including ALMA high spatial resolution maps, to determine the intrinsic properties of the gas reservoirs in great detail and to compare they to the properties of local actively star forming systems.

Contact: Dr. Axel Weiss (aweiss@mpifr.de)

Deuterated molecules in early stages of high-mass star-formation (KM08)

Probing the initial conditions and early stages of high-mass clumps is key for our understanding of the formation of high-mass star clusters. Deuterated molecules might provide a unique view onto the physical conditions in cold molecular clumps. A high deuteration of molecules has been found towards early stages of high-mass star-formation. In particular, the ATLASGAL unbiased inner Galactic Plane survey of the 870~$\mu$m dust continuum provides an ideal hunting ground for massive clumps in early evolutionary stages and our previous 3mm line surveys of flux-limited samples of massive clumps lead to many new detections of bright deuterated ammonia. These clumps were followed up in the deep integrations with the IRAM 30m and APEX 12m telescopes to study a range of deuterated molecules and their excitation. These new observations will be used to constrain chemical models of deuteration and to study the physical conditions and dynamics in early stages of high-mass star-formation.

contact: Dr. Friedrich Wyrowski (fwyrowski@mpifr.de)

The Argelander Institute for Astronomy at the U of Bonn

Research Groups: http://www.astro.uni-bonn.de/en/research/groups/

Pair-instability supernovae in the local universe (UB08)

The most massive stars, assuming mass loss is not too strong, are thought not to form iron cores, but rather to become unstable due to electron-positron-pair formation before central oxygen burning. The collapsing oxygen-rich core will then ignite oxygen explosively, which may lead to pair-instability supernovae, leaving no compact remnant. While such explosions have been predicted since 50 years ago, they were often assumed to only occur in the early universe. However, very recently, pair- instability supernovae have been found observationally in the local universe. This PhD project aims at constructing the first progenitor and explosion models for local, i.e., finite metallicity pair-instability supernovae, using our most modern hydrodynamic stellar evolution code. The idea is to characterize the observable properties of the progenitor and of the supernovae, and to make predictions for the nucleosynthesis yields of pair-instability, which could well dominate the metal production in their host galaxies.

Bibliography:

Gal-Yam, A., et al., 2009, Nature, 462, 624

Langer, N., 2009, Nature, 462, 579

Contact: Prof. N. Langer (nlanger@astro.uni-bonn.de)

Site: Bonn, Bonn, Argelander Institute for Astronomy, University of Bonn

Studying the nature of dark energy with eROSITA X-ray galaxy clusters (UB09)

Dark energy makes up about 70% of the Universe’s energy content and is responsible for its observed acceleration. A powerful method to constrain this enigmatic component is a precise measurement of the evolution of the most massive objects in the Universe: galaxy clusters. For this purpose, the new X-ray telescope eROSITA will be launched in 2019. Its mission is the discovery of ~100,000 galaxy clusters, including ALL massive clusters in the entire observable universe. eROSITA will be the first “Stage IV” dark energy probe world-wide. The PhD candidate will work on preparations for this mission and will enjoy privileged access to the first eROSITA data to study galaxy cluster physics, chemistry, and cosmology.

Bibliography:

Contact: Prof. Dr. Thomas H. Reiprich (reiprich@astro.uni-bonn.de)

Site: Bonn, Argelander Institute for Astronomy, University of Bonn

A next-generation galaxy cluster survey with the CCAT-prime telescope (UB11)

Data from wide-area galaxy cluster surveys are one of the main drivers behind the current “golden era” of cosmology. Among the handful of methods that can reliably detect galaxy clusters and infer their masses, the Sunyaev-Zel’dovich, or SZ, effect is a unique one: its signal is practically undiminished by redshift and at frequencies below 220 GHz galaxy clusters produce a negative signal in the microwave sky. At the University of Bonn we are part of a team preparing for a new-generation SZ cluster survey with the CCAT-prime telescope, that will not only improve the raw detection sensitivity of the SZ signal compared to current generation instruments, but will also extend the measurement of the SZ effect in the sub-millimeter domain. Here the SZ signal is positive and gets mixed with contaminating foreground sources. The challenge of this thesis work will be to optimize some of the existing cluster detection methods, and develop new ones, that will yield unbiased cluster SZ measurements in the sub-millimeter wavebands for applications in cosmology and astrophysics.

Bibliography:

• “CCAT-Prime: science with an ultra-widefield submillimeter
observatory on Cerro Chajnantor”, G. Stacey et al. 2018, SPIE proceedings, arXiv:1807.04354
• “Planck’s view on the spectrum of the Sunyaev-Zeldovich effect”, J. Erler et al. 2018, MNRAS, arXiv:1709.01187

Contact: Dr. Kaustuv moni Basu (kbasu@astro.uni-bonn.de), Prof. Dr. Frank Bertoldi (bertoldi@astr.uni-bonn.de)

Site: Bonn, Argelander Institute for Astronomy, University of Bonn

Measuring the cosmic velocity field with galaxy clusters (UB12)

Studying the number count of galaxy clusters is currently one of the leading methods for cosmological studies, particularly for finding out the nature of dark energy. These cluster surveys are primarily conducted in the optical, X-ray, or millimeter wavebands, where the last option make use of the so-called Sunyaev-Zel’dovich (SZ) effect for detecting and characterizing galaxy clusters out to very high redshifts. But the SZ effect measurements also provide additional benefits like inferring the proper motion of galaxy clusters (sometimes called the peculiar velocity) in the comoving cosmological frame. Measuring this velocity field will be a new and much fruitful method for constraining cosmology and particularly the dark energy models. This research project will focus on improving the cluster velocity measurement techniques based on multi-frequency SZ survey data, both from the currently available Planck satellite and also from the upcoming CCAT-prime telescope. Our group at the Bonn University is strongly involved in the latter project whose data will become available starting from 2021.

Bibliography:

• “CCAT-Prime: science with an ultra-widefield submillimeter
observatory on Cerro Chajnantor”, G. Stacey et al. 2018, SPIE proceedings, arXiv:1807.04354
• “Planck’s view on the spectrum of the Sunyaev-Zeldovich effect”, J. Erler et al. 2018, MNRAS, arXiv:1709.01187

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astr.uni-bonn.de)

Site: Bonn, Argelander Institute for Astronomy, University of Bonn

Resolved SZE observations of galaxy clusters (UB13)

This PhD project will prepare and conduct sensitive, high-resolution interferometric (CARMA, ALMA) and single dish (GBT, IRAM 30m, CCAT) multi-band imaging of galaxy clusters in the Sunyaev-Zel’dovich Effect (SZE).

We expect to benefit in particular from using representative subsamples of eROSITA-detected clusters.

Galaxy clusters can be used as powerful probes to constrain cosmological models. They also represent laboratories to study the baryonic physics and its interplay with structure formation. Especially when observed at X-ray or millimeter/sub-mm (SZE) wavelengths, the hot, diffuse intracluster medium (ICM) allows to infer valuable information on the total mass, dynamical structure and evolutionary status of the cluster, as well as on the thermal and chemical properties of the ICM itself. Resolved SZE imaging of galaxy clusters provides important constraints on the cluster baryonicstate, revealing merger shock fronts or extended regions of shock-heated gas at any temperature. The internal bulk motions induced by mergers contribute to the kinetic SZ signal that can be detected through multi-frequency SZE observation. ALMA and single dish SZE imaging (CCAT, IRAM 30m, GBT) together can resolve all relevant scales of galaxy clusters at all redshifts, delivering accurate estimates of the integrated Comptonization parameter (used as cluster mass proxy) for samples large enough to be of cosmological significance.  This project will therefore also support our efforts within the European ALMA regional center (ARC) to investigate methods and develop software for a optimal combination of ALMA interferometer and single dish imaging data.  The PhD student will participate in the transregional collabroative research center TRR 33 “Tha Dark Universe” and in the activities of the German ARC node.

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astro.uni-bonn.de), Dr. Kaustuv Basu (kbasu@astro.uni-bonn.de)

Site: Argelander-Institute for Astronomy, University of  Bonn

Conditions of star-formation in low redshift galaxies (UB14)

UB14: Conditions of star-formation in low redshift galaxies

Tracing galaxies over the past 8 billion years, we found a correlation between their star formation activity and stellar mass (Noeske et al., 2007, Schreiber et al. 2015). This “main sequence” of star-formation suggests that galaxies evolved through a steady and self-regulated mode of star-formation, sustained by the accretion of cold gas along the cosmic web. Variations of the gas accretion rate on a Gyr timescale and internal gas transport can affect the star formation rate, leading to the apparent 0.3 dex dispersion of the main sequence (Tacchella et al., 2016). To constrain the mechanisms that cause the shape and dispersion of the main-sequence requires detailed observations of large and comprehensive samples of galaxies.

PhD project: We will constrain the star formation, dust content and temperature, and the gas content of several ten thousands of low redshift (z<0.4) galaxies. This requires a careful modelling of the far-infrared spectral energy distribution using the largest optical-to-submillimeter dataset available, the GAMA/H-ATLAS survey (Driver et al. 2016), employing innovative statistical methods (e.g., stacking). We will investigate variations of the dust and gas properties across and along the main-sequence and compare those with theoretical expectations. Combined with size measurements from GAMA, we will study the so called Schmidt-Kennicutt relation across and along the main-sequence, obtaining constraints on the physical conditions of star-formation within these galaxies. We will also explore the effects of large-scale cosmic environment on these relations.

Driver et al. 2016, MNRAS, 455, 3911
Magnelli et al. 2014, A&A, 561, 86
Noeske et al., 2007, ApJ 660, 43
Schreiber et al., 2015, A&A, 575, 74
Tacchella et al., 2016, MNRAS 457, 2790

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de)
Site: Argelander-Institute for Astronomy, University of Bonn

A census on the dust and gas content of galaxies across cosmic time (UB15)

UB15: A census on the dust and gas content of galaxies across cosmic time

Among the most pressing questions in galaxy evolution studies is how the total stellar mass assembled over cosmic time. Our understanding of the regulating cause of star formation in galaxies is poor, but over the past years a picture emerged in which the build-up of new stars is tightly related to the existing stellar mass (e.g. Karim et al. 2011, Magnelli et al. 2014, Schreiber et al. 2015). This suggests that a self-regulated gas-exchange of galaxies with their respective haloes relates to the small-scale, highly inefficient process of star formation (e.g., Bouché et al., 2010; Lilly et al., 2013; Peng et al., 2014; Rathaus et al. 2016). Given the hierarchical, violent assembly of the large-scale dark matter component of the Universe, this link is surprising and needs concrete observational evidence. A future confirmation or falsification of this gas-regulator model relies on better measurements of the gas content of galaxies across cosmic time.

PhD project: We will measure the gas content, gas surface density, and star formation surface density in representative samples of star forming galaxies across cosmic time. The gas content of galaxies is measured using deep and wide area millimeter “surveys” provided by the ALMA archive. Their star forming sizes will be inferred from our 3 GHz, 0.7 arcsec resolution radio continuum survey of the COSMOS field, extending results from Jimenez et al. (in prep.) towards low mass galaxies, using innovative statistical methods (e.g., stacking). By combining the gas and size measurements, we can measure for the first time the Schmidt-Kennicutt relation and its evolution across cosmic time. We will then explore the effects of the galaxy environment on the gas content of galaxies, which was suggested to be separable from the star formation-mass link (e.g. Peng et al. 2010).

This project is to be conducted within the Cologne-Bonn collaborative research center CRC 956, and in close collaboration with our international colleagues E. Schinnerer, D. Liu, V. Smolcic.

Bouché et al., ApJ 718, 1001, 2010
Karim et al. 2011, ApJ, 730, 61
Lilly et al., 2013, ApJ 772, 119
Magnelli et al. 2014, A&A, 561, 86
Peng et al., 2014, MNRAS 438, 262
Rathaus et al., 2016, MNRAS 458, 3168

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de)
Site: Argelander-Institute for Astronomy, University of Bonn

Submillimeter Observations of the highest redshift galaxies and quasars (UB16)

UB16: Submillimeter observations of the highest redshift galaxies and quasars

We will investigate the physical, chemical and dynamical conditions of the star forming gas in high-redshift quasars and far-infrared selected starburst galaxies, and follow how this relates to the processes that shape galaxies and govern the formation of stars in the early universe. For this we will conduct high angular resolution imaging of CO, [CII], [NII] and continuum emission of redshift 2 to 7 quasars, using the IRAM PdBI, JVLA, and ALMA (Swinbank et al. 2012, Riechers et al. 2013, Decarli et al., 2017). We will search for new samples of high-redshift 4 to 6 far-infrared-selected galaxies, applying a blind [CII] line detection method to the ALMA archive. With high angular resolution follow-up of their [CII] line emission with ALMA/NOEMA, together with available observations (Jimenez-Andrade et al. 2018), we will study their dynamics and the gas conditions in which their intense star-formation occurred. We will compare [CII] observations of far-infrared selected galaxies and quasars, and in the long term plan blind spectral surveys of [CII] for the earliest star forming galaxies using ALMA and CCAT-prime (www.ccatobservatory.org), for which we shall define the first survey observations. We will identify and quantify the potential of far-infrared fine structure lines to provide new diagnostics that constrain physical parameters, such as average densities and temperatures in the star forming interstellar medium on sub-kpc scales.

This project is expected to be conducted within the Cologne-Bonn collaborative research center CRC 956, and in close collaboration with our international colleagues F. Walter, C. Carilli, A. Omont, R. Wang, V. Smolcic, X. Fan, et al.

Bibliography:
Decarli et al., 2017, Nature, 545, 457
Swinbank et al. 2012, MNRAS, 427, 1066
Riechers et al. 2013, Nature, 496, 329
Jimenez-Andrade et al. 2018, A&A, 615, 25

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de)
Site: Argelander-Institute for Astronomy, University of Bonn

Ambiguities in gravitational lens modeling (UB17)

Finding mass models for multiple images and giant arcs is a standard problem of gravitational lens theory. These mass models are not only of interest with regards for determining the radial mass profile and angular structure (e.g., ellipticity) of the lensing galaxy or cluster, but they are also needed if lens systems are to be employed for cosmological purposes, such as estimates of the Hubble constant. There is a well-known analytical transformation of mass models, called the mass-sheet transformation, which leaves all observables properties of the lens system invariant; hence, for obtaining a unique mass model, the corresponding degeneracy needs to be broken with external data. Recently, another (almost) invariance transformation was found, the so-called source position transformation (SPT). In a PhD thesis, the properties of the SPT shall be investigated in detail, exploring the freedom in mass models this new transformation allows. For example, how much can the SPT change the angular structure of the lens, which is left unchanged under the mass-sheet transformation and thus considered to be a robust outcome of lens models? What is the impact of the SPT on estimates of the Hubble constant, or asked in a different way: assuming the Hubble constant to be known from other sources, how much freedom is still left in the SPT?

Litterature:

Schneider, P. & Sluse, D. (2013), “Mass-sheet degeneracy, power-law models and external convergence: Impact on the determination of the Hubble constant from gravitational lensing”, A&A 559, A37

Schneider, P. & Sluse, D. (2014), “Source-position transformation: an approximate invariance in strong gravitational lensing”, A&A 564, A103

Contact: Prof. Dr. Peter Schneider (peter@astro.uni-bonn.de)

Site: Bonn, Bonn, Argelander Institute for Astronomy, University of Bonn

The Kilo Degree Survey and the mean mass distribution of galaxies and clusters (UB21)

Using the method of weak gravitational lensing, which employs the effect that light bundles are slightly distorted as they propagate through a tidal gravitational field, one can measure the mean mass profile of luminous objects, such as galaxies, groups or clusters. The accuracy of such a measurement depends on the amount of available data, its imaging quality and on the ability to estimate redshifts of galaxies from multi-band photometry (so-called photometric redshifts). In this respect, the recently started Kilo Degree Survey (KiDS) provides a unique data set, owing to its wide-field coverage of 1500 square degrees, its excellent image quality and (together with the VIKING survey) having photometry in nine optical and near-infrared bands. The thesis project will be a combination of strong participation in the analysis of the optical data from KiDS, and a scientific exploitation of the data with regards to weak lensing measurements of galaxies and clusters. Depending on the preference of the PhD candidate, the emphasis can be chosen to be on photometric redshift techniques, shear measurements, or statistical analysis, in close collaboration with other team members.

References:
Erben, T. et al. (2009), A&A 493, 1197
Reyes, R. (2008) MNRAS 390, 1157
Schneider, P. (2006), “Weak gravitational lensing”, arXiv:astro-ph/0509252

Contact: Dr. Thomas Erben (terben@astro.uni-bonn.de),
Prof. Dr. Peter Schneider (peter@astro.uni-bonn.de)

Site: Bonn, Bonn, Argelander Institute for Astronomy, University of  Bonn

Calibrating high-redshift clusters masses with Hubble Space Telescope weak lensing observations (UB22)

Weak gravitational lensing is a key technique for the calibration of galaxy cluster mass proxies and therefore an essential ingredient for cluster-based cosmological studies. We have a lead role for the weak lensing mass calibration of the cluster survey conducted by the South Pole Telescope (SPT) via the Sunyaev-Zel’dovich effect, which has provided the largest number of massive high-redshift clusters to date. We are seeking for a PhD student to join our team, focusing on the analysis of our new Hubble Space Telescope observations of high-redshift galaxy clusters. It is the goal of this analysis to provide the most accurate and precise calibration of high-redshift cluster masses to date, as crucially required for the full cosmological exploitation of SPT and other cluster surveys such as DES and eROSITA. In addition to the SPT sample we have obtained new HST observations for two further high-mass, high-redshift clusters, which may also be analysed as part of the thesis work. In combination with additional multi-wavelength follow-up this analysis will also aim at a detailed astrophysical characterisation of the clusters.

Contact: Dr. Tim Schrabback (schrabba@astro.uni-bonn.de), Prof. Dr. Peter Schneider (peter@astro.uni-bonn.de)

Site: Bonn, Bonn, Argelander Institute for Astronomy, University of Bonn

Preparing for weak lensing measurements with the Euclid space probe (UB23)

 ESA’s space probe Euclid, currently scheduled for launch in 2020, will constrain the cosmological model and study the nature of dark energy using two techniques: weak gravitational lensing and redshift space distortions. Our group at AIfA is involved in the scientific preparation of the mission, in particular the development of techniques for weak lensing galaxy shape measurements that require careful correction for instrumental distortions such as the image point-spread function (PSF). We are looking for a PhD student to join our team. One focus of our activities is on exploiting the Hubble Space Telescope (HST) archive as training sample for weak lensing shape measurements. As part of the thesis work the student would contribute to the image reductions, account for the impact of HST instrumental effects (e.g. PSF variations), measure intrinsic galaxy shape parameters (which are required for the calibration of shape estimation techniques), and contribute to the development of improved shape estimation algorithms and the generation of mock image simulations.Schneider, P. (2006), “Weak gravitational lensing”, arXiv:astro-ph/0509252 Schrabback, T. et al. (2010) “Evidence of the accelerated expansion of the Universe from weak lensing tomography with COSMOS”, A&A 516, A63Contact: Dr. Tim Schrabback (schrabba@astro.uni-bonn.de), Prof. Dr. Peter Schneider (peter@astro.uni-bonn.de)Site: Bonn, Bonn, Argelander Institute for Astronomy, University of Bonn

1st Physikalisches Institut of the University of Cologne

Research Groups: https://www.astro.uni-koeln.de/

Physics of Low-Luminosity Radio Nuclei (UK01)

 Aim of this project is to address the global radio and molecular gas properties of a representative  sample of galaxies hosting low-luminosity quasi-stellar objects. An abundant supply of gas is  necessary to fuel both the active galactic nucleus and any circum-nuclear star-burst activity of  quasi-stellar objects (QSOs). The connection between ultra-luminous infrared galaxies and the host  properties of QSOs is a subject to a controversial debate. Nearby low-luminosity QSOs are ideally suited to study the properties of their host galaxies because of their higher frequency of occurrence  compared to high-luminosity QSOs in the same comoving volume and because of their small cosmological distance.Representative samples are selected from QSO and radio surveys. The abundance of molecular gas and the importance of star formation is being probed through infrared- and mm-spectroscopy. The nuclear activity is tested through radio interferometric observations that aim at distinguishing between contributions from  non-thermal nuclei and super nova remanents in these low luminosity nuclei.Contact: Prof. Dr. Andreas Eckart (eckart@ph1.uni-koeln.de)Site: Cologne, 1st Physics Institute, University of Cologne

NIR Studies of the Black Hole at the Center of our Galaxy (UK02)

The compact source Sgr A* that can be associated with the massive black hole at the center of the Milky Way shown a strong variability from the radio to the X-ray wavelength domain. The most recent results from a near-infrared observations revealed polarized NIR flare emission of Sgr A*. This can be interpreted as emission from spots which are on relativistic orbits around Sgr A*. Emission from a possible jet or outflow from such a disk can also contribute. We also find that the variable NIR emission of Sgr A* is highly polarized and consists of a contribution of a non- or weakly polarized main flare with highly polarized sub-flares. The flare activity shows a possible quasi-periodicity of 20 min consistent with previous observations. The highly variable and polarized emission supports that the NIR emission is non-thermal and is consistent with emission from a jet or temporary disk. Alternative explanations for the high central mass concentration involving boson or fermion balls are increasingly unlikely. Observations with the VLT, VLTI and in future with the LBT will allow us to better discriminate Sgr A* from the surrounding stars, to register the light curves with a higher signal to noise, and to further develop the theoretical models.

Contact: Prof. Dr. Andreas Eckart (eckart@ph1.uni-koeln.de)

Site: Cologne, 1st Physics Institute, University of Cologne

Young protoplanetary disks at high-angular resolution and contribution to the METIS/ELT project (UK03)

Protoplanetary disks are the birthplace of exoplanets. During their formation process they dynamically interact with the circumstellar material, which alters and shapes the disk. Such formation signatures are detectable in the inner regions of the disk using high angular resolution techniques in the infrared such as AO or interferometry at the VLTI. Other star/disk interaction tracers (e.g. accretion) essential in our understanding of planet formation can be traced by these techniques.
Within this project, the successful candidate will work with existing and new data from VLT/VLTI (Gravity and, in the near future MATISSE) to investigate such processes in YSOs (e.g. Matter, Labadie et al., A&A 586, A11, 2016). This observational project also tackles the question of disk evolution in close binary systems. A second part of the PhD project is focusing on infrared instrumentation and is devoted to contributing the development and building of the Warm Calibration Unit of the METIS instrument on the upcoming European Extremely Large Telescope. The E-ELT is one of the leading project in the field of infrared astronomy. Strong interactions with the METIS team in Cologne and within the international METIS consortium is expected from the candidate.

Site: Cologne, 1st Physics Institute, University of Cologne

Star formation in deep-zoom simulations of molecular clouds (UK04)

The aim of this project is to study star formation in turbulent molecular clouds with high-resolution, three-dimensional, magneto-hydrodynamic adaptive mesh refinement simulations, which include self-gravity, radiative transfer and a chemical network. The state-of-the-art simulations will help us to understand where massive stars form within a molecular cloud and what special conditions must be met in order to allow for massive star formation. Further, we will study the role of the magnetic field in shaping the environment that is about to form a massive star and on the star formation process itself. Massive stars are already the sources of intense radiation before they reach the main sequence and therefore the effects of radiative feedback has to be taken into account. The project is suited for a student with excellent numerical skills and a great interest in star formation.

Contact: Prof. Dr. Stefanie Walch-Gassner (walch@ph1.uni-koeln.de)
Site: Cologne, 1. Physics Institute, University of Cologne

The formation of molecular clouds in galactic disks (UK05)

Molecular clouds form out of the warm, turbulent interstellar medium. In regions where the gas density is increased, for example by sweeping up material through shock waves in galactic spiral arms or on the borders of super-shells which are driven by a massive central cluster, the gas may cool and molecules begin to form until a molecular cloud is assembled. In low-resolution simulations of galactic disks, which set the large-scale environment, we zoom in on regions which are about to form molecular clouds and follow these with high-resolution, three-dimensional adaptive mesh refinement simulations to study the properties of the resulting clouds in different galactic environments. We will be able to address, whether the initial conditions for star formation vary as a function of environment in a typical galaxy.

Contact: Prof. Dr. Stefanie Walch-Gassner (walch@ph1.uni-koeln.de)
Site: Cologne, 1. Physics Institute, University of Cologne

Determining the chemical phases of molecular clouds (UK06)

Stars form in molecular clouds. However, it is to date still a highly debated question how fast molecular clouds evolve and how fast they form stars. Similar, it is of great interest how much stars they form, in which regions and under which physical and chemical conditions. Within this project the PhD candidate will work on assessing the accuracy of observational methods used to answer these questions by direct comparison of (synthetic) observations and simulations. The project is based on state-of-the-art 3D, magneto-hydrodynamical simulations of molecular clouds and their embedded star formation process and include an astrochemical network. In a first step, the PhD candidate will produce synthetic observations from these simulations using modern radiative transfer codes. In a second step, these synthetic observations will be compared to actual observations.
The project aims to tackle several burning questions in modern astronomy:
– Which molecules are most suitable to determine the mix of different (chemical and physical) phases in molecular clouds via observations?
– How accurate can this mix be probed under different conditions?
– What are observational requirements (telescope sensitivity and exposure time) for successful observations?

The candidate will investigate the emission of several molecules. In detail, the project aims at investigating the origin of the emission lines of atomic and ionised carbon in molecular clouds and simulating the emission of OH, ArH$^+$, HF, and HCl. The actual list of molecules will depend on the results achieved by the candidate and can be adapted flexible during the course of the PhD. Furthermore, the candidate will study both the effect of different environmental conditions and observing conditions. Based on the outcomes, the candidate is expected to get involved in observational proposals to test the results obtained in this work. For this purpose we expect the candidate to closely interact with various observational groups at the 1. Physical Institute in Cologne as well as in Bonn.

Bibliography:
– Walch et al., 2015, MNRAS, 454, 238, “The SILCC (SImulating the LifeCycle of molecular Clouds) project – I. Chemical evolution of the supernova-driven ISM”, arXiv:1412.2749
– Seifried et al, 2017, MNRAS, 472, 4797, “SILCC-Zoom: the dynamic and chemical evolution of molecular clouds”, arXiv:1704.06487

Contact: Prof. Dr. Stefanie Walch-Gassner (walch@ph1.uni-koeln.de), Dr. Daniel Seifried (seifried@ph1.uni-koeln.de),
Site: Cologne, 1. Physics Institute, University of Cologne