# PhD Projects

//PhD Projects
PhD Projects 2019-10-25T12:40:11+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. To do this we use the largest radio telescopes around the world, including our own 100-m telescope in Effelsberg, the MeerKAT array in South Africa and FAST in China.

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 processes 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),

#### Optimizing search strategies for fast radio bursts (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. Discovering more sources is key to making progress, but current search techniques are not optimized to distant sources. Furthermore, it is key to make quality detections with as many burst properties as possible.
The goal of this project is to develop new methods for searching for FRBs that are better optimized for extragalactic bursts. One target for the developed pipelines is a state-of-the-art phased array feed receiver on the Effelsberg radio telescope. A successful applicant should have a background in computer programming.
This student will work in the Lise-Meitner independent research group of Dr. Laura Spitler.
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

#### Cosmic Magnetism - from the local to the distant Universe (MK06)

Dynamically important magnetic fields have been shown to play pivotal roles in processes that are intimately linked to galaxy evolution, such as star formation, disk-halo interaction, gas accretion and galactic-scale outflows in local and distant galaxies. However, how galaxies and their magnetic fields have co-evolved since the early Universe remains one of the major unsolved fundamental question in astrophysics. To directly address the origin and evolution of cosmic magnetism, and their impact on galaxy evolution, we need to probe cosmic magnetic fields from the local to the high redshift Universe using broadband polarization measurements from new radio instruments around the world including the MeerKAT (the Square Kilometre Array SKA precursor), Australia SKA pathfinder (the POSSUM survey), Jansky Very Large Array and other SKA pathfinders.
The following research projects are offered:
(1) The Magnetic Milky Way: The MPIfR-MeerKAT Galactic Plane survey at S-band (1.7-3.5 GHz)will provide exquisite polarization data to characterize the large and small-scale Galactic magnetic fields in unprecedented details. The successful candidate will construct a dense Faraday rotation measure grid towards polarized background radio sources, along with the observed diffuse polarized synchrotron emission, to map the Galactic magnetic field in the southern Galactic plane, build a detailed model Milky Way disk field model, and compare the observed magnetic field properties with theoretical predictions.
(2) Magnetic field tomography of nearby galaxies: The recent advent of broadband radio polarimetry has enabled the reconstruction of 3D magnetic fields in nearby galaxies. The successful candidate will use observations of nearby galaxies to model the measured complex polarization as a function of wavelength to derive their 3D magnetic field structures. These field properties will then be correlated with star formation rate, neutral and ionized gas properties to constrain conditions optimal for the galactic dynamo effect.
(3) Broadband polarization properties of extragalactic radio sources:  Faraday rotation measure (RM) towards background polarized radio sources, forming the so-called RM grid, is the major tool to measure magnetized gas in a range of foreground astrophysical objects in the era of the SKA pathfinders and the eventual SKA. Characterizing the intrinsic polarization properties of these sources is the key to successfully conducting the RM grid experiment. The successful candidate will characterize broadband polarization behaviors of a large sample of radio sources and perform a systematic study of their polarization properties as a function of source class and redshift.
(4) Measure galactic magnetic fields in and around high redshift galaxies: Faraday rotation towards radio-bright and polarized quasars enables one to probe magnetized gas in cosmologically distant intervening galaxies. The successful candidate will use broadband radio polarization observations of  quasar absorption line systems to statistically infer magnetic fields associated with intervening galaxies at high redshift. To interpret these observations, the student will compare the observations with predictions from different theoretical models of galactic magnetic field generation mechanisms.
The successful candidate(s) will have opportunities to work in large international collaborations, with close collaborators in South Africa, Australia, the US, Canada and the UK.

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

Contact: Dr. Sui Ann Mao (mao@mpifr-bonn.mpg.de) and Prof. Michael Kramer (mkramer@mpifr.de)

#### Precise constraint of the cosmic microwave background radiation with a new S-Band southern sky survey (MK07)

Precise measurement of the Galactic foreground emission, that
contaminates the faint polarised cosmic microwave background radiation
(CMB), is a major challenge for the next-generation of CMB experiments
and will impact on the precise measurement of anisotropies in its
polarisation states. In particular, the gradient-type E-modes and the
curl-type B-modes are highly affected, which enables us to detect
primordial gravitational waves and the imprints of the reionisation
history of the Universe, that directly impacts on our framework of
the inner workings of the Universe.

In order to improve our understanding of the Galactic foreground,
dedicated sky surveys are necessary to disentangle the various
contributions of the foreground emission, in particular in continuum
emission: synchrotron emission from relativistic electrons gyrating in
ambient magnetic fields, bremsstrahlung (also referred to as the
free–free emission) from thermal electrons scattering off ions,
anomalous microwave emission (AME) and emission from thermal dust
grains; and in polarised emission: synchrotron and thermal dust
emissions. A unique opportunity to measure
these contributions will be provided by the 15-m SKA-MPG telescope in
South Africa operating in the frequency range between 1.7 and 3.5 GHz
(S-Band) and new analysis techniques. In summary, this will allow for better
constrain the foreground cleaning and thus contribute significantly in the
component separation analyses and will increase the usable sky area for
cosmological analysis of the Planck data, and future mission like e.g. t
https://arxiv.org/pdf/1906.04788.pdf

The PhD-Project will cover the full scope of astrophysics, technical
applications, and observational cosmology. In particular, observation,
development and optimisation of calibration, imaging, and analysis
procedures. And a compilation of a first version of the Southern Sky
S-Band survey and a Model of the Galactic foreground will be part of
the thesis.

Contact: Dr. Hans-Rainer Klöckner (hkloeckner@mpifr.de), Dr. Gundolf Wieching (wieching@mpifr.de), Prof. Michael Kramer (mkramer@mpifr.de); in collaboration with Dr. A. Basu and Prof. D. Schwarz (University of Bielefeld)

# Very Long Baseline Interferometry and Radio Astronomy Group at the MPIfR

Director: Prof. Dr. J. Anton Zensus

Group website

#### Multi-wavelength study og the Galactic center's supermassive black hole and its immediate vicinity (AZ01)

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 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 NIR adaptive optics imaging and light curve extraction from Spitzer data, to time series analysis and modeling of radiative processes behind the variability.

#### Bibliography

Collaboration webpage: https://www.cfa.harvard.edu/irac/gc

#### Contact

Prof. Dr. J. Anton Zensus (azensus@mpifr.de), Dr. Gunther Witzel (gwitzel@mpifr.de)

#### Site

Bonn, Max-Planck-Institut für Radioastronomie, VLBI group in collaboration with CfA Harvard, University of California Los Angeles, and the Universität zu Köln.

#### Direct imaging of the extreme vicinity of cosmic black holes with mm-VLBI (AZ02)

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 for the Event Horizon Telescope (EHT), which performes VLBI observations at 1.3mm wavelength.  Both 3.5mm and 1.3mm VLBI observations may include the ALMA telescope. One of the main goals of this effort is to probe the ‘the black hole shadows’ in the Galactic Centre and in the radio-galaxy M 87. Another important topic is to study the origin of jets and the initial jet acceleration and collimation in more distant radio-galaxies and quasars (AGN) with unprecedented resolution.

The PhD candidate is supposed to actively participate in the VLBI imaging of Active Galactic Nuclei (Quasars, BL Lac objects, Radio Galaxies, etc.) with the highest possible  angular and spatial resolution. The research addresses addressing questions related to AGN activity, outburst-ejection relations, the physical origin of jets, the details of the jet launching and the primary jet acceleration  processes. For this the jet kinematics, their spectral and polarimetric properties will be studied using mm-VLBI images obtained from a variety of observing dates, and on spatial scales which are as close as possible to the central engine  (scales of a few 10 – 1000 gravitational radii).

Another science topic is the study of the polarised fine structure of AGN, which probes 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
• Kim, J.-Y., Krichbaum, T., Lu, R.-S., Ros, E., Bach, U., Bremer, M., de Vicente, P., Lindqvist, M., Zensus, J.A.: The limb-brightened jet of M87 down to the 7 Schwarzschild radii scale, A&A 616, A188 (2018) https://doi.org/10.1051/0004-6361/201832921
• 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
• The Event Horizon Telescope Collaboration et al.: First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole, ApJL 875 L4 (2019), https://doi.org/10.3847/2041-8213/ab0e85
• The Event Horizon Telescope Collaboration et al.: First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, ApJL 875 L1 (2019), https://doi.org/10.3847/2041-8213/ab0ec7

Global mm-VLBI Array: www.mpifr-bonn.mpg.de/div/vlbi/globalmm

Event Horizon Telescope: www.eventhorizontelescope.org

#### Contact

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

#### 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 (AZ03)

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

• Fromm, C. M., Younsi, Z., Baczko, A.K., Mizuno, Y., Porth, O., Perucho, M., Olivares, H., Nathanail, A., Angelakis, E., Ros, E., Zensus, J.A., Rezzolla, L.: Using evolutionary algorithms to model relativistic jets – Application to NGC 1052, A&A 629 A4 (2019), https://doi.org/10.1051/0004-6361/201834724
• Fromm, C. M., Perucho, M., Porth, O., Younsi, Z., Ros, E,; Mizuno, Y., Zensus, J. A., Rezzolla, L.: Jet-torus connection in radio galaxies. Relativistic hydrodynamics and synthetic emission, A&A (2018) 609 A80, https://dx.doi.org/10.1051/0004-6361/201731048
• Fromm, C. M.; Perucho, M.; Mimica, P.; Ros, E.: Spectral evolution of flaring blazars from numerical simulations, A&A (2016) 588, A101, https://dx.doi.org/10.1051/0004-6361/201731048
• Perucho, M.; Agudo, I.; Gómez, J. L.; Kadler, M.; Ros, E.; Kovalev, Y. Y.: On the nature of an ejection event in the jet of 3C 111, A&A (2009) 489, L59, https://dx.doi.org/10.1051/0004-6361:200810479

#### Contact

Prof. Dr. Anton Zensus (azensus@mpifr.de), Prof. Dr. Eduardo Ros (ros@mpifr.de), Prof. Dr. Manel Perucho (Univ. València, Spain, perucho@uv.es), Dr. Christian M. Fromm (cfromm@th.physik.uni-frankfurt.de)

#### Site

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

#### Extragalactic jet physics: the radio-gamma connection and monitoring (AZ04)

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/

#### Site

Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group in collaboration with the Universität Würzburg, the NASA Goddard Flight Space Centre, Aalto University, and other centres of the MOJAVE and TANAMI collaborations.

#### Extragalactic jet physics: Testing jet formation in radio galaxies (AZ05)

Nearby radio galaxies are ideal targets for studies aimed at investigating the launching mechanism of relativistic jets.
VLBI observations enable us to obtain a sharp imaging of the jet base in radio galaxies, and to derive the physical properties
of the relativistic jet plasma at short distances from the supermassive black hole. Due to their relative proximity and to
the reduced Doppler boosting affecting the jet emission, these objects also offer us a complete picture of the processes
taking place in active galaxies since, in observations, we are often able to separate the different constituents
of the AGN — the accretion disk, the jet, the torus, the ambient medium — and study their interplay. This is crucial if
we aim at understanding under which conditions a jet is formed, and what determines its power.
This project focuses on the investigation of a well selected sample of radio galaxies, comprising both high-power and
low-power objects. High resolution VLBI observations down to millimeter wavelengths will be combined with lower resolution radio
interferometric data and with observations in other bands (e.g., X-rays), in order to relate the jet properties on small scales
to the characteristics of the nucleus as well as of the large scale jet.

Bibliography:

Boccardi, B., Krichbaum, T.P., Bach, U., Mertens, F., Ros, E., Alef, W., Zensus, J.A.: The stratified two-sided jet of Cygnus A, Astronomy & Astrophysics (2016) 585 A33, https://doi.org/10.1051/0004-6361/201526985

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

Giovannini, G., Savolainen, T., Orienti, M., et al.: A wide and collimated radio jet in 3C84 on the scale of a few hundred gravitational radii, Nature Astronomy (2018) 2 472, https://doi.org/10.1038/s41550-018-0431-

Contacts: Prof. Dr. Anton Zensus (azensus@mpifr.de), Dr. Bia Boccardi (bboccardi@mpifr.de)

#### Magnetic fields in the vicinity of black hole event horizon (AZ06)

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 combine (i) high-resolution multi-frequency polarimetric VLBI radio imaging and (ii )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)

#### Site

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

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

Robust radio interferometric imaging at millimetre and crosubmillimetre 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).

#### Site

Bonn, Max-Planck-Institut für Radioastronomie, VLBI Group, in collaboration with the Event Horizon Telescope Consortium

#### Modeling X-ray Polarization from 3D Relativistic Magnetohydrodynamic Jet Simulations (AZ08)

(i) The PLUTO code is a versatile relativistic magnetohydrodynamic (RMHD) gas dynamics solver that can be used to simulate the plasma of relativistic jets. The latest version of the code contains a new particle module that models the emission emanating from relativistic plasma across the entire electromagnetic spectrum: from low-energy radio waves to high-energy gamma-rays. (ii) The RADMC-3D code is a modularized ray-tracing software designed to interface with 3D gas dynamic simulations in order to create synthetic images of astrophysical plasma simulations. Synthetic imaging of these simulations (via ray-tracing) makes possible more direct comparisons between jet models and the observations of jet synchrotron emission (obtained with interferometric arrays). This PhD project will focus on combining the PLUTO and RADMC-3D codes to produce maps of the polarized synchrotron emission emanating from a 3D RMHD jet simulation in the X-ray pass band. These ray-tracing calculations will help make valuable predictions as to the levels of linear polarization that NASA’s upcoming Imaging X-ray Polarimetry Explorer (IXPE) mission will detect within relativistic jets. The PhD candidate will be introduced to the theory of polarized radiative transfer and numerical RMHD/ray-tracing techniques and will gain valuable (and transferable) skills in high-performance computing through the execution of 3D jet simulations/ray-tracing calculations, all of which are computed ‘in-house’ on the MPIfR’s cluster/correlator.

#### Contact

Prof. Dr. Anton Zensus (azensus@mpifr.de), Dr. Nicholas R. MacDonald (nmacdona@mpifr.de)

Nicholas MacDonald’s Homepage: http://www3.mpifr-bonn.mpg.de/staff/nmacdonald/Home.html

The PLUTO Code: http://plutocode.ph.unito.it/index.html

NASA’s Imaging X-ray Polarimetry Explorer (IXPE): https://ixpe.msfc.nasa.gov/

#### Site

Bonn, Max-Planck-Institut für Radioastronomie, VLBI group

# 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)

#### Imaging the dense molecular gas content of the Magellanic Clouds (KM09)

Mapping the molecular gas content in nearby galaxies has become one of the prime goals to investigate the relation between the molecular gas surface density and the star-formation surface density (the Schmitt-Kennicut law; short KS–law) over the past years. While this field is rapidly evolving  from galaxy integrated survey to spatially resolved  studies from ALMA for relatively massive local galaxies samples, progress for metal poor systems has been much slower. In particular, the question of the extension of the KS-law to low metallicty systems is highly debated and has an important connection to the star formation process in high redshift galaxies. While a lot of effort has been undertaken to study the molecular gas content in nearby dwarf galaxies, the faintness of CO lines has hampered progress to
study the molecular gas content in these low metallicity systems.

With its location in the southern hemisphere, the APEX 12m telescope in combination with the new LASMA 345GHz 7 pixel array receiver will open a new window  to study in great detail two nearest (and therefore brightest) dwarf galaxies in the local Universe – namely  the LMC and SMC. With their significantly low metallicity than spiral galaxies, the LMC and SMC are prime candidates to study the spatially resolve KS-law in low metallicity environments at unmatched spatial resolution.

Site: MPI for Radio Astronomy, Bonn

Contact: Axel Weiss (aweiss@mpifr-bonn.mpg.de)

#### The impact of stellar feedback on the interstellar medium organization of the Milky Way (KM10)

Radiation fields and winds that result from massive star formation play a dominant role into regulating the energy balance and dynamics of the interstellar medium (ISM). These so called “stellar feedback” can drive turbulence, dissipate gas and destroy molecular clouds and, at the same time, compress gas and trigger new star formation. Stellar feedback often manifest themselves into the ISM as round cavities named “bubbles” or “shells”. Nevertheless, bubbles have complex morphologies and are difficult to identify and analyze in a meaningful statistical fashion. “Artificial intelligence” methods are keys to tackle this issue. In this ambitious project unsupervised machine learning techniques (such as dendrograms of emission and clustering) as well as deep learning algorithms (aided by magnetohydrodynamic simulations of turbulent molecular clouds) will be implied to detect stellar feedback regions in large surveys of the Galactic plane. The PhD candidate will work on data acquired by the APEX 12m telescope that is imaging, at high resolution, a large chunk of the Milky Way as part of the “Structure, excitation, and dynamics of the inner Galactic interstellar medium (SEDIGISM)”, “Outer Galaxy High Resolution Survey (OGHReS)”, and “LASMAGAL” projects. The results will be combined with the “Global View of Star Formation in the Milky Way” JVLA survey data to provide the most comprehensive, three dimensional characterization of the Galactic interstellar bubble to date that will help to understand the role of these energetic phenomena on the properties and organization of the interstellar medium of our own Galaxy.

Site: MPIfR, Bonn

Contact: Dario Colombo (dcolombo@mpifr.de), Prof. Dr. K. Menten (menten@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 has been. 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 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
• “SZ spectroscopy” in the coming decade: Galaxy cluster cosmology and astrophysics in the submillimeter”, K. Basu et al., Astro2020 White Paper, arXiv:1903.04944

Contact: Dr. Kaustuv 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:

• “A Space Mission to Map the Entire Observable Universe using the CMB as a Backlight”, K. Basu et al., ESA Voyage 2050 Science Paper, arXiv:1909.01592
• “SZ spectroscopy” in the coming decade: Galaxy cluster cosmology and astrophysics in the submillimeter”, K. Basu et al., Astro2020 White Paper, arXiv:1903.04944

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

Reference: “ALMA-SZ Detection of a Galaxy Cluster Merger Shock at Half the Age of the Universe”, K. Basu et al., ApJ 829, 2016, arXiv:1608.05413

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, TBD)

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?

Literature:

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

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

 ESA’s space probe Euclid, currently scheduled for launch in 2022, 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 the 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. We are also developing a shape measurement method for Euclid based on machine learning. Furthermore, we are characterizing the impact of residual detector effects on the shape analysis. Literature: Schneider, P. (2006), “Weak gravitational lensing”, arXiv:astro-ph/0509252 Laureijs, R. et al. (2011), “Euclid definition study report”, arXiv:1110.3193L Tewes, M. et al. (2019), “Weak-lensing shear measurement with machine learning. Teaching artificial neural networks about feature noise.”,  arXiv:1807.02120 Euclid Collaboration et al. (2019): “Euclid preparation. IV. Impact of undetected galaxies on weak-lensing shear measurements”, arxiv:1902.00044 Contact: Dr. Tim Schrabback (schrabba@astro.uni-bonn.de), Prof. Dr. Peter Schneider (peter@astro.uni-bonn.de)   Site: Bonn, Argelander Institute for Astronomy, University of Bonn

# 1st Physikalisches Institut of the University of Cologne

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