Doctoral Thesis

We currently offer a number of PhD thesis topics that we describe below. If you are interested, please talk to Frank Bertoldi and / or Kaustuv Basu (clusters and cosmology) or Benjamin Magnelli (galaxies). [last update March 2019] 

A next-generation galaxy cluster survey with the CCAT-p telescope 

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.
Literature: “CCAT-Prime: science with an ultra-widefield submillimeter bservatory 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

Measuring the cosmic velocity field with galaxy clusters

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 from  2021 on.
Literature:   “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

Resolved SZE observations of galaxy clusters

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 MYSTERY OF GALAXY CLUSTER RADIO HALO

Giant radio halos inside galaxy clusters are Mpc scale diffuse synchrotron emissions whose formation processes are still poorly understood. These radio halos are associated with galaxy cluster collisions, but we do not know how many of these objects are in the sky or what impact they may have on our understanding of other cluster properties. We have initiated a project to correlate cluster radio halo measurements with X-ray and SZ effect data to understand the powering mechanism and mass dependence of radio halos, as well as determining their true abundance in the sky. New data from several radio telescopes (EVLA, GMRT) have been collected and being analyzed. The goal of this PhD project will be to take a leading position in this work and measure the radio halo properties in several new clusters using this state-of-the-art radio data, aiming towards a comprehensive picture of radio halo origin.
Literature: "A Sunyaev-Zel'dovich take on cluster radio haloes -- I. Global scaling and bi-modality using Planck data", K. Basu; "A comparative study of radio halo occurrence in SZ and X-ray selected galaxy cluster samples", M. Sommer & K. Basu

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

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.
Literature: 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

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, 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. 
Literature:  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