Doctoral Thesis

We  offer a number of PhD thesis topics that we describe below. Funding for doctoral students is limit and may not be available at all time. If you are interested, please talk to Frank Bertoldi and / or Kaustuv Basu. [last update September 2020] 

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