PhD and Master Thesis Themes - Group Bertoldi

We here present a number of possible PhD/Master theses topics. The PhD projects could, in reduced version, serve for as master thesis or Bachelor thesis / internship. We only have a very limited number of paid PhD positions and Master stipends - please contact Frank Bertoldi to find out which topics are currently free and what financial support is available. PhD candidates are commonly expected to apply through the IMPRS.

Master 1: Mapping the diffuse baryon in the local universe

The local group and the Virgo cluster have several well-mapped filaments, derived optical data, that extend tens of degrees on the sky. We shall cross-compare these filaments' optical data with the Sunyaev-Zel'dovich effect maps extracted from the Planck all-sky survey and look for the evidence of diffuse warm-hot intergalactic medium (WHIM). The project involves careful cleaning of the Planck SZ maps and application of several cross-correlation techniques on a sphere.

Master 2 : Study of galaxy cluster accretion shocks from Planck

Galaxy clusters are not isolated objects, they grow in mass from major collisions as well as from a continuous accretion of materials from the surrounding medium. This accretion process leads to infall shocks at the boundary of the bound gas of the clusters. We shall look for the signature of these infall shocks in several nearby galaxy clusters from the Sunyaev-Zel'dovich effect maps prepared from Planck all-sky data.

Master 3: The powering mechanism of radio mini-halos

Radio mini halos are relatively compacts (less than 100 kpc in size) diffuse synchrotron sources residing in the core region of some galaxy clusters. All mini-halo clusters also host powerful AGN at their centers, but the connection between the mini-halo power and AGN activity has not been demonstrated conclusively. Our aim is to check several AGN activity indicators with the associated mini-halo radio luminosity, to establish a definitive causal connection.

PhD1: Physics of galaxy clusters from SZ and X-ray data

In the framework of galaxy cluster cosmology, understanding the physics of the hot, ionized intra-cluster medium (ICM) is a central issue. The ICM plays an important role in the evolution of galaxy clusters and the growth of the cluster member galaxies. The physical condition of the ICM also determines how clusters are selected through the X-ray or Sunyaev-Zel'dovich (SZ) effect surveys. Since clusters can be observed through many different techniques (like X-ray, SZ, optical or radio), a combination of these observables often produce the best results. The APEX-SZ group at the Bonn university is active in this research field, having pioneered some of these joint analysis methods, and the prospective student is expected to become a member of this group. The main research objective will be to develop new methods for studying the cluster morphology and dynamical states through a combination of APEX-SZ, Planck SZ and XMM-Newton data. This project will also prepare the student for future cluster SZ observation using the CCAT telescope, which will start operating in the year 2017 and for which Bonn University will have access through guaranteed time.

"Non-parametric modeling of the intra-cluster gas using APEX-SZ bolometer imaging data", Basu, K. et al. 2010, A&A, 519, 29; "Planck intermediate results. V. Pressure profiles of galaxy clusters from the Sunyaev-Zeldovich effect", Planck collaboration 2013, A&A, 550, 131

Contact: Dr. Kaustuv moni Basu (, Prof. Dr. Frank Bertoldi (

PhD2: The mystery of galaxy cluster radio halos

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.

"A Sunyaev-Zel'dovich take on cluster radio haloes - I. Global scaling and bi-modality using Planck data", Basu, K. 2012, MNRAS, 421, 112; "A comparative study of radio halo occurrence in SZ and X-ray selected galaxy cluster samples", Sommer, M. W. & Basu, K. 2014, MNRAS, 437, 2163

Contact: Dr. Kaustuv moni Basu (, Prof. Dr. Frank Bertoldi (

PhD3: 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 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: Dr. Kaustuv moni Basu (, Prof. Dr. Frank Bertoldi (

PhD4: The cosmic star formation history seen through (sub)mm deep field surveys

Background: Among the most pressing questions in modern galaxy evolution studies is how stellar mass assembled over cosmic time. Our current understanding of the main drivers of star formation within galaxies is still sparse but over the last years a picture emerged in which the build-up of new stars is tightly connected to the existing stellar mass (e.g. Karim et al. 2011). This possibly suggests that simple self-regulated gas-exchange of galaxies and their respective haloes is capable of describing the very localized and highly inefficient process of star formation using only a few parameters (Lilly et al. 2013). Given the highly hierarchical, violent assembly of the large-scale dark matter component of the Universe, this link is surprising and awaits detailed observational confirmation. It has been suggested that the effects fo the galaxy environment are separable from the star formation-mass link (e.g. Peng et al. 2010), a suggestion that also awaits observational confirmation.
PhD project: This project aims at determining the dust-unbiased cosmic star formation rate by measuring the important star formation/stellar mass link out to redshifts z≈2, using individually detected typical galaxies that are responsible for the bulk of the star formation activity. We will also pursue the identification of heavily dust-obscured and most extreme star forming galaxies at z>4. We will make use of the growing (sub)millimeter data in the 2 sqd. COSMOS deep field. Our group is leading observing efforts using the latest bolometer array instrumentation, at 2mm with the GISMO camera and at 0.45/0.87mm with the A-MKID camera. This unique multi-wavelength coverage is needed to systematically constrain the abundance and properties of interstellar dust (predominantly heated by young stars) in distant galaxies. Overall, this PhD project will provide critical pathfinder science for the upcoming CCAT observatory ( and is expected to be conducted within the Cologne-Bonn collaborative research center SFB 956.

Karim et al. 2011, ApJ, 730, 61; Lilly et al. 2013, ApJ, 772, 19; Peng et al. 2010, ApJ, 721, 93

Contact: Prof. Dr. Frank Bertoldi (, Dr. Alexander Karim (, Dr. Benjamin Magnelli (

PhD5: A panoramic millimeter-radio view onto the cosmic star formation history

Among the most pressing questions in modern galaxy evolution studies is how stellar mass assembled over cosmic time. Our current understanding of the main drivers of star formation within galaxies is still sparse but over the last years a picture emerged in which the build-up of new stars is tightly connected to the existing stellar mass (e.g. Karim et al. 2011 and references therein). This possibly suggests that simple self-regulated gas-exchange of galaxies and their respective haloes is capable of describing the very localized and highly inefficient process of star formation using only a few parameters (Lilly et al. 2013). Given the highly hierarchical, violent assembly of the large scale dark matter component of the Universe this link is surprising and awaits detailed observational confirmation. Similarly, it has been suggested that the effects different galaxy environments cause are fully separable with respect to the star formation-mass link (e.g. Peng et al. 2011), a conclusion that clearly awaits observational confirmation at earlier cosmic epochs than probed so far.

High angular resolution radio continuum observations of cosmological deep fields can play an important role in dissecting all these effects and shed light on the related open questions. Radio observations have been highly successful in providing tight constraints for our understanding of star formation over large cosmic timescales (e.g. Karim et al. 2011). The PhD project suggested here will make use of the unique data sets currently observed at the Jansky-VLA in the 2 square degree COSMOS deep field (see Schinnerer et al. 2010 for earlier COSMOS radio observations using the VLA). The angular resolution achieved in this survey will allow us to explore the extent of the star forming regions within galaxies and help to dissect them from the active nuclear regions. Particularly, recently obtained detailed environmental information in the COSMOS field will allow to study the link of star formation and mass in a wide range of galaxy environments, such as clusters, groups and filaments of the large-scale matter distribution in unprecedented detail. The successful candidate will also have the opportunity to use the radio data in combination with newly obtained mm-data (from single-dish as well as interferometric observations, partially from ALMA) to study star formation at very high redshifts z>4 and to explore further research avenues within the vast panchromatic COSMOS survey data sets. Alexander Karim and Frank Bertoldi are full members of the international COSMOS consortium and have access to all the latest deep observations from a large variety of instruments. Critically, they play leading roles in the ongoing field observations using the expanded Very Large Array (Jansky-VLA; USA) as well as the 2mm GISMO bolometer array operating at the IRAM 30m telescope on Pico Veleta (Spain).

Contact: Prof. Dr. Frank Bertoldi (, Dr. Alexander Karim (, Dr. Benjamin Magnelli (

PhD6: 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 throughout cosmic time, 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 and redshift 4-6 submillimeter galaxies, using the IRAM PdBI, JVLA, and ALMA (Swinbank et al. 2012, Riechers et al. 2013). We will compare ALMA/PdBI [CII] observations of submm galaxies and quasars and in the long term plan blind spectral surveys of [CII] for the earliest star forming galaxies using ALMA and CCAT (, 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 medium on sub-kpc scales. For this we shall also pursue dedicated line and continuum modeling to quantify the proprietary and literature data sets. This project is expected to be conducted within the Cologne-Bonn collaborative research center SFB 956, and in close collaboration with our international colleagues F. Walter, C. Carilli, A. Omont, R. Wang, V. Smolcic, X. Fan, et al.

Swinbank et al. 2012, MNRAS, 427, 1066; Riechers et al. 2013, Nature, 496, 329

Contact: Prof. Dr. Frank Bertoldi (

PhD7: A census of active galactic nuclei across cosmic time

Active galactic nuclei manifest themselves in a variety of observational diagnostics. The mechanisms triggering AGN are empirically not well constrained, and it is unclear to what extent distant AGN affect their host galaxies in their ability to form stars. We shall use state-of-the-art deep field far-infrared to radio continuum survey data to study the coeval growth of SMBH and their host galaxies, for AGN samples selected in X-rays, emission lines, near-IR diagnostics, and radio continuum. Determining star formation rates from appropriate far-IR diagnostics and using a large set of deep multi-wavelength ancillary data will allow us to relate AGN host galaxies to the global population of distant star forming galaxies. Specifically, we will identify radio AGN within the 2 sqd COSMOS field (Smolcic et al. 2009; Schinnerer et al. 2010). Radio AGN have been favored by theorists (e.g. Croton et al. 2006) to explain the absence of a very massive galaxy population. In such semi-analytical models the so-called radio-mode feedback controls the gas heating such that a quiescent, non-star forming mode is maintained. This project will constrain the radio AGN luminosity function and determine empirical constraint on the total energy deposit of radio AGN into their hot gas haloes. We make use of the unique multi-frequency radio continuum data coverage of the COSMOS field between 0.3-3GHz.
This project will be carried out in close collaboration with V. Smolcic (Zagreb), E. Schinnerer (Heidelberg) and H. Klöckner (MPIfR). We are members of the international COSMOS deep field survey consortium and have access to all the latest deep observations from a large variety of instruments. We co-lead the ongoing high angular resolution radio continuum observations of this field using the expanded Very Large Array (Jansky-VLA; USA) and low-frequency surveys with the Giant Meterwave Radio Telescope (GMRT; India).

Smolcic et al. 2009, ApJ, 696, 24; Schinnerer et al. 2010, ApJS, 188, 384; Croton et al. 2006, MNRAS, 365, 11

Contact: Prof. Dr. Frank Bertoldi (, Dr. Alexander Karim (, Dr. Benjamin Magnelli (

PhD8: A systematic search for dust excess emission in nearby galaxies

The spectral energy distribution (SED) provides powerful means to study galaxy properties, such as dust content and extinction, star formation activity, ISM heating and cooling balance. The continuum emission in the submm to radio spectral regime originates mostly from a combination of thermal dust, free-free and synchrotron emission. There is growing evidence from observations of nearby starburst galaxies, low-metallicity galaxies, as well as submillimetre galaxies for an additional emission constituent on top of these components (provisionally quoted ``excess'' emission for lack of a conclusive interpretation), which challenges this simple picture. At face value, a very cold dust component with colour temperatures of 4-7K can explain this extra emission. This cold component could contain the bulk of the dust mass, making the mass estimate highly uncertain. Nevertheless, modelling this excess with a very cold dust component usually leads to low gas-to-dust ratios, in contradiction with the low metallicities measured in the gas. Different alternative scenarios for the origin of the additional emission in the submm-cm regime have been suggested, e.g.: (a) a variation of the dust emissivity spectral index related to intrinsic dust properties of amorphous grains resulting in a flattening of the spectral shape towards long wavelengths; (b) emission from very small spinning dust grains with a peak frequency in the submm-cm regime; (c) cosmic microwave background (CMB) fluctuations; (d) abundance increase of the hot, very small, stochastically heated grains with a low dust emissivity; and (e) magnetic dipole radiation from thermal fluctuations in the magnetisation of nanoparticles containing iron.

Up to now the origin of the additional emission component still remains unidentified, and our lack of knowledge to which degree it contributes to the Rayleigh-Jeans tail of dust emission prevents a reliable determination of the amount of dust cooler than that responsible for the FIR peak, i.e., below 15 -- 20K. Crucial for a better understanding are observations of the spatial distribution of the excess emission in nearby galaxies. The advent of Herschel and especially Planck data in the far-infrared and submm regime now enables a systematic search for galaxies showing excess emission in their global spectral energy distributions. The aim of this project is to identify these candidate galaxies and subsequently provide the crucial submm/mm imaging. Ideal for this task are the bolometer arrays GISMO and NIKA at the IRAM 30m telescope (Spain), LABOCA at APEX (Chile), as well as the bolometers at the CCAT telescope, which is foreseen to be operational in 2017 and for which Bonn University has guaranteed time. The obtained imaging will be combined with Spitzer and Herschel images to derive maps of the excess emission and its three-point spectral energy distribution. A comparison with different ISM conditions as traced by auxiliary multi-wavelength data shall then be used to put constraints on the possible origin of the excess emission.

Contact: Prof. Dr. Frank Bertoldi (, Dr. Marcus Albrecht (

PhD9: The role of dust in galaxy evolution and in the intra-group medium of compact groups

A main prediction of the cold dark matter (CDM) paradigm is the hierarchical structure formation and consequently that galaxies are more likely to be clustered than isolated. The clustering comprises scales from small groups to clusters and superclusters, where galaxy groups, including the subclass of compact groups, constitute a crucial part in this hierarchy. It has been established that poor groups host the majority of galaxies in the local universe, which has contributed in shifting our approach of treating galaxies as static entities towards a more dynamical view of considering them as interacting members of their local (baryonic and dark) environment. The low velocity dispersions, high number densities and short crossing times characterising compact groups bring forward interactions and mergers. This makes compact group environments the crucial laboratories to investigate the mechanisms related to interaction-induced star formation, galaxy evolution, morphological transitions and the impact of the intra-group medium (IGM) in the nearby universe. Moreover, they most closely reproduce the interaction environment of the earlier universe when galaxies assembled through hierarchical formation.

Based on the amount and distribution of HI, an evolutionary sequence for compact groups was suggested, where in the final phase the bulk of the neutral gas in the disks of the galaxies has been displaced. This classification of the evolutionary state is corroborated by a bimodality in the distribution of the specific star formation rate and of the mid-infrared (MIR) activity. These bimodalities and a gap in the IRAC colour-colour diagramm give evidence for an accelerated galaxy evolution in the compact group environment. Compact groups show mid-IR colour distributions that put them in close relation to the infall regions of galaxy clusters. Moreover, evolved groups show a higher fraction of early-type galaxies. These observations together led to the conclusion that compact groups constitute local examples of the plausible building blocks of clusters at higher redshifts.

Extrapolation of near-infrared and mid-infrared data of compact groups to the far-infrared regime by fitting spectral energy distributions has been used for a comparison of the resulting synthetic flux densities of the entire group with the observed values of the member galaxies. As a result the existence of cold intra-group dust was revealed, which can be a factor 2-3 higher in emission than the cold dust in the member galaxies. Still, deep mm and submm measurements at resolutions that allow to map the distribution and properties of the cold dust within group members and especially the intergalactic medium (IGM) are yet missing. As a result, very little is known on the role that dust plays in the evolution of compact groups and the physics of the IGM. It is unclear to which amount compact groups can replenish the IGM with reprocessed material in the form of diffuse tails and tidal dwarf galaxies. This is also important in view of changing the spectra of background sources. The aim of this project is to obtain and analyse these crucial observations using the bolometers LABOCA at APEX (Chile) and GISMO and NIKA at the IRAM 30m telescope (Spain).

Contact: Prof. Dr. Frank Bertoldi (, Dr. Marcus Albrecht (