Argelander Institute for Astronomy of the University of Bonn

The Argelander Institute for Astronomy

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

Code: UB08

Pair-instability supernovae in the local universe

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

Code: UB09

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

Dark energy makes up about 70% of the Universe's energy content and is responsible for its observed acceleration. A powerful method to constrain this enigmatic component is a precise measurement of the evolution of the most massive objects in the Universe: galaxy clusters. For this purpose, the new X-ray telescope eROSITA will be launched end of 2018. Its mission is the discovery of ~100,000 galaxy clusters, including ALL massive clusters in the entire observable universe. eROSITA will be the first "Stage IV" dark energy probe world-wide. The PhD candidate will work on preparations for this mission and will enjoy privileged access to the first eROSITA data to study galaxy cluster physics, chemistry, and cosmology.

Bibliography:

Pillepich et al. (2012, http://adsabs.harvard.edu/abs/2012MNRAS.422...44P) Merloni et al. (2012, http://adsabs.harvard.edu/abs/2012arXiv1209.3114M)

Borm et al. (2014, http://adsabs.harvard.edu/abs/2014arXiv1404.5312B)

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

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

Code: UB13

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: 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

Code: UB14

The cosmic star formation history seen through (sub)millimeter 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 (www.ccatobservatory.org) and is expected to be conducted within the Cologne-Bonn collaborative research center SFB 956.

Bibliography:

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 (bertoldi@astro.uni-bonn.de),  Dr. Alexander Karim (karim@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de)

Site: Argelander-Institute for Astronomy, University of  Bonn

Code: UB15

Project Title: 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 (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 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.

Bibliography:

Swinbank et al. 2012, MNRAS, 427, 1066

Riechers et al. 2013, Nature, 496, 329

Contact: Prof. Dr. Frank Bertoldi (bertoldi@astro.uni-bonn.de),  Dr. Alexander Karim (karim@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de), Dr. Yujin Yang (yang@astro.uni-bonn.de)

Site: Argelander-Institute for Astronomy, University of  Bonn

Code: UB16

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).

Bibliography:

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 (bertoldi@astro.uni-bonn.de),  Dr. Alexander Karim (karim@astro.uni-bonn.de), Dr. Benjamin Magnelli (magnelli@astro.uni-bonn.de)

Site: Argelander-Institute for Astronomy, University of Bonn

Code: UB17

Ambiguities in gravitational lens modeling

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

Litterature:

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

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

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

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

Code: UB21

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

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

Code: UB22

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

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


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

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

Code: UB23

Preparing for weak lensing measurements with the Euclid space probe

ESA's space probe Euclid, currently scheduled for launch in 2020, will constrain the cosmological model and study the nature of dark energy using two techniques: weak gravitational lensing and redshift space distortions. Our group at AIfA is involved in the scientific preparation of the mission, in particular the development of techniques for weak lensing galaxy shape measurements that require careful correction for instrumental distortions such as the image point-spread function (PSF). We are looking for a PhD student to join our team. One focus of our activities is on exploiting the Hubble Space Telescope (HST) archive as training sample for weak lensing shape measurements. As part of the thesis work the student would contribute to the image reductions, account for the impact of HST instrumental effects (e.g. PSF variations), measure intrinsic galaxy shape parameters (which are required for the calibration of shape estimation techniques), and contribute to the development of improved shape estimation algorithms and the generation of mock image simulations.


Schneider, P. (2006), ``Weak gravitational lensing'', arXiv:astro-ph/0509252
Schrabback, T. et al. (2010) ``Evidence of the accelerated expansion
of the Universe from weak lensing tomography with COSMOS'', A&A 516, A63


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

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

 
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