Diplomarbeitsthemen / Thesis Projects

              in the astrophysics group at the
          Argelander-Institut f. Astronomie
          Univ. Bonn

A brief outline of the research in our group can be found here. The three, partly overlapping research areas are

Strong and weak gravitational lensing and its cosmological applications,
Wide-field imaging, data analysis and processing, and
Studying the Nature of Dark Energy with Galaxy Clusters

Here you can find a set of pretty up-to-date reviews on gravitational lensing, but this is, at least partly, difficult reading material and contains much more information than you want to know (well, at least for a start). This is a comapact and didactical review on Lensing. You can also get a more basic description of what lensing is, how it can be used to study clusters of galaxies and the large-scale structure of the Universe.

Your Quest for the HOLIGRAIL

HOLIGRAIL stands for the Hoher List Gravitational Lensing Project, a long-term monitoring campaign of multiple-image gravitational lens systems with the 1m-telescope at Hoher List observatory in the Eifel, about 100 km south of Bonn. The project ultimately aims at putting constraints on the value of the Hubble constant by means of time delay measurements.
One of the systems monitored by HOLIGRAIL is shown on the right, the four blue objects being images of the same background quasar, gravitationally lensed by the reddish galaxy in the center and the surrounding galaxy group. Intrinsic variability in the light curve of this quasar shows up in all four images, but at different epochs due to the different travel times along the light paths. If one is able to measure the time delay between the images, the Hubble constant can be calculated, provided that a sufficiently well-determined model of the lens is known.
Several telescopes of the 1m- to 2m-class around the world are currently dedicated to the observation of such gravitational lens systems, the 1m-telescope at Hoher List being a potentially valuable supplement due to its location at high latitude. Regular observations started in September 2007; for the time being the monitoring at Hoher List is observer-based, a successive automation of observations is planned and already initiated.
The thesis project covers the reduction and in-depth analysis of observational data, aimed at evaluating whether HOLIGRAIL is competitive, compared to other monitoring projects. You will take on responsibility in observations and the development of software concerning automated observing and data reduction. Furthermore, first steps in a science analysis could be performed. For more information about the HOLIGRAIL project klick here.
Contact person: Peter Schneider, Tel.: 3671

Detection efficiency of galaxy clusters from lensing

Gravitational light deflection by a mass concentration distorts the images of background galaxies. From this distortion, one can determine the mass and study the matter distribution in galaxy clusters. One can also search for galaxy clusters with this weak lensing effect, by investigating the shapes of faint galaxies on the sky and searching for coherent alignment patterns. The advantage of this search method is that galaxy clusters are selected without any reference to their luminous properties, like their abundance of bright galaxies or the X-ray luminosity of the intracluster gas. By applying this technique to wide-field imaging data, quite a number of genuine clusters have already been found, and the detection rate is increasing rapidly. In addition, we have discovered a very strong candidate for a mass concentration which is not associated with a concentration of galaxies, and which has been termed `dark clump'; even a detailed investigation with the Hubble Space Telescope has not fully resolved its physical nature. The method is, however, affected by projection effects of the large-scale matter distribution in the Universe. In this thesis work, quantitative methods shall be investigated which aim towards minimizing these projection effects; these should then be applied to a set of 100 cluster candidates that have been found in the course of M. Schirmer's dissertation. Furthermore, these methods will be applied to the data from the upcoming KIDS survey (see below), in which our group is strongly involved.
Contact person: Peter Schneider, Tel.: 3671

Mapping the dark matter in high-redshift X-ray galaxy clusters

In the last years, observations have shown that the expansion of the Universe is accelerating, despite all the massive galaxies in it attracting each other gravitationally. The reason for this acceleration is unknown; it may be related to a missing link in fundamental physics (the ``problem'' of Einstein's cosmological constant). Currently, the best way to study the nature of this ``dark energy'' component is through astronomical observations. Large efforts are being undertaken presently across many countries for this study. One method uses the evolution with time of the most massive clearly defined structures in the Universe: galaxy clusters. With the ultimate goal of constraining this evolution, the diploma thesis work would concentrate on reduction and weak lensing analysis of optical galaxy cluster data taken at the 6.5m MMT telescope with the new 36-CCD Megacam camera (travel to Mt. Hopkins in Arizona may also be required for obtaining observations for additional clusters). The clusters are selected from a complete X-ray flux-limited sample of distant galaxy clusters. All of these clusters have been reobserved with the X-ray satellite observatory Chandra, allowing detailed comparisons of the dark (weak lensing) and luminous (X-ray) cluster components.
Contact person: Thomas Reiprich, Tel.: 3642

A related project is based on image mosaics of medium- to high-redshift galaxy clusters with the Advanced Camera for Surveys (ACS) onboard Hubble Space Telescope. These fields cover (6 arcmin)^2, centered on clusters known to exhibit strong and weak lensing effects. The goal of the project would be to conduct a combined strong and weak lensing analysis of these clusters, employing methods that have been developed in our group. In addition, the mass density profile would be combined with a detailed study of the galaxy properties in these clusters, carried out by the group of Bodo Ziegler in Goettingen, to correlate galaxy properties with the local surface mass density, e.g., to study the morphology-density relation.
Contact person: Peter Schneider, Tel.: 3671

Cosmology from wide-field imaging: The KIDS survey

Weak gravitational lensing, the distortion of faint galaxies by the intervening mass distribution in the Universe, is one of the prime science targets of the upcoming KIlo Degree Survey (KIDS). It is an imaging survey, carried out with the new OmegaCAM instrument, a 16000 times 16000 pixel camera, at the newly built VLT Survey Telescope on Paranal. The survey will be carried out by a team of five groups in Leiden/Groningen, Munich, Paris, Naples, and Bonn. It will cover about 1500 square degrees in 5 optical bands; in addition, near-IR data will be available from the UKIDSS survey and, somewhat later, from VISTA. These multi-band data will allow us to determine very accuracte (photometric) redshifts and classification of objects, which, among other aspect, is very important for a quantitative lensing analysis, as well as for investigating the 3-dimensional structure of the galaxy distribution in the Universe. In preparation for KIDS, and for intensive work with the Wide-Field Imager on the ESO/MPG 2.2 meter telescope on La Silla, we have developed a pipeline for the reduction of multi-chip CCD cameras. About 450 nights of observing time, spread over three to five years, will be used for the survey, which will be the (one of the) largest surveys ever carried out by ESO. The resulting data set will be a unique resource for many years.
Several thesis projects within the frame of KIDS are available, which will be a mixture of practical work, such as data reduction, image analysis etc., as well as scientific exploitation of these data, e.g., with respect to cosmic shear, galaxy-galaxy lensing, the search for clusters, etc. Even before the start of the survey, such projects can begin, as we have available a large set of survey data from our GaBoDS Survey. We have recently released one part of that survey (see also here).
Contact persons: Thomas Erben, Tel.: 3646, or Peter Schneider, Tel.: 3671

Theoretical investigation of cosmic shear

Cosmic shear is the weak lensing action of the large-scale matter distribution in the Universe on the image shapes of faint and distant galaxies. First detected in 2000, it has by now become one of the pillars on which our cosmological standard model rests. By investigating the statistical properties of the image distortions, one infers information on the statistical properties of the large-scale structure, which in turn depende on the cosmological model. In this way, the cosmological parameters can be estimated. In particular, cosmic shear is seen as one of the few methods which can probe the properties of the Dark Energy in the Universe. Ongoing and future surveys, such as KIDS, will turn cosmic shear into a precision tool. For this to happen, we need to obtain precise predictions of the shear signal from cosmological models - the current accuracy of such predictions will not suffice for a quantitive analysis of the future surveys. This statement is particularly true when considering higher-order statistics (see, e.g., Martin Kilbinger's recent dissertation) which will be measured with great precision in these large surveys. This project aims at analyzing simulation results, which are obtained by tracing light bundles through numerical models of the large-scale matter distribution, to obtain mock data from these simulation which resemble the data from real surveys, and to apply the same statistical methods to both data sets. This analysis will then yield predictions of the lensing signal, as well as estimates on the accuracy of parameter determinations from these surveys, and can be used also to define optimal survey strategies.
Contact person: Peter Schneider, Tel.: 3671

The mass of galaxies, groups and galaxy clusters

Weak gravitational lensing offers the possibility to determine the mass of individual clusters of galaxies, but is much more powerful in obtaining the mean mass and mass profile of clusters, groups and individual galaxies with a specified property; for example, the mean mass of groups with a given richness (number of member galaxies) has been obtained from the Sloan Digital Sky Survey. These determinations are affected by a number of possible systematic effects, like foreground-background contamination, misidentification of the group's center etc. In order to study the effects of these systematics, and their redshift dependence, on the derived mass properties of groups and clusters, high-resolution ray-tracing simulations through the largest existing cosmological N-body simuation can be combined with the group-halo identification in these simulations. The result of this project will be of great interest for the interpretation of observational results derived from the next generation of wide-field imaging surveys.
Contact person: Peter Schneider, Tel.: 3671