Gravitational Lensing
Gravitational Lensing

Gravitational lensing is sensitive to all matter inhomogeneities along the line of sight, irrespective of their nature (baryonic or dark matter) and their dynamical state (relaxed or merging systems). Also, analysis of the lensing effect does not require any assumptions about the symmetry of the interveining mass distributions. Therefore, the gravitational lens effect can be used to investigate mass distributions on all scales and almost any distance along the line of sight:
Since a lensing effect always goes with magnification (at least one image of a source is always magnified relative to the unlensed source), one can investigate faint sources (e.g. individual microlensed stars, or distant QSOs and galaxies lensed by galaxies and galaxy clusters).
Search for MACHOS towards M31
A major effort of our lensing activities is WeCAPP (Wendelstein Calar Alto Pixellensing Project), a search for MACHOS using the 0.8-m Wendelstein and 1.23-m Calar-Alto telescopes. Four years of monitoring of M31, the Andromeda galaxy, have been carried out and data have been reduced. Since individual stars can not be resolved in the central region of M31, we use a difference imaging analysis to obtain light curves of every pixel within these 4 years. The light curves and the lensing analysis of the two brightest, short-term lensing events identified have been published. In both cases, lensing by a (full) MACHO-halo is more likely than self-lensing by the bulge. The most likely masses of the lenses are close to the hydrogen-burning-limit. In order to separate longer term lensing events (which should not be recurrent !) from variables with similar light curves (in particular Mira) we analyzed all intrinsically variable sources found in the 4 years. The properties of the 23770 variables in the 16'×16' field are described in a paper submitted to A&A. The identification, analysis of Novae and determination of the M31-bulge-Nova-rate is subject of a diploma thesis (paper close to submission).
In parallel we carry out the estimatations of expected MACHO and self-lensing event rates, using realistic models for the density and kinematics of the bulge, disk and halo and the stellar populations in M31 as well as the observational limitations (eg. minimal excess flux of the lensing event and minimal fwhm-timescale of the event).
Dark matter profiles in cluster of galaxies
The mass profiles of collapsed structures like cluster of galaxies are of large interest. Their measurements will help to reveal the nature of dark matter, e.g. by determining if a dark matter systems behave indeed like collisionless systems or not. Numerical similations predict for that case a universal dark matter profile (NFW-profiles), which however, can be modified by presence of baryons (e.g. central galaxies in galaxy clusters).
We use the multi waveband public ACS-data of the cluster A1689 to identify multiple images and measure their photo-zs and thus distances. We then do a lens-model fitting for a NFW- and cored isothermal dark matter profile and compare their performances. Using simulations we determine the confidence regions for the best fitting DM-parameters. Results can be seen on a conference poster; a paper with improved analysis is close to submission.
Nature of lensed SMM-sources
The nature of Sub-mm sources (SMM) is still unclear: dusty star forming galaxies or heavily star forming progenitors of todays most massive ellipticals. Once the fraction of the latter among the Sub-mm sources is determined, a census of most massive galaxies can be performed up to fairly large redshifts. The results of that might challange `semianalytical' galaxy formation models. SMM-sources are most sucessfully optically identified and spectroscopied if they are lensed and thus magnified by a foreground cluster of galaxies. The lensing effect then has to be evaluated to obtain the fluxes and sizes of the unlensed SMM-sources. In collaboration with the MPE-Genzel group we examined this `unlensing' for the SMM_J14011+0252 galaxy at redshift above two. The magnification is around 5; the maps for the 2-dimensional emission in the optical and NIR-bands will be shown in a forthcoming publication.
Together with T. Erben at Uni-Bonn, we have started to analyze the galaxy-galaxy-lensing effect in the FORS-Deep-Field (FDF). We plan to analyze the Omegacam-data obtained in the frame of the GTO-time and the planned Public-Survey wrt. to the galaxy-galaxy-signal and to measure the parameters of the dark halos as a function of the galaxy SEDs (spectral energy distribution).

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On 29 Oct 2004, 13:08.