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:
- one can measure dark matter
fluctuations along the line of sight (Cosmic Shear)
- one can obtain the
mass distribution of galaxy clusters and groups
(using multiple images in cluster centers, weak lensing mass reconstructions
or profile analysis);
- the central mass distributions in galaxies are constrained
by
image splitting of QSOs; the
galaxy-galaxy-lensing signal can be used to determine the
halo profile and extent statistically;
- the
clumpiness of galactic and
intergalactic medium can be investiged using
flux ratios of multiple lensed QSOs,
microlensing variability, strong lensing frequencies and lens
properties.
-
finally, one can determine the MACHO fraction
in the local group galaxies from microlensing events that these
objects cause for sources in their background.
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.
Galaxy-Galaxy-Lensing
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.