We also probe the dark matter halos of galaxies through gravitational lensing: for the small scale, central mass density constraints we analyze the strong lensing effect. In some lensing systems the lensed sources have HST-resolvable surface brightness substructure and are so extended that their images cover a fair fraction of the Einstein circle. Their images can then be used to study their relative surface brightness mapping and to obtain upper limits on the dark matter substructure, eg., on the subhalo fraction above a mass of 10^7 solar masses. We have studied such a case in Bauer et al., submitted , and showed (at a 2 sigma level) that the lens contains less than 4 percent of mass in substructure within the Einstein-cylinder (for a CDM subhalo mass function slope of about 1.9).
At the same time one can use these extended source systems and also systems with several distinct sources at the same redshift to measure the steepness of the mass density profile around one effective radius. In Grillo et al.2010 we identified and analyzed such a "golden lens" system.
The galaxies' outer mass profile (scales of 20kpc to a few 100 kpc) and, more general, the galaxy-mass correlation function (scales up to several couple of 100kpc) can be obtained from the galaxy-galaxy weak lensing effect. We study galaxies in the CFTHLS-Wide survey for that, with redshifts between 0 and 1 and apparent I-band magnitudes brighter than about 24 mags. We show results of this ongoing analysis here.
Most of the even brightest stars in nearby galaxies can not be resolved even with HST because they are heavily crowded and the flux of them is small compared to the sum of the fluxes of all stars in the same resolution element (PSF). However, if stars vary in time and if their variation is strong enough to exceed the photon noise in the same resolution element, their variation can be identified with the difference imaging method. This difference imaging method can be used to find intrinsically variable stars and stars where the variability is induced by the microlensing effect. Microlensing events are achromatic and have a unique light curve which should make discrimination from intrinsically variable stars possible, if sampled well enough. The microlensing effect itself can be caused by stars in the foreground of stars ("self-lensing") or by compact dark matter "machos" in the haloes of galaxies. Thus a monitoring of nearby galaxies can provide upper limits to the compact dark matter fraction in haloes. We have performed such a monitoring for about 10 years towards M31 and show events found. We also demonstrate that the analysis of individual events like WECAPP GL1 can already point to a halo lensing event, provided that the stellar properties (size, colors, brightness, spatial distribution and velocities) of stars in M31 are well understood. This understanding can be improved by investigtions of M31 as detailed in the dynamical/upper part of this webpage (see also Montalto et al. 2009 for our recent determination of the dust properties in M31).