Gravitational lens chromaticity

Resumo

The gravitational lensing effect is independent of wavelength, However, in many lens systems differences in color between the images are observed. These chromatic variations could be produced by two effects: differential extinction in the lens galaxy and/or chromatic microlensing. When each image's light crosses the interstellar medium of the lens galaxy it may be affected in different amounts by patchily distributed dust. This results in differential extinction between pairs of images, and makes possible the determination of the extinction law of the lens galaxy. The other chromatic phenomenon arises when a star or a compact object in the lens galaxy lines up perfectly along the line of sight between the quasar and the observer. The quasar image then undergoes an amplification or demagnification known as microlensing. This effect depends on the angular size of the source, in this case on the accretion disk of the quasar. Because the accretion disk is hotter closer to the black hole, and because the emission of the accretion disk depends on temperature, different amplifications may be observed at different wavelengths. This effect is known as chromatic microlensing, and it offers unprecedented perspectives into the physical properties of the lensed accretion disks. In my thesis work both aspects of gravitational lens chromaticity have been deeply studied from an observational and a theoretical point of view. Initially, we analyzed the chromaticity of a particular lens system, Q2237+0305, reporting the unambiguous detection of chromatic microlensing and making a preliminary study of the size-wavelength scaling to test the existing accretion disk models. Moreover, we have made a statistical analysis of the chromatic effect, studying the dependence of the probability of its occurrence with the parameters that characterize a lens system to build a chromatic atlas. That will allow a quick analysis of the mass distribution in the lensing galaxy when chromatic microlensing is detected and, for the first time, precise measurements of the mass density in stars in the lens galaxy will be made. Finally, we also studied the differential extinction of high redshift galaxies, counting for that with ground-based data as well as with Hubble Space Telescope images.