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The baryonic mass-to-light ratio in elliptical galaxies measured from the static gravitational micro-lensing of multiply imaged quasars

Astronomy Spring Colloquia March 30, 2010 Tuesday, 4:30pm-5:30pm, Peyton Hall auditorium:

By Paul Schechter

Many gravitationally lensed quasars exhibit flux ratio "anomalies"  that cannot be explained under the hypothesis that the lensing  potential is smooth on scales smaller than one kpc.  Micro-lensing by  stars is a natural source of granularity in the lens potential.  The  character of the expected fluctuations due to micro-lensing depends  sensitively on the relative surface densities of micro-lenses (stars)  and smoothly distributed (dark) matter.  Observations of flux ratios  may in principle be used to infer the mass-to-light (M/L) ratio of  stellar component along the line of sight -- typically at impact  parameters 1.5 times the half light radius.  In practice the optical  emission of quasars appears to come from regions comparable in size to  the Einstein rings of the stellar microlenses, greatly complicating  the M/L analysis.  Fortunately, the X-ray emission from quasars  appears to be point-like.  Chandra observations of a dozen lensed  quasars yield an M/L ratio accurate to 15%.