Probing the Interstellar Dust in Galaxies over > 10 Gyr of Cosmic
  History
  This article is based on an invited talk given by V. P. Kulkarni at the 8th
Cosmic Dust meeting. Dust has a profound effect on the physics and chemistry of
the interstellar gas in galaxies and on the appearance of galaxies.
Understanding the cosmic evolution of dust with time is therefore crucial for
understanding the evolution of galaxies. Despite the importance of interstellar
dust, very little is known about its nature and composition in distant
galaxies. We summarize the results of our ongoing programs using observations
of distant quasars to obtain better constraints on dust grains in foreground
galaxies that happen to lie along the quasar sightlines. These observations
consist of a combination of mid-infrared data obtained with the Spitzer Space
Telescope and optical/UV data obtained with ground-based telescopes and/or the
Hubble Space Telescope. The mid-IR data target the 10 $\mu$m and 18 $\mu$m
silicate absorption features, while the optical/UV data allow determinations of
element depletions, extinction curves, 2175 {\AA} bumps, etc. Measurements of
such properties in absorption-selected galaxies with redshifts ranging from
$z\sim0$ to $z>2$ provide constraints on the evolution of interstellar dust
over the past $> 10$ Gyr. The optical depth of the 10 $\mu$m silicate
absorption feature ($\tau_{10}$) in these galaxies is correlated with the
amount of reddening along the sightline. But there are indications [e.g., based
on the $\tau_{10}$ /$E(B-V)$ ratio and possible grain crystallinity] that the
dust in these distant galaxies differs in structure and composition from the
dust in the Milky Way and the Magellanic Clouds. We briefly discuss the
implications of these results for the evolution of galaxies and their star
formation history.
