2012ApJ...760...42W

We report the discovery of an unusual, extremely dust-rich and metal-strong damped Lyα absorption system (DLA) at a redshift z_a= 2.4596 toward the quasar SDSS J115705.52+615521.7 with an emission-line redshift z_e= 2.5125. The quasar spectrum, taken in the Sloan Digital Sky Survey, shows a very red color and a number of metal absorption lines, including C II, Al II, Si II, Fe II, and Zn II, which are confirmed and further characterized by follow-up spectroscopy made with the Multiple Mirror Telescope. Its neutral hydrogen column density N_ H I_= 10^21.8±0.2/cm2 is among the highest values measured in quasar DLAs. The measured metal column density is N_Zn II_~ 10^13.8/cm2, which is about 1.5 times larger than the largest value in any previously observed quasar DLAs. We derive the extinction curve of the dusty DLA using a new technique, which is an analog of the "pair method" widely used to measure extinction curves in the Milky Way (MW). The best-fit curve is an MW-like law with a significant broad feature centered around 2175 Å in the rest frame of the absorber. The measured extinction A_V~ 0.92 mag is unprecedentedly high in quasar DLAs. After applying an extinction correction, the i-band absolute magnitude of the quasar is as high as M_i ~ -29.4 mag, placing it as one of the most luminous quasars ever known. The large gas-phase relative abundance of [Zn/Fe] ~1.0 indicates that metals are heavily depleted onto dust grains in the absorber. The dust depletion level is between that of the warm and cool clouds in the MW. This discovery is suggestive of the existence of a rare yet important population of dust-rich DLAs with both high metallicities and high column densities, which may have significant impact on the measurement of the cosmic evolution of neutral gas mass density and metallicity.