DETECTION OF A SUPERSTRONG 2175 A ABSORPTION GALAXY AT z = 0.8839 TOWARD THE QUASAR SDSS J100713.68+285348.4

We report a superstrong 2175 A absorption galaxy at a redshift of zG = 0.8839 toward the background quasar SDSS J100713.68+285348.4 at zQ = 1.047 identified from the Sloan Digital Sky Survey (SDSS) data. The bump is the strongest ever known, about 2.5 times the average in the Galactic interstellar extinction curves. About two dozens of narrow absorption lines are identified in this system in the SDSS and the follow-up Multiple Mirror Telescope (MMT) spectra, including Zn II, Cr II, Mn II, Si II, Fe II, Ti II, Ca II, Al III, Mg II, and Mg I. We have derived accurate measurements of the gas-phase column densities of most of these ions through their weak absorption lines. The combination of unprecedentedly strong 2175 A absorption bump strength with the measured zinc column density suggests that carbon is likely to be the main carrier of the 2175 A absorber assuming that the total abundance of the absorber relative to zinc is similar to the solar values and zinc is not heavily depleted onto dust grains. Under the same assumption, we found that dust depletion in this absorption galaxy is much less than that in the Milky Way, indicating possible different nucleosynthesis and/or chemical and interstellar medium evolution history.