Detections of the 2175 Å Dust Feature at 1.4 ≲ z ≲ 1.5 from the Sloan Digital Sky Survey

The strongest spectroscopic dust extinction feature in the Milky Way, the broad absorption bump at 2175 Å, is generally believed to be caused by aromatic carbonaceous materials, very likely a mixture of polycyclic aromatic hydrocarbon molecules, the most abundant and widespread organic molecules in the Milky Way. In this paper, we report identifications of this absorption feature in three galaxies at 1.4 ≲ z ≲ 1.5 that produce intervening Mg II absorption toward quasars discovered by the Sloan Digital Sky Survey. The observed spectra can be fitted using Galactic-type extinction laws, characterized by parameters [RV,E(B - V)] ≃ [0.7,0.14], [1.9, 0.13], and [5.5, 0.23], respectively, where RV ≡ AV/E(B - V) is the total-to-selective extinction ratio and E(B - V) ≡ AB - AV is the color excess. These discoveries imply that the dust in these distant quasar absorption systems is similar in composition to that of the Milky Way, but with a range of different grain size distributions. The presence of complex aromatic hydrocarbon molecules in such distant galaxies is important for both astrophysical and astrobiological investigations.