Detection of the 2175 Å dust feature from The Sloan Digital Sky Survey first and second data releases

Abstract

We summarise our detections of the 2175 Å feature due to dust extinction in intervening galaxies toward quasars with redshifts $z>0.9$ from the Sloan Digital Sky Survey first and second data release. For the 15 Mg II absorption systems selected from $\sim$22,000 quasar spectra, the observed extinction 2175 Å “bump” with various strengths can be well fitted using template quasar spectra reddened with dust located in the Mg II absorber redshift. The dust extinctions are characterised by either a parameterised Galactic extinction law, or empirical extinction curves for the Large Magellanic Cloud and the Small Magellanic Cloud. We observe that our sample of intervening galaxies exhibiting dust features are all strong Mg II absorbers ($EW_{\lambda 2796}>1.7$Å). The various strengths of the absorption features imply a diversity of dust properties at high redshift. The Mg II systems showing the 2175 Å absorption are characterised by their large Mg II EW, high H I column density, and heavy reddening.