Extinction curve template for intrinsically reddened quasars

Abstract

We analyze the near-infrared to UV data of 16 quasars with redshifts ranging from 0.71 $<$ $z$ $<$ 2.13 to investigate dust extinction properties. The sample presented in this work is obtained from the High $A_V$ Quasar (HAQ) survey. The quasar candidates were selected from the Sloan Digital Sky Survey (SDSS) and the UKIRT Infrared Deep Sky Survey (UKIDSS), and follow-up spectroscopy was carried out at the Nordic Optical Telescope (NOT) and the New Technology Telescope (NTT). To study dust extinction curves intrinsic to the quasars, from the HAQ survey we selected 16 cases where the Small Magellanic Cloud (SMC) law could not provide a good solution to the spectral energy distributions (SEDs). We derived the extinction curves using Fitzpatrick & Massa 1986 (FM) law by comparing the observed SEDs to the combined quasar template from Vanden Berk et al. 2001 and Glikman et al. 2006. The derived extinction, $A_V$, ranges from 0.2-1.0 mag. All the individual extinction curves of our quasars are steeper ($R_V=2.2$-2.7) than that of the SMC, with a weighted mean value of $R_V=2.4$. We derive an `average quasar extinction curve' for our sample by fitting SEDs simultaneously by using the weighted mean values of the FM law parameters and a varying $R_V$. The entire sample is well fit with a single best-fit value of $R_V=2.2\pm0.2$. The `average quasar extinction curve' deviates from the steepest Milky Way and SMC extinction curves at a confidence level $\gtrsim95\%$. Such steep extinction curves suggest a significant population of silicates to produce small dust grains. Moreover, another possibility could be that the large dust grains may have been destroyed by the activity of the nearby active galactic nuclei (AGN), resulting in steep extinction curves.