Extinction curve template for intrinsically reddened quasars ^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: tzafar@eso.org
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ Department of Space Science, University of the Punjab, Quaid-i-Azam Campus, 54590 Lahore, Pakistan
⁴ Dipartimento di Fisica Universitá di Roma La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁵ Instituto de Astrofísica de Canarias, C/ Vía Lxctea, s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, 38206, La Laguna, Tenerife, Spain
⁷ Gran Telescopio Canarias (GRANTECAN), 38205 San Cristóbal de La Laguna, Tenerife, Spain
⁸ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 21 May 2015
Accepted: 8 October 2015

Abstract

We analyzed 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 was 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, we selected 16 cases from the HAQ survey for which the Small Magellanic Cloud (SMC) law could not provide a good solution to the spectral energy distributions (SEDs). We derived the extinction curves using the Fitzpatrick & Massa (1986, ApJ, 307, 286, FM) law by comparing the observed SEDs to a combined previously published quasar template. 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 derived an average quasar extinction curve for our sample by simultaneously fitting SEDs 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 ± 0.2. The average quasar extinction curve deviates from the steepest Milky Way and SMC extinction curves at a confidence level ≳95%. Such steep extinction curves suggest that a significant population of silicates is involved in producing small dust grains. Another possibility might 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.