The Reddest DR3 SDSS/XMM Quasars

TL;DR

This study cross-matched SDSS DR3 quasars with XMM-Newton data to identify the causes of red colors, revealing a subset of intrinsically red quasars with unique spectral properties and low accretion rates.

Contribution

It is the first to classify red quasars into dust-reddened and intrinsically red categories using combined optical and X-ray data, highlighting low accretion rates as a possible cause.

Findings

7 quasars are likely intrinsically red with broad MgII lines and low accretion rates.

8 quasars are dust-reddened with significant X-ray absorption.

Some quasars show high X-ray absorption, indicating dust and gas obscuration.

Abstract

We have cross-correlated the SDSS DR3 Schneider et al. (2005) quasar catalog with the XMM-Newton archive. Color and redshift selections (g - r > 0.5 and 0.9 z < 2.1) result in a sample of 17 red, moderate redshift quasars. The redshift selection minimizes possible contamination due to host galaxy emission and Lyalpha forest absorption. Both optical and X-ray information are required to distinguish between the two likely remaining causes of the red colors: 1) dust-reddening and 2) an intrinsically red continuum. We find that 7 of 17 quasars can be classified as probable `intrinsically red' objects. These 7 quasars have unusually broad MgII emission lines (<FWHM>=10,500 km s^{-1}), moderately flat, but unabsorbed X-ray spectra <Gamma>=1.66+/-0.08), and low accretion rates (mdot/mdot_{Edd}} ~ 0.01). We suggest low accretion rates as a possible physical explanation for quasars with intrinsically red optical continua. We find that 8 of 17 quasars can be classified as dust-reddened. Three of these have upper-limits on the absorption column from X-ray spectral fits of N_H = 3-13 x 10^{22} cm^2, while the other five quasars must be absorbed by at least N_H = 10^{23} cm^2 in order to be consistent with a comparably selected alpha_{ox}-l_{uv} distribution. Two objects in the sample are unclassified.