What makes red quasars red?

Dohyeong Kim,Myungshin Im

doi:10.1051/0004-6361/201731963

Abstract

Red quasars are very red in the optical through near-infrared (NIR) wavelengths, which is possibly due to dust extinction in their host galaxies as expected in a scenario in which red quasars are an intermediate population between merger-driven star-forming galaxies and unobscured type 1 quasars. However, alternative mechanisms also exist to explain their red colors: (i) an intrinsically red continuum; (ii) an unusual high covering factor of the hot dust component, that is, CFHD=LHD∕Lbol, where theLHDis the luminosity from the hot dust component and theLbolis the bolometric luminosity; and (iii) a moderate viewing angle. In order to investigate why red quasars are red, we studied optical and NIR spectra of 20 red quasars atz~ 0.3 and 0.7, where the usage of the NIR spectra allowed us to look into red quasar properties in ways that are little affected by dust extinction. The Paschen to Balmer line ratios were derived for 13 red quasars and the values were found to be ~10 times higher than unobscured type 1 quasars, suggesting a heavy dust extinction withAV> 2.5 mag. Furthermore, the Paschen to Balmer line ratios of red quasars are difficult to explain with plausible physical conditions without adopting the concept of the dust extinction. The CFHDof red quasars are similar to, or marginally higher than, those of unobscured type 1 quasars. The Eddington ratios, computed for 19 out of 20 red quasars, are higher than those of unobscured type 1 quasars (by factors of 3−5), and hence the moderate viewing angle scenario is disfavored. Consequently, these results strongly suggest the dust extinction that is connected to an enhanced nuclear activity as the origin of the red color of red quasars, which is consistent with the merger-driven quasar evolution scenario.

Highlights

We find that the observed Balmer line luminosity for a given Paschen line luminosity is 1.5–290 (∼12 on average) times weaker for the red quasars than for unobscured type 1 quasars
Several explanations for the red colors of red quasars have been suggested; for example, (i) there is dust in their host galaxy, (ii) these quasars are intrinsically red, (iii) the redness is attributable to high CFHD, and (iv) the red color can be attributed to a moderate viewing angle
We find that the line luminosity ratios of red quasars are almost six times higher than those of unobscured type 1 quasars, and the extinction correction based on E(B − V) estimated from the continuum shape can bring the similar mean Pβ/Hβ luminosity ratio to that of unobscured type 1 quasars

Summary

Introduction

Large area surveys in X-ray, ultraviolet (UV), optical, and radio wavelengths have uncovered nearly a half million quasars to date (Grazian et al 2000; Becker et al 2001; Anderson et al 2003; Croom et al 2004; Risaliti & Elvis 2005; Schneider et al 2005; Véron-Cetty & Véron 2006; Im et al 2007; Lee et al 2008; Young et al 2009; Pâris et al 2014; Kim et al 2015c). In the UV to optical wavelength range, the spectra of quasars show a blue power-law continuum with broad and narrow emission lines or a host galaxy continuum with only narrow emission lines, which are classified as type 1 and 2 quasars, respectively. A quasar is composed of a black hole (BH), accretion disk, dust torus, broad line regions (BLRs), and narrow line regions (NLRs). The type 1 and 2 quasars are physically the same, but an obscuring dust torus prevents us from seeing the accretion disk and the BLR in a certain line-of-sight direction for type 2 quasars.

Results

Discussion

Conclusion