The Impact of Dust in Host Galaxies on Quasar Luminosity Functions

TL;DR

This study examines how dust in host galaxies affects quasar brightness measurements, revealing that dust significantly dims quasars and exploring mechanisms to mitigate this effect in models.

Contribution

It introduces a self-consistent method to estimate dust attenuation in quasars and investigates mechanisms to reduce its impact on luminosity functions.

Findings

Dust dims bright quasars by about 2 magnitudes in B-band.

Standard dust attenuation models overpredict quasar dimming, conflicting with observed luminosity functions.

Reducing dust effects requires delayed black hole fueling or a small dust covering factor.

Abstract

We have investigated effects of dust attenuation on quasar luminosity functions using a semi-analytic galaxy formation model combined with a large cosmological N-body simulation. We estimate the dust attenuation of quasars self-consistently with that of galaxies by considering the dust in their host bulges. We find that the luminosity of the bright quasars is strongly dimmed by the dust attenuation, about 2 mag in the B-band. Assuming the empirical bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we find that this dust attenuation is too strong to explain the B-band and X-ray quasar luminosity functions simultaneously. We consider two possible mechanisms that weaken the dust attenuation. As such a mechanism, we introduce a time delay for AGN activity, that is, gas fueling to a central black hole starts some time after the beginning of the starburst induced by a major merger. The other is the anisotropy in the dust distribution. We find that in order to make the dust attenuation of the quasars negligible, either the gas accretion into the black holes has to be delayed at least three times the dynamical timescale of their host bulges or the dust covering factor is as small as 0.1.