Unveiling the True Colours of Red Quasars

Abstract

The archetypal signature of an unobscured quasar, which constitute the majority of the optically-selected quasar population, is broad emission lines superimposed onto a luminous blue thermal continuum. Consequently, it was long believed that quasars are only blue in colour. However, the discovery of a small, but significant, subset of quasars with redder continuum emission over 20 years ago, has challenged the conventional view. In this thesis I have investigated the fundamental differences between red and blue quasars, using SDSS- and NuSTAR-selected quasar samples, where the latter quasars represent the less luminous end of the population. Consequently, we have found that the majority NuSTAR red quasars ascribe their redness to dominant host galaxy emission. On the other hand, we have shown that the luminous SDSS quasars are outshine their host galaxy. The evidence presented here for the SDSS-selected quasars supports dust-reddening as the key ingredient for the observed colours in the intermediate-radio, compact red quasars, with jet- and wind-driven outflows. This is consistent with the picture of red quasars being in a young, evolutionary phase, enshrouded by a cocoon of dust, with strong winds and compact or frustrated jets. Our ongoing investigations appear to be confirming this phenomenological picture, although the key studies to conclusively confirm or refute this view are still to come!