How are Red and Blue Quasars Different? The Radio Properties

TL;DR

Red quasars exhibit enhanced small-scale radio emission likely due to jets or dusty winds, revealing fundamental differences from blue quasars and linking dust obscuration with radio properties.

Contribution

This study presents new analyses of radio properties of red quasars across multiple surveys, highlighting their enhanced small-scale radio emission and exploring potential mechanisms.

Findings

Red quasars have increased small-scale radio emission.

Radio emission peaks around the radio-quiet threshold.

Star-formation is ruled out as the primary cause.

Abstract

A non-negligible fraction of quasars are red at optical wavelengths, indicating (in the majority of cases) that the accretion disc is obscured by a column of dust which extinguishes the shorter-wavelength blue emission. In this paper, we summarize recent work by our group, where we find fundamental differences in the radio properties of SDSS optically-selected red quasars. We also present new analyses, using a consistent color-selected quasar parent sample matched to four radio surveys (FIRST, VLA Stripe 82, VLA COSMOS 3 GHz, and LoTSS DR1) across a frequency range 144 MHz-3 GHz and four orders of magnitude in radio flux. We show that red quasars have enhanced small-scale radio emission (~kpc) that peaks around the radio-quiet threshold (defined as the ratio of 1.4 GHz luminosity to 6 micron luminosity) across the four radio samples. Exploring the potential mechanisms behind this enhancement, we rule out star-formation and propose either small-scale synchrotron jets, frustrated jets, or dusty winds interacting with the interstellar medium; the latter two scenarios would provide a more direct connection between opacity (dust; gas) and the production of the radio emission. In our future study, using new multi-band uGMRT data, we aim to robustly distinguish between these scenarios.