A striking excess of red quasars with steep radio spectral slopes: a dusty blow-out phase revealed through AGN-driven shocks?

Abstract

Red quasars exhibit a higher incidence of compact (galaxy-scale or smaller) radio emission than blue quasars, arising from systems near the radio-loud/radio-quiet threshold. In this paper we select quasars from SDSS ($0.2 <z <2.4$), and use archival radio data (FIRST, VLASS, LoTSS) to visually determine the radio morphologies of 573 red quasars compared to a control sample of 1278 typical blue quasars. We find an excess of steep-slope radio emission ($\alpha_{1.4-3\text{ GHz}}\sim-1$, where $S_\nu \propto \nu^\alpha$) from red quasars with compact ($<6''$) radio morphologies over 144 MHz, 1.4 GHz, and 3 GHz. This excess steep radio emission signature is not seen in normal blue quasars (radio compact or extended) or red quasars with extended low-frequency radio emission, which instead show a broad range of radio spectral slopes consistent with a range of different physical processes. We show that the strength of the excess steep spectral slope component increases with dust extinction, along with an overall increase in the radio-detection fraction. We argue that this excess steep-slope radio emission is due to shocks between quasar-driven winds/jets and the dusty nuclear-host galaxy environment. The majority ($86^{+5}_{-21}\%$) of the dustiest quasars ($E(B-V)>0.4$ mag) with steep slopes have radio luminosities consistent with the prediction from a wind-shock model with wind efficiencies of up to 7%. This agrees with the scenario where these compact red quasars are undergoing a "dusty blow-out" phase, where compact jets and/or AGN-driven winds interact with a dusty ISM, causing shocks, leading to steep spectral slopes and enhanced radio detection rates.