Winds as the origin of radio emission in z = 2.5 radio-quiet extremely red quasars

Abstract

Most active galactic nuclei (AGNs) are radio-quiet, and the origin of their radio emission is not wellunderstood. One hypothesis is that this radio emission is a by-product of quasar-driven winds. In this paper, we present the radio properties of 108 extremely red quasars (ERQs) at z = 2 − 4. ERQs are among the most luminous quasars (L bol ∼ 10 47−48 erg s −1 ) in the Universe, with signatures of extreme ( 1000 km s −1 ) outflows in their [O III]λ5007Å emission, making them the best subjects to seek the connection between radio and outflow activity. All ERQs but one are unresolved in the radio on ∼ 10 kpc scales, and the median radio luminosity of ERQs is νL ν [6 GHz] = 10 41.0 erg s −1 , in the radio-quiet regime, but one to two orders of magnitude higher than that of other quasar samples. The radio spectra are steep, with a mean spectral index α = −1.0. In addition, ERQs neatly follow the extrapolation of the low-redshift correlation between radio luminosity and the velocity dispersion of [O III]-emitting ionized gas. Uncollimated winds, with a power of one per cent of the bolometric luminosity, can account for all these observations. Such winds would interact with and shock the gas around the quasar and in the host galaxy, resulting in acceleration of relativistic particles and the consequent synchrotron emission observed in the radio. Our observations support the picture in which ERQs are signposts of extremely powerful episodes of quasar feedback, and quasar-driven winds as a contributor of the radio emission in the intermediate regime of radio luminosity νL ν = 10 39 − 10 42 erg s −1 .

Host galaxies of high-redshift extremely red and obscured quasars

“…These observations are consistent with the picture in which their radio emission is a by-product of quasar-driven winds, produced when relativistic particles are accelerated in the resulting shocks. Radio data reinforce our hypothesis that ERQs are hosting some of the most extreme galactic winds known (Alexandroff et al 2016;Hwang et al 2018).…”

“…As a population, ERQs are radio-quiet but not radiosilent (Hwang et al 2018). Their radio luminosities and [OIII] velocity widths lie on the extreme end of the radio/[OIII] relationship found in low redshift quasars (Zakamska & Greene 2014).…”

“…Such extremely high star formation rates are well above those seen in sub-millimeter galaxies (Blain et al 2002;Chapman et al 2005), but have been seen in the most extreme high-redshift starbursts (Rowan-Robinson et al 2018) and may not necessarily be out of place in the hosts of ERQs, which are after all some of the most rapidly accreting active nuclei in the universe. Nonetheless, we previously argued that such high star formation rates are unlikely because the radio luminosities of ERQs are dominated by the active nuclei, not by the host galaxies (Hwang et al 2018). Furthermore, the co-evolution models are not yet well probed at L bol > 10 47 erg/sec, and the existing observations do not support the predicted extremely high star formation rates at the high luminosity end (Hickox et al 2014).…”

Quantifying the thermal Sunyaev–Zel’dovich effect and excess millimetre emission in quasar environments

“…The origin of radio emission in radio quiet quasars remains a topic of some debate (Laor & Behar 2008;Zakamska et al 2016;Hwang et al 2018;Laor et al 2019;Panessa et al 2019), and quantifying the radio spectral indices of a statistical sample of radio-quiet quasars has not been done. In this study, we include the stacked radio flux densities at 1.4 and 3 GHz because if there is significant synchrotron and/or free-free emission in the quasar population, then it has a potential to be present at 95 GHz where we also expect to see a decrement in the emission due to the SZ effect.…”

“…The origin of radio emission from the hosts of radio quiet quasars remains unknown (Laor & Behar 2008;Zakamska et al 2016;Hwang et al 2018;Laor et al 2019;Panessa et al 2019). Emission in this regime may be due to optically thin synchrotron and free-free emission from star formation in the host galaxy (e.g., Condon et al 2013), synchrotron and free-free emission from quasar-driven outflows (e.g., Faucher-Giguère & Quataert 2012; Zubovas & King 2012;Nims et al 2015), or synchrotron from a low power jet (Falcke et al 2004;Leipski et al 2006).…”