An ASKAP survey for H i absorption towards dust-obscured quasars

Abstract

Obscuration of quasars by accreted gas and dust, or dusty intervening galaxies, can cause active galactic nuclei (AGN) to be missed in optically-selected surveys. Radio observations can overcome this dust bias. In particular, radio surveys searching for Hi absorption inform us on how the AGN can impact on the cold neutral gas medium within the host galaxy, or the population of intervening galaxies through the observed line of sight gas kinematics. We present the results of a Hi absorption line survey at 0.4 < z < 1 towards 34 obscured quasars with the Australian SKA Pathfinder (ASKAP) commissioning array. We detect three Hi absorption lines, with one of these systems previously unknown. Through optical follow-up for two sources, we find that in all detections the Hi gas is associated with the AGN, and hence that these AGN are obscured by material within their host galaxies. Most of our sample are compact, and in addition, are either gigahertz peaked spectrum (GPS), or steep spectrum (CSS) sources, both thought to represent young or recently re-triggered radio AGN. The radio spectral energy distribution classifications for our sample agree with galaxy evolution models in which the obscured AGN has only recently become active. Our associated Hi detection rate for GPS and compact SS sources matches those of other surveys towards such sources. We also find shallow and asymmetric Hi absorption features, which agrees with previous findings that the cold neutral medium in compact radio galaxies is typically kinematically disturbed by the AGN.

Australian square kilometre array pathfinder: I. system description

“…Many publications arose from the early science programme, as a result of identifying projects that the telescope could excel at with only a third of its collecting area. These include a continuum survey and detection of cold gas outflows from the small magellanic cloud (McClure-Griffiths et al 2018;Joseph et al 2019), studies of neutral hydrogen in nearby galaxy groups (Reynolds et al 2019;Lee-Waddell et al 2019;Elagali et al 2019;Kleiner et al 2019;For et al 2019), studies of absorption lines (Glowacki et al 2019;Allison et al 2020), continuum observations of the galaxy and mass assembly (GAMA) G23 field (Leahy et al 2019), searches for transient and variable sources (Bhandari et al 2018), and a few targeted studies of individual radio galaxies (e.g. Seymour et al 2020), including the polarisation characteristics of Centaurus A (Anderson et al 2018).…”

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

“…Several recent results from observations of radio galaxies and quasars during ASKAP commissioning have demonstrated feasibility (e.g. Allison et al 2015Allison et al , 2016aMoss et al 2017;Allison et al 2017;Glowacki et al 2019;Allison et al 2019). Here, we have used observations of the 23 hr field of the Galaxy And Mass Assembly survey (GAMA; Liske et al 2015) 1 , a 50 deg 2 area of the southern sky that contains spectroscopic information for galaxies to an i-band magnitude limit of i < 19.2 mag.…”

Spectral variability of radio sources at low frequencies

“…Recent simulations have also proposed GPS and CSS are the result of relativistic jet feedback and interactions with the surrounding warm ISM (Bicknell et al 2018b). Several observations of absorption features linked with dust surrounding AGN have also found a strong connection with PSS (Grasha et al 2019;Glowacki et al 2019;Jarvis et al 2019). However, as several studies have noted (O'Dea 1998;Callingham et al 2017;Bicknell et al 2018a), distinguishing between synchrotron self absorption (SSA) and FFA for the PSS population as a whole has thus far yielded inconclusive results as both SSA and FFA spectral models are consistent with current observations for most PSSs.…”