Discovery of H i gas in a young radio galaxy at z = 0.44 using the Australian Square Kilometre Array Pathfinder

Abstract

<p>We report the discovery of a new 21-cm H <span class="sc">i</span> absorption system using commissioning data from the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP). Using the 711.51015.5MHz band of ASKAP we were able to conduct a blind search for the 21-cm line in a continuous redshift range between <i>z</i>=0.4 and 1.0, which has, until now, remained largely unexplored. The absorption line is detected at <i>z</i>=0.44 towards the GHz-peaked spectrum radio source PKSB1740−517 and demonstrates ASKAP's excellent capability for performing a future wide-field survey for H <span class="sc">i</span> absorption at these redshifts. Optical spectroscopy and imaging using the Gemini-South telescope indicates that the H <span class="sc">i</span> gas is intrinsic to the host galaxy of the radio source. The narrow [O <span class="sc">iii</span>] emission lines show clear double-peaked structure, indicating either large-scale outflow or rotation of the ionized gas. Archival data from the <i>XMMNewton</i> satellite exhibit an absorbed X-ray spectrum that is consistent with a high column density obscuring medium around the active galactic nucleus. The H <span class="sc">i</span> absorption profile is complex, with four distinct components ranging in width from 5 to 300kms<sup>−1</sup> and fractional depths from 0.2 to 20 per cent. In addition to systemic H <span class="sc">i</span> gas, in a circumnuclear disc or ring structure aligned with the radio jet, we find evidence for a possible broad outflow of neutral gas moving at a radial velocity of <i>v</i>∼300kms<sup>−1</sup>. We infer that the expanding young radio source (<em>t</em><sub>age</sub>≈2500yr) is cocooned within a dense medium and may be driving circumnuclear neutral gas in an outflow of ∼1M<sub>⊙</sub>yr<sup>−1</sup>.</p><p>DOI for Erratum: <a href="http://dx.doi.org/10.1093/mnras/stv2908" rel="nofollow">doi:10.1093/mnras/stv2908</a></p>