Detection of emission lines from<i>z</i>∼ 3 DLAs towards the QSO J2358+0149

Abstract

Using VLT/X-shooter, we searched for emission line galaxies associated with four damped Lyman α systems (DLAs) and one sub-DLA at 2.73 ≤&lt;it&gt;z&lt;/it&gt; ≤3.25 towards QSO J2358+0149. We detect [O &lt;scp&gt;iii&lt;/scp&gt;] emission from a ‘low-cool’ DLA at &lt;it&gt;z&lt;/it&gt;&lt;inf&gt;abs&lt;/inf&gt; = 2.9791 (having log &lt;it&gt;N&lt;/it&gt;(H &lt;scp&gt;i&lt;/scp&gt;) = 21.69 ± 0.10, [Zn/H] = −1.83 ± 0.18) at an impact parameter of, ρ ∼ 12 kpc. The associated galaxy is compact with a dynamical mass of (1–6) × 109 M&lt;inf&gt;&amp;odot;&lt;/inf&gt;, very high excitation ([O &lt;scp&gt;iii&lt;/scp&gt;]/[O &lt;scp&gt;ii&lt;/scp&gt;] and [O &lt;scp&gt;iii&lt;/scp&gt;]/[Hβ] both greater than 10), 12+[O/H]≤8.5 and moderate star formation rate (SFR ≤2 M&lt;inf&gt;&amp;odot;&lt;/inf&gt; yr−1). Such properties are typically seen in the low-&lt;it&gt;z&lt;/it&gt; extreme blue compact dwarf galaxies. The kinematics of the gas is inconsistent with that of an extended disc and the gas is part of either a large scale wind or cold accretion. We detect Lyα emission from the &lt;it&gt;z&lt;/it&gt;&lt;inf&gt;abs&lt;/inf&gt; = 3.2477 DLA [having log &lt;it&gt;N&lt;/it&gt;(H &lt;scp&gt;i&lt;/scp&gt;) = 21.12 ± 0.10 and [Zn/H] = −0.97 ± 0.13]. The Lyα emission is redshifted with respect to the metal absorption lines by 320 km s−1, consistent with the location of the red hump expected in radiative transport models. We derive SFR ∼0.2–1.7 M&lt;inf&gt;&amp;odot;&lt;/inf&gt; yr−1 and Lyα escape fraction of ≥10 per cent. No other emission line is detected from this system. Because the DLA has a small velocity separation from the quasar (∼500 km s−1) and the DLA emission is located within a small projected distance (ρ &lt; 5 kpc), we also explore the possibility that the Lyα emission is being induced by the QSO itself. QSO-induced Lyα fluorescence is possible if the DLA is within a physical separation of 340 kpc to the QSO. Detection of stellar continuum light and/or the oxygen emission lines would disfavour this possibility. We do not detect any emission line from the remaining three systems.