Discovery of a compact gas-rich damped Lyman-α galaxy at z = 2.2: evidence of a starburst-driven outflow

We present the detection of Ly α, [O iii], and H α emission associated with an extremely strong damped Lyman-α (DLA) system (N(H i) = 10^22.10 cm^-2) at z = 2.207 towards the quasar SDSS J113520.39 - 001053.56. This is the largest H I column density ever measured along a quasi-stellar object (QSO) line of sight, though typical of those often found in DLAs associated to gamma-ray bursts (GRBs). This absorption system can also be classified as an ultra-strong Mg II system with W_r^λ2796≃3.6 Å. The mean metallicity of the gas ( [Zn/H] = -1.1) and dust depletion factors ( [Zn/Fe] = 0.72, [Zn/Cr] = 0.49) are consistent with (and only marginally larger than) the mean values found in the general QSO-DLA population. The [O iii]-Hα emitting region has a very small impact parameter with respect to the QSO line of sight, b ≈ 0.1'' (0.9 kpc proper distance), and is unresolved. From the H α line, we measure a significant star formation rate (SFR) ≈ 25 M_⊙ yr^-1 (uncorrected for dust). The shape of the Ly α line is double-peaked, which is the signature of a resonant scattering of Ly α photons, and the Ly α emission is spatially extended. More strikingly, the blue and red Ly α peaks arise from distinct regions extended over a few kpc on either side of the star-forming region. We propose that this is the consequence of a Ly α transfer in outflowing gas. The presence of starburst-driven outflows is also in agreement with the high SFR together with the small size and low mass of the galaxy (M_vir ~ 10¹⁰ M_⊙). By placing constraints on the stellar UV continuum luminosity of the galaxy, we estimate an age of at most a few 10⁷ yr, again consistent with a recent starburst scenario. We interpret these data as the observation of a young, gas-rich, compact starburst galaxy, from which material is expelled through collimated winds powered by the vigorous star formation activity. We substantiate this picture by modelling the radiative transfer of Ly α photons in the galactic counterpart. Though our model (a spherical galaxy with bipolar outflowing jets) is a simplistic representation of the true gas distribution and velocity field, the agreement between the observed and simulated properties is particularly good (spectral shape and width of the Lyman-α emission, spatial configuration, escape fraction as well as absorption kinematics, H I column density, and dust reddening). Finally, we propose that selecting DLAs with very high H I column densities may be an efficient way of detecting star-forming galaxies at small impact parameters from the background QSO lines of sight.

Based on data obtained with MagE at the Clay telescope of the Las Campanas Observatory (CNTAC Prgm. ID 2011B-90) as well as X-shooter (Prgm. ID 286.A-5044) and UVES (archive: Prgm ID. 385.A-0778) at the Very Large Telescope of the European Southern Observatory.