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

¹ UPMC-CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
e-mail: noterdaeme@iap.fr
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Oskar Klein Centre, Dept. of Astronomy, Stockholm University, 10691 AlbaNova, Stockholm, Sweden
⁴ Dark Cosmology Centre, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen O, Denmark
⁵ Instituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁶ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁸ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, 411 007 Pune, India

Received: 20 December 2011
Accepted: 1 February 2012

Abstract

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  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.