PDF《Astronomy》

A&

A 540, A63 (2012) DOI: 10.

1051/0004-6361/201118691 c ESO

2012 Astronomy &

Astrophysics Discovery of a compact gas-rich damped Lyman-α galaxy at z = 2.2: evidence of a starburst-driven out?ow P. Noterdaeme1,2, P. Laursen3,4, P. Petitjean1, S. D. Vergani5, M. J. Maureira2,6, C. Ledoux7, J. P. U. Fynbo4, S. López2, and R. Srianand8

1 UPMC-CNRS, UMR7095, Institut d'

Astrophysique de Paris,

75014 Paris, France e-mail: [email protected]

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 Astro?sica, 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) = 1022.10 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 classi?ed as an ultra-strong Mg ii system with Wλ2796 r 3.6 ?. 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 signi?cant 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 out?owing gas. The presence of starburst-driven out?ows is also in agreement with the high SFR together with the small size and low mass of the galaxy (Mvir ?

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 out?owing jets) is a simplistic represen- tation of the true gas distribution and velocity ?eld, the agreement between the observed and simulated properties is particularly good (spectral shape and width of the Lyman-α emission, spatial con?guration, 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 e?cient way of detecting star-forming galaxies at small impact parameters from the background QSO lines of sight. Key words. galaxies: ISM C galaxies: high-redshift C galaxies: star formation C quasars: individual: SDSS J113520.39?001053.56 C quasars: absorption lines 1. Introduction Detecting and performing detailed studies of high-z (i.e. z >