Galaxy counterparts of metal-rich damped Lyα absorbers - I. The case of the z= 2.35 DLA towards Q 2222−0946★

Abstract

We have initiated a survey using the newly commissioned X-shooter spectrograph to target candidate relatively metal-rich damped Ly absorbers (DLAs). Our rationale is that highmetallicity DLAs due to the luminosity-metallicity relation likely will have the most luminous galaxy counterparts. In addition, the spectral coverage of X-shooter allows us to search for not only Ly emission, but also rest-frame optical emission lines. We have chosen DLAs where the strongest rest-frame optical lines ([O II], [O III], H and H) fall in the nearinfrared atmospheric transmission bands. In this first paper resulting from the survey, we report on the discovery of the galaxy counterpart of the z abs = 2.354 DLA towards the z = 2.926 quasar Q 2222-0946. This DLA is amongst the most metal-rich z > 2 DLAs studied so far at comparable redshifts and there is evidence for substantial depletion of refractory elements on to dust grains. We measure metallicities from Zn II, Si II, Ni II, Mn II and Fe II of -0.46 0.07, -0.51 0.06, -0.85 0.06, -1.23 0.06 and -0.99 0.06, respectively. The galaxy is detected in the Ly, [O III] 4959, 5007 and H emission lines at an impact parameter of about 0.8 arcsec (6 kpc at z abs = 2.354). Based on the H line, we infer a star formation rate of 10 M yr -1 , which is a lower limit due to the possibility of slit loss. Compared to the recently determined H luminosity function for z = 2.2 galaxies, the DLAgalaxy counterpart has a luminosity of L 0.1L * H . The emission-line ratios are 4.0 (Ly/H) and 1.2 ([O III]/H). In particular, the Ly line shows clear evidence for resonant scattering effects, namely an asymmetric, redshifted (relative to the systemic redshift) component and a much weaker blueshifted component. The fact that the blueshifted component is relatively weak indicates the presence of a galactic wind.