Galaxy counterparts of metal-rich damped Lyα absorbers - I. The case of the z = 2.35 DLA towards Q2222-0946

We have initiated a survey using the newly commissioned X-shooter spectrograph to target candidate relatively metal-rich damped Lyα absorbers (DLAs). Our rationale is that high-metallicity 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 Lyα emission, but also rest-frame optical emission lines. We have chosen DLAs where the strongest rest-frame optical lines ([O II], [O III], Hβ and Hα) fall in the near-infrared 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 Lyα, [O III]λλ4959, 5007 and Hα emission lines at an impact parameter of about 0.8 arcsec (6 kpc at z_abs= 2.354). Based on the Hα 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 Hα luminosity function for z= 2.2 galaxies, the DLA-galaxy counterpart has a luminosity of L∼ 0.1L*_Hα. The emission-line ratios are 4.0 (Lyα/Hα) and 1.2 ([O III]/Hα). In particular, the Lyα 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.The properties of the galaxy counterpart of this DLA are consistent with the prediction that metal-rich DLAs are associated with the most luminous of the DLA-galaxy counterparts.