Comprehensive study of a z = 2.35 DLA Galaxy: mass, metallicity, age, morphology and SFR from HST and VLT★

Abstract

We present a detailed study of the emission from a z = 2.35 galaxy that causes damped Lyman α absorption in the spectrum of the background quasar (QSO), SDSS J2222−0946. We present the results of extensive analyses of the stellar continuum covering the rest frame optical–UV regime based on broad-band Hubble Space Telescope (HST) imaging, and of spectroscopy from VLT/X-Shooter of the strong emission lines: Lyα, [O ii], [O iii], [N ii], Hα and Hβ. We compare the metallicity from the absorption lines in the QSO spectrum with the oxygen abundance inferred from the strong-line methods (R23 and N2). The two emission-line methods yield consistent results: [O/H] = −0.30 ± 0.13. Based on the absorption lines in the QSO spectrum a metallicity of −0.49 ± 0.05 is inferred at an impact parameter of 6.3 kpc from the centre of the galaxy with a column density of hydrogen of log (NHI/cm−2 = 20.65 ± 0.05. The star formation rates (SFRs) of the galaxy from the UV continuum and Hα line can be reconciled assuming an amount of reddening of E(B − V) = 0.06 ± 0.01, giving an inferred SFR of 13 ± 1 M⊙ yr−1 (Chabrier initial mass function). From the HST imaging, the galaxy associated with the absorption is found to be a compact (re = 1.12 kpc) object with a disc-like, elongated (axis ratio 0.17) structure indicating that the galaxy is seen close to edge-on. Moreover, the absorbing gas is located almost perpendicularly above the disc of the galaxy suggesting that the gas causing the absorption is not corotating with the disc. We investigate the stellar and dynamical masses from spectral energy distribution-fitting and emission-line widths, respectively, and find consistent results of 2 × 109 M⊙. We suggest that the galaxy is a young proto-disc with evidence for a galactic outflow of enriched gas. This galaxy hints at how star-forming galaxies may be linked to the elusive population of damped Lyα absorbers.