[O II] nebular emission from Mg II absorbers: star formation associated with the absorbing gas
Résumé
We present nebular emission associated with 198 strong Mg II absorbers at 0.35 ≤z ≤ 1.1 in the fibre spectra of quasars from the Sloan Digital Sky Survey. Measured [O II] luminosities (L_[O II]) are typical of sub-L⋆ galaxies with derived star formation rate (uncorrected for fibre losses and dust reddening) in the range of 0.5-20 M⊙ yr-1. Typically less than ∼3 per cent of the Mg II systems with rest equivalent width, W2796≥2Å, show L_[O II] ≥0.3 L^{\star }_[O II]. The detection rate is found to increase with increasing W2796 and z. No significant correlation is found between W2796 and L_[O II] even when we restrict the samples to narrow z ranges. A strong correlation is seen between L_[O II] and z. While this is expected from the luminosity evolution of galaxies, we show that finite fibre size plays a very crucial role in this correlation. The measured nebular line ratios (like [O III]/[O II] and [O III]/H β) and their z evolution are consistent with those of galaxies detected in deep surveys. Based on the median stacked spectra, we infer the average metallicity (log Z ∼8.3), ionization parameter (log q∼7.5) and stellar mass (log (M/M⊙) ∼ 9.3). The Mg II systems with nebular emission typically have W2796 ≥2 Å, Mg II doublet ratio close to 1 and W(Fe II λ2600)/W2796∼0.5 as often seen in damped Ly α and 21-cm absorbers at these redshifts. This is the biggest reported sample of [O II] emission from Mg II absorbers at low-impact parameters ideally suited for probing various feedback processes at play in z ≤ 1 galaxies.
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