Average [O II] nebular emission associated with Mg II absorbers: dependence on Fe II absorption

Abstract

We investigate the effect of Fe II equivalent width (W 2600 ) and fibre size on the average luminosity of [O II] 3727, 3729 nebular emission associated with Mg II absorbers (at 0.55 z 1.3) in the composite spectra of quasars obtained with 3 and 2 arcsec fibres in the Sloan Digital Sky Survey. We confirm the presence of strong correlations between [O II] luminosity (L [O II] ) and equivalent width (W 2796 ) and redshift of Mg II absorbers. However, we show L [O II] and average luminosity surface density suffer from fibre size effects. More importantly, for a given fibre size, the average L [O II] strongly depends on the equivalent width of Fe II absorption lines and found to be higher for Mg II absorbers with R W 2600 /W 2796 0.5. In fact, we show the observed strong correlations of L [O II] with W 2796 and z of Mg II absorbers are mainly driven by such systems. Direct [O II] detections also confirm the link between L [O II] and R. Therefore, one has to pay attention to the fibre losses and dependence of redshift evolution of Mg II absorbers on W 2600 before using them as a luminosity unbiased probe of global star formation rate density. We show that the [O II] nebular emission detected in the stacked spectrum is not dominated by few direct detections (i.e. detections 3 significant level). On an average, the systems with R 0.5 and W 2796 2 are more reddened, showing colour excess E(B -V) 0.02, with respect to the systems with R < 0.5 and most likely trace the high H I column density systems.