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]$\hat{I}»\hat{I}»$3727,3729 nebular emission associated with Mg II absorbers (at $0.55\le z\le 1.3$) 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${}_{[\mathrm{O}\mathrm{II}]}$) and equivalent width ($W_{2796}$) and redshift of Mg II absorbers. However, we show L${}_{[\mathrm{O}\mathrm{II}]}$ and average luminosity surface density suffers from fibre size effects. More importantly, for a given fibre size the average L${}_{[\mathrm{O}\mathrm{II}]}$ strongly depends on the equivalent width of Fe II absorption lines and found to be higher for Mg II absorbers with $R\equiv$ $W_{2600}/W_{2796}$ $\ge 0.5$. In fact, we show the observed strong correlations of L${}_{[\mathrm{O}\mathrm{II}]}$ 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${}_{[\mathrm{O}\mathrm{II}]}$ 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 $\ge 3\ddot{I}ƒ$ significant level). On an average the systems with $R$ $\ge 0.5$ and $W_{2796}$ $\ge 2$ à are more reddened, showing colour excess E($B-V$) $\sim$ 0.02, with respect to the systems with $R$ $<0.5$ and most likely traces the high H I column density systems.

16 Pages, 5 Tables, 13 Figures (Accepted for the publication in MNRAS main Journal)