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

We investigate the effect of Fe II equivalent width (W₂₆₀₀) and fibre size on the average luminosity of [O II] λλ3727, 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 (W₂₇₉₆) 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 ≡W₂₆₀₀/W₂₇₉₆ ≥ 0.5. In fact, we show the observed strong correlations of L_{[O II]} with W₂₇₉₆ 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 W₂₆₀₀ 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 ≥3σ significant level). On an average, the systems with R ≥ 0.5 and W₂₇₉₆ ≥ 2 Å 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.