On the Observed W_MgII--L_[OII] Correlation in SDSS QSO Spectra

TL;DR

This study explains the observed correlation between MgII absorption strength and [OII] luminosity in SDSS QSO spectra as a result of aperture effects and galaxy halo properties, rather than a direct physical link.

Contribution

It demonstrates through simulations that the correlation arises from aperture loss and halo scaling, challenging the idea of a direct physical connection.

Findings

The correlation can be reproduced by models with extended MgII halos and aperture effects.

Larger fibers reduce the observed correlation, indicating it is not intrinsic.

MgII absorbers mainly trace halo gas, not star-forming disks.

Abstract

This paper investigates the effect of differential aperture loss with SDSS fibers and examines whether such selection bias would result in the observed correlation between rest-frame absorption equivalent width of MgII absorbers, Wr(2796), and mean associated [OII] luminosity, L_[OII], in SDSS QSO spectra. We demonstrate based on a Monte Carlo simulation that the observed Wr(2796) vs. L_[OII] correlation of MgII absorbers can be well-reproduced, if all galaxies found in deep surveys possess extended MgII halos and if the extent of MgII halos scales proportionally with galaxy mass as shown in previous studies. The observed correlation can be explained by a combination of (1) the known Wr(2796) vs. rho anti-correlation in galaxy and MgII absorber pairs and (2) an increasing aperture loss in the 3" diameter SDSS fiber for galaxies at larger rho. Galaxies at larger projected distances produce on average weaker MgII absorbers and weaker (or zero) L_[OII] in SDSS QSO spectra. We show that such correlation diminishes when larger fibers are adopted and is therefore not physical. While under a simple halo model the majority of MgII absorbers do not directly probe star-forming disks, they trace photo-ionized halo gas associated with galaxies. We show that because of the scaling relation between extended gas cross-section and galaxy mass, the number density evolution of the MgII absorber population as a whole provides a good measure of the cosmic star formation history.