On the selection of damped Lyman alpha systems using MgII absorption at 2<z<4

TL;DR

This study examines how selecting damped Lyman alpha systems based on MgII absorption at redshifts 2 to 4 affects sample completeness and derived cosmic gas density, revealing potential evolutionary trends and biases.

Contribution

It demonstrates that low MgII equivalent width DLAs are common at high redshift and their exclusion biases the measurement of cosmic HI density.

Findings

20% of DLAs have W2796 < 0.6A at 2<z<4

Excluding low W2796 DLAs increases estimated cosmic HI density by ~7%

Low W2796 DLAs tend to have low metallicity and narrow velocity widths

Abstract

The XQ-100 survey provides optical and near infrared coverage of 36 blindly selected, intervening damped Lyman alpha systems (DLAs) at 2 < z < 4, simultaneously covering the MgII doublet at 2796A, 2803A, and the Ly-alpha transition. Using the XQ-100 DLA sample, we investigate the completeness of selecting DLA absorbers based on their MgII rest-frame equivalent width (W2796) at these redshifts. Of the 29 DLAs with clean MgII profiles, we find that six (20% of DLAs) have W2796 < 0.6A. The DLA incidence rate of W2796 < 0.6A absorbers is a factor of ~5 higher than what is seen in z~1 samples, indicating a potential evolution in the MgII properties of DLAs with redshift. All of the W2796 < 0.6A DLAs have low metallicities (-2.5 < [M/H] < -1.7), small velocity widths (v90 < 50 km/s), and tend to have relatively low N(HI). We demonstrate that the exclusion of these low W2796 DLAs results in a higher mean N(HI) which in turn leads to a ~7% increase in the cosmological gas density of HI of DLAs at 2 < z < 4; and that this exclusion has a minimal effect on the HI-weighted mean metallicity.