Physical Properties of Weak MgII Absorbers at z~2

TL;DR

This study models the physical properties of weak MgII absorbers at redshifts around 2, revealing consistent characteristics over a broad redshift range and suggesting a decline in their occurrence due to reduced activity rather than formation processes.

Contribution

First detailed photoionization analysis of weak MgII absorbers at z~2, comparing their properties across redshifts and proposing a new explanation for their decreasing number density.

Findings

Absorber properties show no significant variation from z=0.4 to 2.4.

The decrease in weak MgII absorber density is likely due to reduced activity, not formation changes.

Presence of aligned low and high density clouds indicates specific absorber geometry.

Abstract

We present the results of photoionization modeling of nine weak MgII (W_r<0.3 Ang) quasar absorption line systems with redshifts 1.4<z<2.4 obtained with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. These systems have been chosen because they provide access to a regime of red-shift space that previous weak MgII studies have not looked at. The densities, metallicities, Doppler parameters, and column densities of these systems are compared to those of other weak MgII systems at lower redshift. There is no significant statistical variation in the properties of the absorbers over the redshift range 0.4<z<2.4. The number density per unit redshift is known to decrease for weak MgII absorbers between z~1 and z~2 by a greater amount than predicted from cosmological effects and changes in the extragalactic ionizing background alone. We suggest that, because the physical properties of the absorber population are not seen to change significantly across this range, that the evolution in dN/dz is due to a decrease in the activity that gives rise to weak MgII absorption, and not due to a change in the processes that form weak MgII absorbers. The presence of separate, but aligned (in velocity) low and high density clouds in all single cloud weak MgII absorbers provides an important diagnostic of their geometry. We discuss possible origins in dwarf galaxies and in extragalactic analogs to high velocity clouds.