We present detailed measurements of the redshift path density , equivalent width distribution , column density distribution , and redshift evolution of Mg ii absorbers as measured in archival spectra from the UVES spectrograph at the Very Large Telescope ( VLT/UVES ) and the HIRES spectrograph at the Keck Telescope ( Keck/HIRES ) to equivalent width detection limits below 0.01 Å .
This survey examines 432 VLT/UVES spectra from the UVES SQUAD collaboration and 170 Keck/HIRES spectra from the KODIAQ group , allowing for detections of intervening Mg ii absorbers spanning redshifts 0.1 < z < 2.6 .
We employ an accurate , automated approach to line detection which consistently detects redshifted absorption lines .
We measure the equivalent widths , apparent optical depth column densities , and velocity widths for each absorbing system .
Using our complete sample of all detectable Mg ii absorbers , we can accurately determine the redshift path density of absorbers across cosmic time .
We measure evolution in the comoving Mg ii line density , dN / dX , finding more high equivalent width absorbers at z = 2 than at present .
We also measure evolution in the equivalent width distribution , parameterized by a Schechter function fit , finding a shallower weak-end slope ( \alpha ) for absorbers at redshifts between 1.53 < z < 2.64 , with \alpha = -0.81 \pm 0.12 , compared to absorbers between 0.14 < z < 0.78 , where \alpha = -1.09 \pm 0.09 .
Finally , we calculate the cosmic mass fraction of Mg ii using the column density distribution , finding that \Omega _ { \hbox { \scriptsize { \hbox { { Mg } \kern 1.0 pt { \sc ii } } } } } increases from \Omega _ { \hbox { \scriptsize { \hbox { { Mg } \kern 1.0 pt { \sc ii } } } } } = ( 0.9 \pm 0.2 ) % \times 10 ^ { -8 } at < z > = 0.49 to \Omega _ { \hbox { \scriptsize { \hbox { { Mg } \kern 1.0 pt { \sc ii } } } } } = ( 1.4 \pm 0.2 ) % \times 10 ^ { -8 } at < z > = 2.1 .
We find that weak Mg ii absorbers , those with equivalent widths less than 0.3 Å , are physically distinct and evolve separately from very strong Mg ii absorbers , which have equivalent widths greater than 1.0 Å .
We compare our observed evolutionary trends in the distributions of Mg ii absorbers to previously studied cosmic trends in metallicty , the ionizing ultraviolet background , and star formation in order to conclude that galaxies eject more metal enriched gas into their halos around z = 2 than at lower redshifts in the form of high equivalent width Mg ii -absorbing outflows .
Over time from z = 2 to the present , these feedback processes decline and the evolving conditions in the circumgalactic medium give rise to a population of low equivalent width , passive Mg ii absorbers .