The Mass Density of Mg II Absorbers from the Australian Dark Energy Survey

We present an all-southern sky survey for Mg II λλ2796, 2803 doublet absorbers in 951 z < 4 active galactic nucleus/quasar spectra from the Australian Dark Energy Survey. The spectral resolution ranges from R = 1400 to 1700 over the wavelengths 3700–8800 Å. The survey has a 5σ detection completeness of 50% and above for rest-frame equivalent widths W _r (2796) ≥ 0.3 Å. We studied 656 Mg II absorption systems over the redshift range 0.33 ≤ z ≤ 2.19 with equivalent widths 0.3 ≤ W _r (2796) ≤ 3.45 Å. The equivalent width distribution is well fit by an exponential function with W _* = 0.76 ± 0.04 Å, and the redshift path density exhibits very little evolution. Overall, our findings are consistent with the large, predominantly northern sky, surveys of Mg II absorbers. We developed and implemented a Monte Carlo model informed by a high-resolution Mg II survey for determining the Mg II mass density, Ω_{Mg II} . We found Ω_{Mg II} ∼ 5 × 10^‑7 with no evidence of evolution over a ∼7 Gyr time span following Cosmic Noon. Incorporating measurements covering 2.0 ≤ z ≤ 6.4 from the literature, we extended our insights into Mg II mass density evolution from the end of reionization well past the Cosmic Noon epoch. The presented Monte Carlo model has potential for advancing our knowledge of the evolution of mass densities of metal ions common to quasar absorption line studies, as it exploits the efficiency of large low-resolution surveys while requiring only small samples from expensive high-resolution surveys.