13 Billion Years of MgII Absorber Evolution

TL;DR

The paper develops a framework to constrain the redshift evolution of the Mg II equivalent width distribution $f(z,W)$ by deriving it from the measured co-moving incidence $d{\cal N}/dX$ using a Schechter function $f(z,W) = \Phi^*(z) (W/W^*(z))^{\alpha(z)} e^{-(W/W^*)} \!\, dW/W^*$ with redshift-dependent parameters. It tests two data-driven scenarios for weak absorbers from $z\sim7$ to $z\sim4$, finding that the scenario where weak systems evolve away post-reionization best matches ancillary metal-line data and the evolution of the strongest systems. The results provide a unified, quantitative description of Mg II absorber evolution over $0<z<7$, revealing distinct paths for weak and strong systems and offering insights into the galactic baryon cycle. The study also underscores the need for deeper $z>5$ Mg II surveys and has implications for detecting a potential Mg II forest at $z>7$.

Abstract

Applying "apportioned integrals," we use dN/dX measurements to determine the MgII absorber equivalent width distribution function for Wr > 0.03 [angstroms] and 0 < z < 7. Adopting a Schechter distribution, f(z,W)dW = Phi* (W/W*)^alpha e^{-W/W*} dW/W*, we present the normalization, Phi*(z), the characteristic equivalent width, W*(z), and the weak-end slope, alpha(z), as smooth functions of redshift. Measurements of dN/dX are robust for z < 4 but less so at z > 4 for weaker absorbers (Wr < 0.3 [angstroms]). We bracketed two data-driven scenarios: from z ~ 7 to z ~ 4, dN/dX of weak absorbers is (1) constant, or (2) decreasing. For scenario #1, the evolution of Phi*(z), W*(z), and alpha(z) show that in the post-reionization universe, weak systems are nonevolving while the incidence of the strongest systems increases until Cosmic Noon; following Cosmic Noon, the strongest absorbers slowly evolve away while the incidence of weak absorbers rapidly increases. For scenario #2, the parameter evolution is such that, in the post-reionization universe, weak systems evolve away while the incidence of the strongest systems increases until Cosmic Noon; following Cosmic Noon, the behavior tracks the same as scenario #1. We argue in favor of scenario #2 based on corroborating OI, CII, and SiII measurements at z > 4. Our results provide a unified, quantitative description for MgII absorber evolution spanning 13 billion years of cosmic time and offer deeper insights into galactic baryon cycle physics. They also highlight the need for deep z > 5 MgII surveys and have implications for detectability of a MgII forest at z > 7.