The Curious Case of Lyman Alpha Emitters: Growing Younger from z ~ 3 to z ~ 2?

TL;DR

This study compares the properties of Lyman Alpha Emitting galaxies at redshifts 3.1 and 2.1 using consistent analysis methods, revealing significant differences that challenge the idea of direct evolutionary connection between these galaxy populations.

Contribution

The paper applies a uniform SED fitting approach to two galaxy samples, providing clearer evidence that LAEs at different redshifts have distinct physical properties.

Findings

LAEs at z=3.1 are old (~1 Gyr) and metal-poor.

LAEs at z=2.1 are young (~50 Myr) and metal-rich.

Direct evolution from z=3.1 to z=2.1 LAEs is unlikely.

Abstract

Lyman Alpha Emitting (LAE) galaxies are thought to be progenitors of present-day L* galaxies. Clustering analyses have suggested that LAEs at z ~ 3 might evolve into LAEs at z ~ 2, but it is unclear whether the physical nature of these galaxies is compatible with this hypothesis. Several groups have investigated the properties of LAEs using spectral energy distribution (SED) fitting, but direct comparison of their results is complicated by inconsistencies in the treatment of the data and in the assumptions made in modeling the stellar populations, which are degenerate with the effects of galaxy evolution. By using the same data analysis pipeline and SED fitting software on two stacked samples of LAEs at z = 3.1 and z = 2.1, and by eliminating several systematic uncertainties that might cause a discrepancy, we determine that the physical properties of these two samples of galaxies are dramatically different. LAEs at z = 3.1 are found to be old (age ~ 1 Gyr) and metal-poor (Z < 0.2 Z_Sun), while LAEs at z = 2.1 appear to be young (age ~ 50 Myr) and metal-rich (Z > Z_Sun). The difference in the observed stellar ages makes it very unlikely that z = 3.1 LAEs evolve directly into z = 2.1 LAEs. Larger samples of galaxies, studies of individual objects and spectroscopic measurements of metallicity at these redshifts are needed to confirm this picture, which is difficult to reconcile with the effects of 1 Gyr of cosmological evolution.