The BRIDGE Survey. I. Metallicities and Physical Properties of Gas Probing the Circumgalactic Medium at Cosmic Noon — E8 Intelligence Research

We present BRIDGE-I, a metallicity survey of \HI-selected absorbers probing ``CGM-like'' gas ($16.4\leq \log N_{\rm HI}<20.3$) at Cosmic Noon ($1\leq z \leq2.2$), the epoch of peak star formation and AGN activity. Combining archival HST UV spectra with Keck/HIRES and VLT/UVES optical spectra, we estimate metallicities and physical conditions for 105 absorbers. The Cosmic Noon metallicity distribution (median $[\rm{X/H}]=-1.19$) is statistically indistinguishable from that at $z<1$ ($-1.18$) but differs significantly from $z>2.2$ ($-2.02$). This plateau is driven by the most diffuse absorbers ($16.4\leq \log N_{\rm HI}<17.2$), which reach their $z<1$ enrichment level by Cosmic Noon, whereas denser absorbers remain more metal-poor than their $z<1$ counterparts and continue to be enriched at later times. Unlike at $z>2.2$, no absorber falls below $[\rm{X/H}]=-3.5$, indicating no pristine CGM-like gas at Cosmic Noon. Absorbers in BRIDGE-I trace overdensities of $400\lesssim δ_b\lesssim2200 Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com