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

Abstract: We present BRIDGE-I, a metallicity survey of \HI-selected absorbers probing ``CGM-like'' gas ($16.4\leq \log N_{\rm HI}&lt;20.3$) 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 $z&lt;1$ ($-1.18$) but differs significantly from $z&gt;2.2$ ($-2.02$). This plateau is driven by the most diffuse absorbers ($16.4\leq \log N_{\rm HI}&lt;17.2$), which reach their $z&lt;1$ enrichment level by Cosmic Noon, whereas denser absorbers remain more metal-poor than their $z&lt;1$ counterparts and continue to be enriched at later times. Unlike at $z&gt;2.2$, 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 $400\lesssim δ<em>b\lesssim2200$, higher than at $z&gt;2.2$. Mean $N</em>{\rm HI}$ differs by a factor of 400 across absorber types, but mean total hydrogen column by only 2.5, as higher $N_{\rm HI}$ is offset by a higher neutral fraction. The absorber classes thus differ in ionization state and metallicity rather than the total gas content. A metallicity floor of $[\rm{X/H}]\sim-2.0$ emerges for $\log N_{\rm HI}&gt;19.0$ absorbers, indicating that denser, more ISM-like gas is well mixed as a population, while more diffuse gas retains a metal-poor population that is not yet fully mixed.