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≤ log NHI<20.3) at Cosmic Noon (1≤ z ≤2.2), 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 [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≤ log NHI<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 [X/H]=−3.5, indicating no pristine CGM-like gas at Cosmic Noon. Absorbers in BRIDGE-I trace overdensities of 400≲ δb≲2200, higher than at z>2.2. Mean NHI differs by a factor of 400 across absorber types, but mean total hydrogen column by only 2.5, as higher NHI 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 [X/H]∼−2.0 emerges for log NHI>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.
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BibTeX
@misc{deepak2026bridge,
title = {{The BRIDGE Survey. I. Metallicities and Physical Properties of Gas Probing the Circumgalactic Medium at Cosmic Noon}},
author = {Saloni Deepak and Nicolas Lehner and J. Christopher Howk and John O'Meara and Celine Peroux and J. Xavier Prochaska},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.26785},
eprint = {2609.26785},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.26785},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The BRIDGE Survey. I. Metallicities and Physical Properties of Gas Probing the Circumgalactic Medium at Cosmic Noon
AU - Saloni Deepak
AU - Nicolas Lehner
AU - J. Christopher Howk
AU - John O'Meara
AU - Celine Peroux
AU - J. Xavier Prochaska
PY - 2026
DA - 2026/09/22
PB - arXiv
DO - 10.48550/arxiv.2609.26785
UR - https://arxiv.org/abs/2609.26785
AB - We present BRIDGE-I, a metallicity survey of \HI-selected absorbers probing ``CGM-like'' gas (16.4≤ log N_HI<20.3) at Cosmic Noon (1≤ z ≤2.2), 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 [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≤ log N_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 [X/H]=−3.5, indicating no pristine CGM-like gas at Cosmic Noon. Absorbers in BRIDGE-I trace overdensities of 400≲ δ_b≲2200, higher than at z>2.2. Mean N_HI differs by a factor of 400 across absorber types, but mean total hydrogen column by only 2.5, as higher N_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 [X/H]∼−2.0 emerges for log N_HI>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.
ER -