The Metal Enrichment History of the CGM and IGM Across ~12 Gyr: The BRIDGE Survey
Saloni Deepak
Department of Physics and Astronomy
University of Notre Dame
The growth and evolution of galaxies are regulated by their exchange of gas with the circumgalactic and intergalactic medium (CGM/IGM). These multiphase reservoirs supply the fuel for star formation and carry the imprint of stellar and AGN feedback, so their metal content constrains feedback and accretion in cosmological simulations, the enrichment timescales of different gas phases, and the cosmic baryon cycle. Metallicity surveys of H I-selected absorbers exist at z < 1 and z > 2.2, but at Cosmic Noon (1 <= z <= 2.2), the peak epoch of star formation and AGN activity, the cool CGM and IGM traced by absorbers with log N_HI < 20.3 remain largely unconstrained. I will present BRIDGE, a survey that fills this gap by combining archival HST UV spectra with Keck/HIRES and VLT/UVES optical spectra. From BRIDGE-I, I report metallicities, densities, and ionization conditions for 105 absorbers with 16.4 ≤ log N_HI < 20.3, whose column densities at Cosmic Noon correspond to overdensities typical of the CGM. Comparing with lower and higher redshifts, I find that lower-overdensity CGM gas has already reached its z < 1 metallicity by Cosmic Noon, while higher-overdensity gas continues to be enriched at later times, with no absorber below [X/H] ~ -2 in the densest gas. I will show that metallicity correlates with overdensity, consistent with metals tracing the large-scale collapse of baryons. I will then turn to a complementary study of a lensed quasar pair whose sightlines intersect three Cosmic Noon absorbers, resolving metallicity and density variations on kpc scales within individual CGM structures. This work shows that metals are poorly mixed within individual CGM structures on kpc scales, and demonstrates the power of lensed sightlines for resolving that patchiness directly.