COSHalos

COSHalos, commonly written as COS-Halos, is a spectroscopic survey that studied the circumgalactic medium (CGM) of galaxies in the low-redshift universe using ultraviolet absorption-line spectroscopy with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. The project aimed to characterize the gas content, metallicity, ionization state, and spatial extent of halo gas around ordinary (L*) galaxies and to understand how CGM properties relate to star formation and galaxy evolution.

Methods and scope: The program used background quasars and active galactic nuclei whose sightlines pass near foreground galaxies within projected separations of roughly 100 to 150 kiloparsecs. Ultraviolet spectra revealed absorption features from hydrogen and multiple metal ions, enabling measurements of column densities, covering fractions, and metallicities across different halo regions. By combining absorption data with the properties of the associated galaxies, the survey sought to infer the mass, enrichment, and physical state of CGM gas.

Key findings: COS-Halos demonstrated that halo gas around L*-type galaxies is rich in both neutral and ionized species, with substantial metal content detected in the CGM out to large radii. The ionization structure and metal enrichment showed dependence on the star-formation activity of the host galaxy, with differences observed between star-forming and quiescent systems. The results indicated that a significant fraction of a galaxy’s baryons and metals reside in the CGM and highlighted the role of feedback and gas flows in shaping halo gas.

Impact: The COS-Halos results provided critical empirical constraints on models of the baryon cycle and feedback in galaxy evolution, influencing subsequent CGM surveys and simulations and shaping our understanding of how galaxies acquire and lose gas.

See also: Circumgalactic medium; Hubble Space Telescope; Cosmic Origins Spectrograph; galaxy evolution. References include key publications from Tumlinson and colleagues describing the survey’s methodology and findings.