The COS-Halos survey: physical conditions and baryonic mass in the low-redshift circumgalactic medium
- UCO/Lick Observatory, University of California, Santa Cruz, CA (United States)
- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD (United States)
- Department of Astronomy, University of Massachusetts, Amherst, MA (United States)
- Department of Physics and Astronomy, University of Notre Dame, South Bend, IN (United States)
- Department of Chemistry and Physics, Saint Michael's College, Colchester, VT (United States)
- Astronomy Department, University of Arizona, Tucson, AZ 85721 (United States)
- Leiden Observatory, Leiden University, NL-2300 RA Leiden (Netherlands)
- University of the Western Cape, Bellville, Cape Town 7535 (South Africa)
We analyze the physical conditions of the cool, photoionized (T ∼10{sup 4} K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of L ∼ L* galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n {sub H} {sub II}/n {sub H} ≳ 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, R {sub vir}, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n {sub H} = (10{sup –4.2±0.25})(R/R {sub vir}){sup –0.8±0.3}. Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the H I column densities, we estimate a lower limit to the cool gas mass M{sub CGM}{sup cool}>6.5×10{sup 10} M {sub ☉} for the volume within R < R {sub vir}. Allowing for an additional warm-hot, O VI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10{sup 12} M {sub ☉} scale.
- OSTI ID:
- 22365244
- Journal Information:
- Astrophysical Journal, Vol. 792, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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