A Galaxy Redshift Survey Near HST/COS AGN Sight Lines

Abstract

To establish the connection between galaxies and UV-detected absorption systems in the local universe, a deep (g ≤ 20) and wide (∼ 20 radius) galaxy redshift survey is presented around 47 sight lines to UV-bright AGN observed by the Cosmic Origins Spectrograph (COS). Specific COS science team papers have used this survey to connect absorbers to galaxies, groups of galaxies, and large-scale structures, including voids. Here we present the technical details of the survey and the basic measurements required for its use, including redshifts for individual galaxies and uncertainties determined collectively by spectral class (emission-line, absorption-line, and composite spectra) and completeness for each sight line as a function of impact parameter and magnitude. For most of these sight lines the design criteria of > 90% completeness over a > 1 Mpc region down to 0.1 L * luminosities at z ≤ 0.1 allows a plausible association between low-z absorbers and individual galaxies. Lyα covering fractions are computed to approximate the star-forming and passive galaxy populations using the spectral classes above. In agreement with previous results, the covering fraction of star-forming galaxies with L ≥ 0.3 L * is consistent with unity inside one virial radius and declines slowly to > 50% at 4 virial radii. On the other hand, passive galaxies have lower covering fractions (∼ 60%) and a shallower decline with impact parameter, suggesting that their gaseous halos are patchy but have a larger scale-length than star-forming galaxies. All spectra obtained by this project are made available electronically for individual measurement and use.

The Warm Circumgalactic Medium: 105−6 K Gas Associated with a Single Galaxy Halo or with an Entire Group of Galaxies?

“…It is possible that the O VI absorber is photoionized at a temperature more typical of the Lyα forest rather than WHIM gas. CLOUDY simulations (e.g., those used in Keeney et al 2017) show that the ratio of ionization fractions of O VI and H I (f HI /f OV I ) remain fairly constant over a wide range of ionization parameters at T < 10 5 K. Adopting the observed N OVI , we infer a characteristic N HI expected for photoionized gas of N HI ∼ 10 14 /Z 0.1 for a canonical Z/Z ⊙ = 0.1. The observed O VI profile must be broadened almost entirely by non-thermal motions for gas at these low temperatures, so this nonthermal broadening would be present in the inferred Lyα profiles as well.…”

“…Stocke et al (2013); Stern et al (2016); Keeney et al (2017) estimate a CGM cool, photo-ionized gas mass somewhat less than or comparable to the mass in stars and disk gas (10-20% baryon fraction compared to ∼ 20% in stars and disk gas; see Table 8 in Stocke et al 2013), while Werk et al (2014); Prochaska et al (2017, COS Halos project) obtain a limit and a value approximately a factor of 2 higher. While the COS-Halos results suggest that all the spiral galaxy baryons may have been identified already, the other results suggest that up to half the baryons are still "missing".…”

“…Recent accountings by our group suggest that 30-50% of spiral galaxy baryons are still "missing" (Stocke et al 2013;Keeney et al 2017) in the sense that they have not been directly detected as yet (Fukugita et al 1998;Bregman 2007;Shull et al 2012). Are these baryons still present in the CGM of massive spirals or have they been ejected out into the Inter-Galactic Medium (IGM) which is enriched in metals by this process?…”

“…For our Cycle 23 HST/COS GO program we rejected sightlines which passed within 1.5 virial radii of an individual group galaxy to make sure we were observing group gas, not gas associated with individual galaxies (see e.g., Prochaska et al 2011;Stocke et al 2013). This impact parameter is large enough that association of an absorber with a single galaxy is problematical (see Paper 2 and Keeney et al 2017). But this proved too restrictive a criterion given the limited number of high-S/N spectra in the current COS archive (Danforth et al 2016).…”

“…Recent studies using the ultraviolet spectrographs of the Hubble Space Telescope (HST) have proven critical to recent advances in the field (Tripp et al 1998;Penton, Stocke, & Shull 2004;Tumlinson et al 2011;Prochaska et al 2011;Stocke et al 2013;Werk et al 2013;Bordoloi et al 2014), including the recognition that the CGM likely contains a comparable number of baryons as found in all the stars and gas in the disks of luminous (L≥ 0.1L * ) late-type galaxies (Tumlinson et al 2011;Stocke et al 2013;Werk et al 2014;Stern et al 2016;Prochaska et al 2017;Keeney et al 2017). Stocke et al (2013); Stern et al (2016); Keeney et al (2017) estimate a CGM cool, photo-ionized gas mass somewhat less than or comparable to the mass in stars and disk gas (10-20% baryon fraction compared to ∼ 20% in stars and disk gas; see Table 8 in Stocke et al 2013), while Werk et al (2014); Prochaska et al (2017, COS Halos project) obtain a limit and a value approximately a factor of 2 higher.…”