An HST/Cos Survey of the Low-Redshift Intergalactic Medium. I. Survey, Methodology, and Overall Results*

Abstract

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright AGN at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths ∆z HI = 21.7 and ∆z OVI = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively-defined as possible. The cumulative column-density distribution of H I systems can be parametrized dN (> N )/dz = C 14 (N/10 14 cm −2 ) −(β−1) , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ (1 + z) 2.3±0.1 , and slope β(z) = 1.75 − 0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N HI . The distribution of O VI absorbers appear to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of ∆v = 50 − 300 km s −1 with no significant clustering at ∆v 1000 km s −1 . Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N HI 10 13.5 cm −2 , metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at

An HST/COS legacy survey of intervening Si III absorption in the extended gaseous halos of low-redshift galaxies

“…This value is more than twice the value derived in this study. The same authors recently published another absorption-line catalog of intervening absorption systems at z < 0.4, this time based on HST/COS data of 82 AGN (Danforth et al 2016). In this new survey, the authors derive a number density of dN/dz = 5.9 +0.9 −0.6 for log N(Si iii) > 12.2.…”

“…We also have identified one candidate Si iii absorber that is not listed in the Danforth et al (2016) paper. In Table A.4 in the Appendix we list these discrepant absorption systems together with a short description of the absorption characteristics.…”

“…Their new value again is substantially higher than the value derived by us. To investigate the origin for this discrepancy we have carefully compared their absorber list with ours, as our COS data sample includes all of the 82 QSOs presented in Danforth et al (2016). We sus- Kilborn et al 2005, and references therein).…”

Bimodality of low-redshift circumgalactic O vi in non-equilibrium eagle zoom simulations

“…Thom & Chen 2008;Tripp et al 2008;Wakker & Savage 2009;Tilton et al 2012;Danforth et al 2016). These observations could be confronted with cosmological hydrodynamical simulations that included metal enrichment from galactic superwinds to learn how the nucleosynthetic products of star formation enrich the diffuse CGM and IGM (e.g.…”

Characterization of the warm–hot intergalactic medium near the Coma cluster through high-resolution spectroscopy of X Comae

“…Cen & Ostriker 1999;Davé et al 2001). Far ultraviolet (FUV) absorption-line spectroscopy, probing the lower-temperature phase of the WHIM (log T (K) = 5 − 6) with transitions from species such as O VI, has so far detected only a fraction of the expected baryons (e.g., Danforth & Shull 2008;Tripp et al 2008;Tilton et al 2012;Shull et al 2012;Danforth et al 2016). The remaining low-redshift baryons are likely to be found at higher temperature (log T (K) = 6 − 7) where X-ray observations can be used to detect their high-ionization absorption lines (e.g., O VII and O VIII Kα).…”