An Ultraviolet Survey of Low-Redshift Partial Lyman-Limit Systems with the HST Cosmic Origins Spectrograph

Abstract

We present an ultraviolet spectroscopic survey of strong H I absorbers in the intergalactic medium, probing their evolution over the last 6-7 Gyr at redshifts $0.24\le z\le 0.84$. We measure column densities $N_{\mathrm{HI}}({\mathrm{cm}}^{-2})$ from the pattern of Lyman-series absorption lines and flux decrement at the Lyman limit (LL) when available. We analyzed 220 H I absorbers in ultraviolet spectra of 102 active galactic nuclei (AGN) taken by the Cosmic Origins Spectrograph aboard the Hubble Space Telescope with G130M/G160M gratings (1134-1795 Å). For 158 absorbers with $\log N_{\mathrm{HI}}\ge 15$, the mean frequency is $d\mathcal{N}/dz=4.95\pm 0.39$ over pathlength $\Delta z=31.94$ ($0.24\le z\le 0.84)$. We identify 8 Lyman Limit Systems (LLS, $\log N_{\mathrm{HI}}\ge 17.2$) and 54 partial systems (pLLS) with $16.0\le \log N_{\mathrm{HI}}<17.2$. Toward 159 AGN between $0.01<z_{\mathrm{abs}}<0.84$ with $\Delta z\approx 48$, we find four damped Ly$\alpha$ absorbers (DLA) with $(d\mathcal{N}/dz{)}_{\mathrm{DLA}}=0.083_{-0.040}^{+0.066}$ at $⟨z⟩=0.18$. The mean LLS frequency between $z=0.24-0.48$ is $(d\mathcal{N}/dz{)}_{\mathrm{LLS}}=0.36_{-0.13}^{+0.20}$ fitted to $N(z)=(0.25_{-0.09}^{+0.13})(1+z{)}^{1.14}$. For 54 pLLS we find $(d\mathcal{N}/dz{)}_{\mathrm{pLLS}}=1.69\pm 0.23$ at $⟨z⟩=0.39$, a frequency consistent with gaseous halo sizes $R\approx 100h^{-1}\mathrm{kpc}$ for ($0.3-3L^{\ast }$) galaxies. A maximum-likelihood analysis yields a distribution $f(N,z)=C_0{N}^{-\beta }(1+z{)}^{\gamma }$ with $\beta =1.48\pm 0.05$ and $\gamma =1.14_{-0.89}^{+0.88}$ for $15\le \log N_{\mathrm{HI}}\le 17.5$. The far-UV opacity gradient is $d{\tau }_{\mathrm{eff}}/dz\approx (0.444)(1+z{)}^{1.14}$ over the range $15\le \log N_{\mathrm{HI}}\le 17$, implying mean LyC optical depth ${\tau }_{\mathrm{eff}}\approx 0.3-0.5$ toward sources at $z=1-2$.

Accepted for publication (Astrophys J), 34 pages, 13 figures, 2 appendices

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