AHUBBLE SPACE TELESCOPESTUDY OF LYMAN LIMIT SYSTEMS: CENSUS AND EVOLUTION

Abstract

We present a survey for optically thick Lyman limit absorbers at z < 2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z < 2.6 by a factor of ∼10. We compile a statistical sample of 50 τ LLS ≥ 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z) = dn/dz, at these redshifts is well described by a single power law, l(z) ∝ (1 + z) γ , with γ =1.33 ± 0.61 at z < 2.6, or with γ = 1.83 ± 0.21 over the redshift range 0.2 ≤ z ≤ 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ∼1.5 over the ∼0.6 Gyr from z = 4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ∼8 Gyr from z = 2.6 to 0.25. We show that the column density distribution function, f (N HI ), at low redshift is not well fitted by a single power law index (f (N HI ) ∝ N −β HI ) over the column density range 13 ≤ log N HI ≤ 22 or log N HI ≥ 17.2. While low and high redshift f (N HI ) distributions are consistent for log N HI > 19.0, there is some evidence that f (N HI ) evolves with z for log N HI 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ∼9 from z ∼5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L * galaxies.

The low-metallicity QSO HE 2158 − 0107: a massive galaxy growing by accretion of nearly pristine gas from its environment?

“…Birnboim & Dekel 2003;Kereš et al 2005), which would then light up as an EELR. This process is theoretically predicted, but observational evidence for its existence is still rare with only a few candidate detections (Giavalisco et al 2011;Ribaudo et al 2011), also because the phenomenon is expected to be more common at high redshift than in the local universe. To shed more light on this question, much deeper spectra covering the entire optical window are needed to turn our current limits into robust abundance measurements that can then be directly compared with those of dwarf galaxies.…”

ULTRA-FAINT ULTRAVIOLET GALAXIES ATz∼ 2 BEHIND THE LENSING CLUSTER A1689: THE LUMINOSITY FUNCTION, DUST EXTINCTION, AND STAR FORMATION RATE DENSITY

“…The values we have adopted (from the literature) for k, N 0 and γ are given in Table 2 (Janknecht et al 2006;Rao et al 2006;Ribaudo et al 2011;O'Meara et al 2013). …”

Triple-ionized carbon associated with the low-density neutral hydrogen gas at 1.7 <z< 3.3: the integratedN H I–N C ivrelation

“…Low-ionisation metal transitions, such as Mg ii and C ii, are mostly found at NH i 10 16 cm −2 . Absorbers with NH i 10 17.2 cm −2 (Lyman limit systems or LLSs) display a wide range of metal species and ionisations (Steidel 1990;Levshakov et al 2003;Prochaska et al 2006;Lehner et al 2013) and are thought to be associated with outflows/infall in outer halos (Jenkins et al 2005;Faucher-Giguère & Kereŝ 2011;Kacprzak et al 2011;Ribaudo et al 2011;Bouché et al 2012;Lehner et al 2013), or with extended disks/inner halos at 10-20 kpc, e.g. an analogue of intermediate/high velocity clouds of the Milky Way or merger remnants (Thilker et al 2004;Lehner et al 2009;Stocke et al 2010).…”