Galaxies Probing Galaxies in PRIMUS - I. Sample, Spectroscopy, and Characteristics of the z~0.5 MgII-Absorbing Circumgalactic Medium

Spectroscopy of background QSO sightlines passing close to foreground galaxies is a potent technique for studying the circumgalactic medium (CGM). QSOs are effectively point sources, however, limiting their potential to constrain the size of circumgalactic gaseous structures. Here we present the first large Keck/LRIS and VLT/FORS2 spectroscopic survey of bright (B_AB < 22.3) background galaxies whose lines of sight probe MgII 2796, 2803 absorption from the CGM around close projected foreground galaxies at transverse distances 10 kpc < R_perp < 150 kpc. Our sample of 72 projected pairs, drawn from the PRIsm MUlti-object Survey (PRIMUS), includes 48 background galaxies which do not host bright AGN, and both star-forming and quiescent foreground galaxies with stellar masses 9.0 < log M_*/M_sun < 11.2 at redshifts 0.35 < z_f/g < 0.8. We detect MgII absorption associated with these foreground galaxies with equivalent widths 0.25 Ang 2sigma significance in 20 individual background sightlines passing within R_perp < 50 kpc, and place 2sigma upper limits on W_2796 of <0.5 Ang in an additional 11 close sightlines. Within R_perp < 50 kpc, W_2796 is anticorrelated with R_perp, consistent with analyses of MgII absorption detected along background QSO sightlines. Subsamples of these foreground hosts divided at log M_*/M_sun = 9.9 exhibit statistically inconsistent W_2796 distributions at 30 kpc < R_perp < 50 kpc, with the higher-M_* galaxies yielding a larger median W_2796 by 0.9 Ang. Finally, we demonstrate that foreground galaxies with similar stellar masses exhibit the same median W_2796 at a given R_perp to within <0.2 Ang toward both background galaxies and toward QSO sightlines drawn from the literature. Analysis of these datasets constraining the spatial coherence scale of circumgalactic MgII absorption is presented in a companion paper.Comment: 36 pages, 18 figures, 5 tables. Accepted to Ap

The CGM and IGM at z∼\sim5: metal budget and physical connection

We present further results of a survey for absorption line systems in the spectra of four high redshift quasars (5.79 $\le$ z$_{\textrm{em}}$ $\le$ 6.13) obtained with the ESO Very Large Telescope X-Shooter. We identify 36 $\textrm{CIV}$ and 7 $\textrm{SiIV}$ systems with a $\ge$ 5$\sigma$ significance. The highest redshift $\textrm{CIV}$ and $\textrm{SiIV}$ absorbers identified in this work are at z = 5.80738 $\pm$ 0.00017 and z = 5.77495 $\pm$ 0.00038, respectively. We compute the comoving mass density of $\textrm{SiIV}$ ($\Omega_{\textrm{SiIV}}$) and find that it evolves from $\Omega_{\textrm{SiIV}}$ = 4.3$^{+2.1}_{-2.1}$ $\times$10$^{-9}$ at = 5.05 to $\Omega_{\textrm{SiIV}}$ = 1.4$^{+0.6}_{-0.4}$ $\times$10$^{-9}$ at = 5.66. We also measure $\Omega_{\textrm{CIV}}$ = 1.6$^{+0.4}_{-0.1}$ $\times$10$^{-8}$ at = 4.77 and $\Omega_{\textrm{CIV}}$ = 3.4$^{+1.6}_{-1.1}$ $\times$10$^{-9}$ at = 5.66. We classify our $\textrm{CIV}$ absorber population by the presence of associated $\textit{low}$ and/or $\textit{high ionisation}$ systems and compute their velocity width ($\Delta$v$_{90}$). We find that all $\textrm{CIV}$ systems with $\Delta$v$_{90}$ > 200 kms$^{-1}$ have associated $\textit{low ionisation}$ systems. We investigate two such systems, separated by 550 physical kpc along a line of sight, and find it likely that they are both tracing a multi-phase medium where hot and cold gas is mixing at the interface between the CGM and IGM. We further discuss the \textrm{MgII} systems presented in a previous work and we identify 5 $\textrm{SiII}$, 10 $\textrm{AlII}$, 12 $\textrm{FeII}$, 1 $\textrm{CII}$, 7 $\textrm{MgI}$ and 1 $\textrm{CaII}$ associated transitions. We compute the respective comoving mass densities in the redshift range 2 to 6, as allowed by the wavelength coverage.Comment: Accepted for publication in MNRAS 22 pages, 19 figures, 6 table

Discovery of three new near-pristine absorption clouds at z=2.6z=2.6-4.4

We report the discovery of three new "near-pristine" Lyman Limit Systems (LLSs), with metallicities ~1/1000 solar, at redshifts 2.6, 3.8 and 4.0, with a targeted survey at the Keck Observatory. High resolution echelle spectra of eight candidates yielded precise column densities of hydrogen and weak, but clearly detected, metal lines in seven LLSs; we previously reported the one remaining, apparently metal-free LLS, to have metallicity <1/10000 solar. Robust photoionisation modelling provides metallicities [Si/H] = -3.05 to -2.94, with 0.26 dex uncertainties (95% confidence) for three LLSs, and [Si/H] >~ -2.5 for the remaining four. Previous simulations suggest that near-pristine LLSs could be the remnants of PopIII supernovae, so comparing their detailed metal abundances with nucleosynthetic yields from supernovae models is an important goal. Unfortunately, at most two different metals were detected in each new system, despite their neutral hydrogen column densities (10^{19.2-19.4} cm^{-2}) being two orders of magnitude larger than the two previous, serendipitously discovered near-pristine LLSs. Nevertheless, the success of this first targeted survey for near-pristine systems demonstrates the prospect that a much larger, future survey could identify clear observational signatures of PopIII stars. With a well-understood selection function, such a survey would also yield the number density of near-pristine absorbers which, via comparison to future simulations, could reveal the origin(s) of these rare systems.Comment: Accepted by MNRAS. 21 pages, 27 figure

Metal-enriched, subkiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy

We report the serendipitous detection of a 0.2 L*, Lyα emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system (N[subscript Hi] = 10[superscript 16.94±0.10] cm[superscript −2]) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalize over uncertainties in the shape and normalization of the ionizing radiation spectrum, we derive the total hydrogen column density N[subscript H] = 10[superscript 19.4±0.3] cm[superscript −2], and show that all the absorbing clouds are metal enriched, with Z = 0.1–0.6 Z[subscript ⊙]. These metallicities and the system's large velocity width (436 km s[superscript −1]) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of ~5 M[subscript ⊙] yr[superscript −1]. Our photoionization model yields extremely small sizes (<100–500 pc) for the absorbing clouds, which we argue is typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a 1000-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM

Probing the Circumgalactic Medium at High-Redshift Using Composite BOSS Spectra of Strong Lyman-alpha Forest Absorbers

We present composite spectra constructed from a sample of 242,150 Lyman-alpha (Lya) forest absorbers at redshifts 2.4<z<3.1 identified in quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS) as part of Data Release 9 of the Sloan Digital Sky Survey III. We select forest absorbers by their flux in bins 138 km/s wide (approximately the size of the BOSS resolution element). We split these absorbers into five samples spanning the range of flux -0.05 < F<0.45. Tests on a smaller sample of high-resolution spectra show that our three strongest absorption bins would probe circumgalactic regions (projected separation < 300 proper kpc and |Delta v| < 300km/s) in about 60% of cases for very high signal-to-noise ratio. Within this subset, weakening Lya absorption is associated with decreasing purity of circumgalactic selection once BOSS noise is included. Our weaker two Lya absorption samples are dominated by the intergalactic medium. We present composite spectra of these samples and a catalogue of measured absorption features from HI and 13 metal ionization species, all of which we make available to the community. We compare measurements of seven Lyman series transitions in our composite spectra to single line models and obtain further constraints from their associated excess Lyman limit opacity. This analysis provides results consistent with column densities over the range 14.4 <~ Log (N_HI) <~ 16.45. We compare our measurements of metal absorption to a variety of simple single-line, single-phase models for a preliminary interpretation. Our results imply clumping on scales down to ~30 pc and near-solar metallicities in the circumgalactic samples, while high-ionization metal absorption consistent with typical IGM densities and metallicities is visible in all samples.Comment: 23 pages, 15 figures, 5 tables, link to downloadable data included. Accepted by MNRAS 2014 March 20. New sections 3.4 and 6.1 limiting the occurrence and impact of Lyman limit system

Metal-enriched, subkiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy★

We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$ cm$^{-2}$) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalise over uncertainties in the shape and normalisation of the ionizing radiation spectrum, we derive the total hydrogen column density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all the absorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_\odot$. These metallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of $\sim5 M_\odot\,$yr$^{-1}$. Our photoionization model yields extremely small sizes ($<$100-500 pc) for the absorbing clouds, which we argue are typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a $1000$-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.Comment: Fixed an incorrect reference to D'Odorico & Petitjean 2001, A&A, 370, 729. Data and code used for the paper are at https://github.com/nhmc/LA

The neutral hydrogen cosmological mass density at z = 5

We present the largest homogeneous survey of $z>4.4$ damped Lyman-$\alpha$ systems (DLAs) using the spectra of 163 QSOs that comprise the Giant Gemini GMOS (GGG) survey. With this survey we make the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, $\Omega_{\rm HI}$. At such high redshift important systematic uncertainties in the identification of DLAs are produced by strong intergalactic medium absorption and QSO continuum placement. These can cause spurious DLA detections, result in real DLAs being missed, or bias the inferred DLA column density distribution. We correct for these effects using a combination of mock and higher-resolution spectra, and show that for the GGG DLA sample the uncertainties introduced are smaller than the statistical errors on $\Omega_{\rm HI}$. We find $\Omega_{\rm HI}=0.98^{+0.20}_{-0.18}\times10^{-3}$ at $\langle z\rangle=4.9$, assuming a 20% contribution from lower column density systems below the DLA threshold. By comparing to literature measurements at lower redshifts, we show that $\Omega_{\rm HI}$ can be described by the functional form $\Omega_{\rm HI}(z)\propto(1+z)^{0.4}$. This gradual decrease from $z=5$ to $0$ is consistent with the bulk of HI gas being a transitory phase fuelling star formation, which is continually replenished by more highly-ionized gas from the intergalactic medium, and from recycled galactic winds.Comment: Accepted by MNRAS. Comments and reference suggestions are still welcome. Data and code for the paper will appear at https://github.com/nhmc/GGG_DLA after the final version has been publishe