We present an absorption-line survey of optically thick gas clouds -- Lyman Limit Systems (LLSs) -- observed at high dispersion with spectrometers on the Keck and Magellan telescopes. We measure column densities of neutral hydrogen NHI and associated
metal-line transitions for 157 LLSs at z=1.76-4.39 restricted to 10^17.3 < NHI < 10^20.3. An empirical analysis of ionic ratios indicates an increasing ionization state of the gas with decreasing NHI and that the majority of LLSs are highly ionized, confirming previous expectations. The Si^+/H^0 ratio spans nearly four orders-of-magnitude, implying a large dispersion in the gas metallicity. Fewer than 5% of these LLSs have no positive detection of a metal transition; by z~3, nearly all gas that is dense enough to exhibit a very high Lyman limit opacity has previously been polluted by heavy elements. We add new measurements to the small subset of LLS (~5-10) that may have super-solar abundances. High Si^+/Fe^+ ratios suggest an alpha-enhanced medium whereas the Si^+/C^+ ratios do not exhibit the super-solar enhancement inferred previously for the Lya forest.
We survey the incidence and absorption strength of the metal-line transitions CII 1334 and CIV from the circumgalactic medium (CGM) surrounding z~2 quasars, which act as signposts for massive dark matter halos M_halo~10^12.5 Msun. On scales of the vi
rial radius (Mvir~160kpc), we measure a high covering fraction fC=0.73+/-0.10 to strong CII absorption (rest equivalent width W1334>0.2A), implying a massive reservoir of cool (T~10^4K) metal enriched gas. We conservatively estimate a metal mass exceeding 10^8 Msun. We propose these metals trace enrichment of the incipient intragroup/intracluster medium that these halos eventually inhabit. This cool CGM around quasars is the pinnacle amongst galaxies observed at all epochs, as regards covering fraction and average equivalent width of HI Lya and low-ion metal absorption. We argue that the properties of this cool CGM primarily reflect the halo mass, and that other factors such as feedback, star-formation rate, and accretion from the intergalactic medium are secondary. We further estimate, that the CGM of massive, z~2 galaxies accounts for the majority of strong MgII absorption along random quasar sightlines. Lastly, we detect an excess of strong CIV absorption (W1548>0.3A) over random incidence to 1Mpc physical impact parameter and measure the quasar-CIV cross-correlation function: xi(r)=(r/r0)^-g with r0 = 7.5Mpc and g=1.7. Consistent with previous work on larger scales, we infer that this highly ionized CIV gas traces massive (10^12 Msun) halos.
With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasars environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse to luminous
, z~2 quasars at proper separations of 30kpc < R < 1Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced HI Lya absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We also observe a high (~60%) covering factor of strong, optically thick HI absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases to ~20% at R~1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with a large correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma = 1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these massive halos are highly biased towards producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z~2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight, which we interpret in terms of the same obscuration effects frequently invoked in unified models of active galactic nuclei.
We analyze the association of galaxies to Lya and OVI absorption, the most commonly detected transitions in the low-z intergalactic medium (IGM), in the fields of 14 quasars with z_em = 0.06-0.57. Confirming previous studies, we observe a high coveri
ng fraction for Lya absorption to impact parameter rho = 300kpc: 33/37 of our L>0.01L* galaxies show Lya equivalent width W_Lya>50mA. Galaxies of all luminosity L>0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~10^(10.5 +/- 0.3) Msun for a constant N_H. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lya absorbers (W_Lya > 300mA) but cannot reproduce the majority of observed lines in the Lya forest. We conclude that the majority of Lya absorption with W_Lya=30-300mA occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ~400kpc. Regarding OVI, we observe a near unity covering fraction to rho=200kpc for L>0.1L* galaxies and to rho = 300kpc for sub-L* (0.1 L*<L<L*) galaxies. Similar to our Lya results, stronger OVI systems (W_OVI > 70mA) arise in the virialized halos of L>0.1L* galaxies. Unlike Lya, the weaker OVI systems (W_OVI~30mA) arise in the extended CGM of sub-L* galaxies. The majority of OVI gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L~0.3L*, and rarely originates in the so-called warm-hot IGM (WHIM) predicted by cosmological simulations.
We publish the survey for galaxies in 20 fields containing ultraviolet bright quasars (with z_em 0.1 to 0.5) that can be used to study the association between galaxies and absorption systems from the low-z intergalactic medium (IGM). The survey is ma
gnitude limited (R~19.5 mag) and highly complete out to 10 from the quasar in each field. It was designed to detect dwarf galaxies (L ~ 0.1 L*) at an impact parameter rho 1Mpc (z=0.1) from a quasar. The complete sample (all 20 fields) includes R-band photometry for 84718 sources and confirmed redshifts for 2800 sources. This includes 1198 galaxies with 0.005 < z < (z_em - 0.01) at a median redshift of 0.18, which may associated with IGM absorption lines. All of the imaging was acquired with cameras on the Swope 40 telescope and the spectra were obtained via slitmask observations using the WFCCD spectrograph on the Dupont 100 telescope at Las Campanas Observatory (LCO). This paper describes the data reduction, imaging analysis, photometry, and spectral analysis of the survey. We tabulate the principal measurements for all sources in each field and provide the spectroscopic dataset online.
We analyze the absorption and emission-line profiles produced by a set of simple, cool gas wind models motivated by galactic-scale outflow observations. We implement monte carlo radiative transfer techniques that track the propagation of scattered an
d fluorescent photons to generate 1D spectra and 2D spectral images. We focus on the MgII 2796,28303 doublet and FeII UV1 multiplet at ~2600A, but the results are applicable to other transitions that trace outflows (e.g. NaI, Lya, SiII). By design, the resonance transitions show blue-shifted absorption but one also predicts strong resonance and fine-structure line-emission at roughly the systemic velocity. This line-emission `fills-in the absorption reducing the equivalent width by up to 50%, shift the absorption-lin centroid by tens of km/s, and reduce the effective opacity near systemic. Analysis of cool gas outflows that ignores this line-emission may incorrectly infer that the gas is partially covered, measure asignificantly lower peak optical depth, and/or conclude that gas at systemic velocity is absent. Because the FeII lines are connected by optically-thin transitions to fine-structure levels, their profiles more closely reproduce the intrinsic opacity of the wind. Together these results naturally explain the absorption and emission-line characteristics observed for star-forming galaxies at z<1. We also study a scenario promoted to describe the outflows of z~3 Lyman break galaxies and find prfiles inconsistent with the observations due to scattered photon emission. Although line-emission complicates the analysis of absorption-line profiles, the surface brightness profiles offer a unique means of assessing the morphology and size of galactic-scale winds. Furthermore, the kinematics and line-ratios offer powerful diagnostics of outflows, motivating deep, spatially-extended spectroscopic observations.
We discuss two mistreatments of damped Lya (DLA) kinematic analysis that were first performed by Haehnelt, Steinmetz, & Rauch (1998; hereafter HSR98) and have recently been repeated by Hong et al. (2010; arXiv:1008.4242v1, arXiv:1008.4242v2; hereafte
r H10). Each mistreatment led to the improper excising of simulated absorption profiles. Specifically, their analyses are strictly biased against DLA sightlines that have low HI column density log NHI < 20.5, very high NHI values, and (for all NHI) sightlines with low velocity width Dv (<30 km/s for HSR98; <[20-30] km/s for H10). None of these biases exist in the observational analysis. We suspect these mistreatments compromise the results that followed. Hopefully this posting will prevent their repetition in the future.
With the Blue Channel Spectrograph (BCS) on the MMT telescope, we have obtained spectra to the atmospheric cutoff of quasars previously known to show at least one absorption system at z>1.6 with very strong metal lines (candidate metal-strong damped
Lya systems; cMSDLAs). The BCS/MMT spectra yield precise estimates of the HI column densities (NHI) of the systems through Voigt profile analysis of their Lya transitions. Nearly all of the cMSDLAs (41/43) satisfy the NHI criterion of DLAs, 10^20.3. As a population, these systems have systematically higher NHI values than DLAs chosen randomly from quasar sightlines. Combining our NHI measurements with previously measured metal column densities, we estimate metallicities for the MSDLAs. These systems have significantly higher values than randomly selected DLAs; at z~2, the MSDLAs show a median metallicity [M/H] ~ -0.67 that is 0.6dex higher than a corresponding control sample. This establishes MSDLAs as having amongst the most metal-rich gas in the high z universe. Our measurements extend the observed correlation between SiII 1526 equivalent width and the gas metallicity to higher values. If interpreted as a mass-metallicity relation, this implies the MSDLAs are the high mass subset of the DLA population. We demonstrate that dust in the MSDLAs reddens their background quasars, with a median shift in the spectral slope of Da = 0.29. Assuming an SMC extinction law, this implies a median reddening E(B-V)=0.025mag and visual extinction A_V=0.076mag. Future studies of MSDLAs offer the opportunity to study the extinction, nucleosynthesis, and kinematics of the most chemically evolved, gas-rich galaxies at high z. [abridged]
We report on strong H2 and CO absorption from gas within the host galaxy of gamma-ray burst (GRB) 080607. Analysis of our Keck/LRIS afterglow spectrum reveals a very large HI column density (NHI = 10^22.70 cm^-2) and strong metal-line absorption at z
_GRB = 3.0363 with a roughly solar metallicity. We detect a series of A-X bandheads from CO and estimate N(CO) = 10^16.5 cm^-2 and T_ex^CO > 100K. We argue that the high excitation temperature results from UV pumping of the CO gas by the GRB afterglow. Similarly, we observe H2 absorption via the Lyman-Werner bands and estimate N(H2) = 10^21.2 cm^-2 with T_ex^H2 = 10--300K. The afterglow photometry suggests an extinction law with R_V=4 and A_V=3.2 mag and requires the presence of a modest 2175A bump. Additionally, modeling of the Swift/XRT X-ray spectrum confirms a large column density with N(H) = 10^22.58 cm^-2. Remarkably, this molecular gas has extinction properties, metallicity, and a CO/H2 ratio comparable to those of translucent molecular clouds of the Milky Way, suggesting that star formation at high z proceeds in similar environments as today. However, the integrated dust-to-metals ratio is sub-Galactic, suggesting the dust is primarily associated with the molecular phase while the atomic gas has a much lower dust-to-gas ratio. Sightlines like GRB 080607 serve as powerful probes of nucleosynthesis and star-forming regions in the young universe and contribute to the population of dark GRB afterglows.
We present new results on the frequency distribution of projected HI column densities f(N,X), total comoving covering fraction, and integrated mass densities rho_HI of high redshift, HI `disks from a survey of damped Lya systems (DLAs) in the Sloan D
igital Sky Survey, Data Release 5. For the full sample spanning z=2.2 to 5 [738 DLAs], f(N,X) is well fitted by a double power-law with a break column density N_d = 10^(21.55 +/- 0.04) and low/high-end exponents alpha = -2.00 +/- 0.05, -6.4^{+1.1}_{-1.6}. The shape of f(N,X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day HI disks as inferred from 21cm observations. We conclude that HI gas has been distributed in a self-similar fashion for the past 12Gyr. The normalization of f(N,X), in contrast, decreases by a factor of two during the ~2Gyr interval from z=4 to 2.2 giving corresponding decreases in both the total covering fraction and rho_HI. At z~2, these quantities match the present-day values suggesting no evolution during the past ~10Gyr. We argue that the evolution at early times is driven by `violent processes that removes gas from nearly half the galaxies at z~3 establishing the antecedants of current early-type galaxies. The perceived constancy of rho_HI, meanwhile, implies that HI gas is a necessary but insufficient pre-condition for star formation and that the global star-formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.
