Keck Baryonic Structure Survey Research Articles

Resolving a merger in a hyperluminous submillimeter galaxy at z = 2.82

ABSTRACT We present the resolved properties of the z = 2.82 hyperluminous infrared Galaxy (HyLIRG) HS1700.850.1, the brightest 850 $\mu$m source found in the SCUBA-2 followup to the Keck Baryonic Structure Survey fields (S850$\mu$m =19.5 mJy), and amongst the most luminous starbursts known at any redshift.- Using the IRAM-NOEMA interferometer in the highest resolution A-configuration, we resolve the source into two components separated by ∼8 kpc, visible as blue shifted and red shifted 12CO(5–4) lines, exhibiting the expected kinematic properties of a major merger between two gas rich Galaxies. The combined merger system is traced over 2.3 arcsec or 18.4 kpc. Each component of the merger shows ordered gas motions suggestive of a massive turbulent disc. We measure the dynamical masses of the blue and red discs as (1.5 ± 0.2) × 1011 and (0.71 ± 0.22) × 1011 M⊙, respectively. The more massive disc component shows broad wings in the CO line, offset by ∼3 kpc from the disc centroid along the major axis, and extending to velocities ∼±1000 km $\rm {s^{-1}}$ from systemic velocity. We interpret this as either a possible bipolar outflowing component, or more likely a warping or tidal structure in the CO disc. Comparing the properties of HS1700.850.1 to other submillimeter detected galaxies with comparably bright 850 $\mu$m luminosities suggests that ongoing gas-rich mergers, or at least a clustered/group environment lead to these most extreme starburst phases.

The Nebular Properties of Star-forming Galaxies at Intermediate Redshift from the Large Early Galaxy Astrophysics Census

We present a detailed study of the partial rest-optical (λ obs ≈ 3600–5600 Å) spectra of N = 767 star-forming galaxies at 0.6 < z < 1.0 from the Large Early Galaxy Astrophysics Census (LEGA-C). We compare this sample with low-redshift (z ∼ 0) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift (z ∼ 1.6) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS Survey, and high-redshift (z ∼ 2) galaxies from the Keck Baryonic Structure Survey (KBSS). At a look-back time of 6–8 Gyr, galaxies with stellar masses appear remarkably similar to z ∼ 0 galaxies in terms of their nebular excitation, as measured using [O iii] λ5008/Hβ. There is some evidence that 0.6 < z < 1.0 galaxies with lower M * have higher [O iii] λ5008/Hβ than z ∼ 0 galaxies and are more similar to less evolved z ∼ 1.6 and z ∼ 2 galaxies, which are offset from the z ∼ 0 locus at all M *. We explore the impact of selection effects, contributions from active galactic nuclei, and variations in physical conditions (ionization parameter and gas-phase oxygen abundance) on the apparent distribution of [O iii] λ5008/Hβ and find somewhat higher ionization in 0.6 < z < 1.0 galaxies with lower M * relative to z ∼ 0 galaxies. We use new near-infrared spectroscopic observations of a subsample of LEGA-C galaxies to investigate other probes of enrichment and excitation. Our analysis demonstrates the importance of obtaining complete rest-optical spectra of galaxies in order to disentangle these effects.

Chemical Abundance Scaling Relations for Multiple Elements in z ≃ 2–3 Star-forming Galaxies

Abstract The chemical abundance patterns of gas and stars in galaxies are powerful probes of galaxies’ star formation histories and the astrophysics of galaxy assembly but are challenging to measure with confidence in distant galaxies. In this paper, we report the first measurements of the correlation between stellar mass (M *) and multiple tracers of chemical enrichment (including O, N, and Fe) in individual z ∼ 2–3 galaxies, using a sample of 195 star-forming galaxies from the Keck Baryonic Structure Survey. The galaxies’ chemical abundances are inferred using photoionization models capable of reconciling high-redshift galaxies’ observed extreme rest-UV and rest-optical spectroscopic properties. We find that the M *–O/H relation for our sample is relatively shallow, with moderately large scatter, and is offset ∼0.35 dex higher than the corresponding M *–Fe/H relation. The two relations have very similar slopes, indicating a high level of α-enhancement—O/Fe ≈ 2.2 × (O/Fe)⊙—across two decades in M *. The M *–N/H relation has the steepest slope and largest intrinsic scatter, which likely results from the fact that many z ∼ 2 galaxies are observed near or past the transition from “primary” to “secondary” N production, and may reflect uncertainties in the astrophysical origin of N. Together, these results suggest that z ∼ 2 galaxies are old enough to have seen substantial enrichment from intermediate-mass stars, but are still young enough that Type Ia supernovae have not had time to contribute significantly to their enrichment.

The KBSS–KCWI survey: the connection between extended Ly α haloes and galaxy azimuthal angle at z ∼ 2–3

ABSTRACT We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $\rm Ly\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $\rm Ly\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $\rm Ly\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling.

The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2

ABSTRACT We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\,{\small I}}$$\rm Ly\,\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\,{\small I}}$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\,{\small I}}$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$^{-1}\,$ dominate the kinematics of circumgalactic $\rm{H\,{\small I}}$ out to Dtran ≃ 50 kpc, while $\rm{H\,{\small I}}$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $\rm{H\,{\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\,\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.

Column Density, Kinematics, and Thermal State of Metal-bearing Gas within the Virial Radius of z ∼ 2 Star-forming Galaxies in the Keck Baryonic Structure Survey

Abstract We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the virial radius (35–100 physical kpc) of eight ∼L* galaxies at z ∼ 2. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad O vi and C iv absorbers are detected at velocities similar to the lower-ionization gas but with a very different kinematic structure, indicating that the circumgalactic medium (CGM) is multiphase. There is a high covering fraction of metal-bearing gas within 100 kpc, including highly ionized gas such as O vi; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (&lt;400 pc for all but the O vi-bearing gas). The mass in metals found within the halo is substantial, equivalent to ≳25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at z ∼ 2. When modeled assuming that ions with different ionization potentials can originate within a single gaseous structure, significant thermal broadening is detected in CGM absorbers that dominates the internal energy of the gas. Some 40% of the detected gas has temperatures in the range 104.5–5.5 K where cooling times are short, suggesting the CGM is dynamic, with constant heating or cooling to produce this short-lived thermal phase.

Dust Attenuation, Star Formation, and Metallicity in z ∼ 2–3 Galaxies from KBSS-MOSFIRE

Abstract We present a detailed analysis of 317 2.0 ≤ z ≤ 2.7 star-forming galaxies from the Keck Baryonic Structure Survey. Using complementary spectroscopic observations with Keck/LRIS and Keck/MOSFIRE, as well as spectral energy distribution (SED) fits to broadband photometry, we examine the joint rest-UV and rest-optical properties of the same galaxies, including stellar and nebular dust attenuation, metallicity, and star formation rate (SFR). The inferred parameters of the stellar population (reddening, age, SFR, and stellar mass) are strongly dependent on the details of the assumed stellar population model and the shape of the attenuation curve. Nebular reddening is generally larger than continuum reddening, but with large scatter. Compared to local galaxies, high-redshift galaxies have lower gas-phase metallicities (and/or higher nebular excitation) at fixed nebular reddening, and higher nebular reddening at fixed stellar mass, consistent with gas fractions that increase with redshift. We find that continuum reddening is correlated with 12 + log(O/H)O3N2 at 3.0σ significance, whereas nebular reddening is correlated with only 1.1σ significance. This may reflect the dependence of both continuum reddening and O3N2 on the shape of the ionizing radiation field produced by the massive stars. Finally, we show that Hα-based and SED-based estimates of SFR exhibit significant scatter relative to one another, and on average agree only for particular combinations of spectral synthesis models and attenuation curves. We find that the SMC extinction curve predicts consistent SFRs if we assume the subsolar (0.14 Z ⊙) binary star models that are favored for high-redshift galaxies.

Measuring the Physical Conditions in High-redshift Star-forming Galaxies: Insights from KBSS-MOSFIRE

Abstract We use photoionization models that are designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies to self-consistently infer the gas chemistry and nebular ionization and excitation conditions for ∼150 galaxies from the Keck Baryonic Structure Survey (KBSS), using only observations of their rest-optical nebular spectra. We find that the majority of z ∼ 2–3 KBSS galaxies are moderately O-rich, with an interquartile range in 12 + log(O/H) = 8.29–8.56, and have significantly sub-solar Fe enrichment, with an interquartile range of [Fe/H] = [−0.79, −0.53], which contributes additional evidence in favor of super-solar O/Fe in high-z galaxies. The model-inferred ionization parameters and N/O are strongly correlated with common strong-line indices (such as O32 and N2O2), with the latter exhibiting similar behavior to local extragalactic H ii regions. In contrast, diagnostics commonly used for measuring gas-phase O/H (such as N2 and O3N2) show relatively large scatter with the overall amount of oxygen present in the gas and behave differently than observed at z ∼ 0. We provide a new calibration for using R23 to measure O/H in typical high-z galaxies, although it is most useful for relatively O-rich galaxies; combining O32 and R23 does not yield a more effective calibration. Finally, we consider the implications for the intrinsic correlations between physical conditions across the galaxy sample and find that N/O varies with O/H in high-z galaxies in a manner that is almost identical to local H ii regions. However, we do not find a strong anti-correlation between ionization parameter and metallicity (O/H or Fe/H) in high-z galaxies, which is one of the principal bases for using strong-line ratios to infer oxygen abundance.

A comparison of observed and simulated absorption from H i, C iv, and Si ivaroundz ≈ 2 star-forming galaxies suggests redshift–space distortions are due to inflows

AbstractWe study H i and metal-line absorption around z ≈ 2 star-forming galaxies by comparing an analysis of data from the Keck Baryonic Structure Survey to mock spectra generated from the Evolution and Assembly of Galaxies and their Environments (EAGLE) cosmological, hydrodynamical simulations. We extract sightlines from the simulations and compare the properties of the absorption by H i, C iv, and Si iv around simulated and observed galaxies using pixel optical depths. We mimic the resolution, pixel size, and signal-to-noise ratio of the observations, as well as the distributions of impact parameters and galaxy redshift errors. We find that the EAGLE reference model is in excellent agreement with the observations. In particular, the simulation reproduces the high metal-line optical depths found at small galactocentric distances, the optical depth enhancements out to impact parameters of 2 proper Mpc, and the prominent redshift–space distortions which we find are due to peculiar velocities rather than redshift errors. The agreement is best for halo masses ∼1012.0 M⊙, for which the observed and simulated stellar masses also agree most closely. We examine the median ion mass-weighted radial gas velocities around the galaxies, and find that most of the gas is infalling, with the infall velocity depending on halo rather than stellar mass. From this, we conclude that the observed redshift–space distortions are predominantly caused by infall rather than outflows.

Nebular Emission Line Ratios in z ≃ 2–3 Star-forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio∗

Abstract We present a detailed study of the rest-optical (3600–7000 Å) nebular spectra of ∼380 star-forming galaxies at , obtained with Keck/Multi-object Spectrometer for Infrared Exploration (MOSFIRE) as part of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample is representative of star-forming galaxies at these redshifts, with stellar masses and star formation rates SFR = 3–1000 yr−1. We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O ii]λλ3727, 3729, [Ne iii]λ3869, Hβ, [O iii] 4960, 5008, [N ii]λλ 6549, 6585, Hα, and [S ii]λλ6718, 6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey and between subsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number of significant differences in addition to the well-known offset in log([O iii] /Hβ) and log([N ii] /Hα). We argue that the primary difference between H ii regions in galaxies and those at is an enhancement in the degree of nebular excitation, as measured by [O iii]/Hβ and [([O iii] +[O ii] )/Hβ]. At the same time, KBSS-MOSFIRE galaxies are ∼10 times more massive than galaxies with similar ionizing spectra and have higher N/O (likely accompanied by higher O/H) at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed N/O and O/H relative to typical galaxies, consistent with Fe-poor stellar population models that include massive binaries, and highlight a population of massive, high-specific star formation rate galaxies at high redshift with systematically different star formation histories than galaxies of similar stellar mass today.

Detection of hot, metal-enriched outflowing gas around z ≈ 2.3 star-forming galaxies in the Keck Baryonic Structure Survey

We use quasar absorption lines to study the physical conditions in the circumgalactic medium of redshift $z\approx 2.3$ star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). In Turner et al. 2014 we used the pixel optical depth technique to show that absorption by HI and the metal ions OVI, NV, CIV, CIII and SiIV is strongly enhanced within $|\Delta v|\lesssim170$ km/s and projected distances $|d|\lesssim180$ proper kpc from sightlines to the background quasars. Here we demonstrate that the OVI absorption is also strongly enhanced at fixed HI, CIV, and SiIV optical depths, and that this enhancement extends out to $\sim350$ km/s. At fixed HI the increase in the median OVI optical depth near galaxies is 0.3-0.7 dex and is detected at 2--3-$\sigma$ confidence for all seven HI bins that have $\log_{10}\tau_{\rm HI}\ge-1.5$. We use ionization models to show that the observed strength of OVI as a function of HI is consistent with enriched, photoionized gas for pixels with $\tau_{\rm HI}\gtrsim10$. However, for pixels with $\tau_{\rm HI} \lesssim 1$ this would lead to implausibly high metallicities at low densities if the gas were photoionized by the background radiation. This indicates that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionized ($T > 10^5\,$K) and that a substantial fraction of the hot gas has a metallicity $\gtrsim 10^{-1}$ of solar. Given the high metallicity and large velocity extent (out to $\sim1.5\times v_{\rm circ}$) of this gas, we conclude that we have detected hot, metal enriched outflows arising from star-forming galaxies.

STRONG NEBULAR LINE RATIOS IN THE SPECTRA ofz∼ 2-3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE

We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z < 2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 < log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight, relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared to local galaxies. Using photoionization models, we argue that the offset of the z~2.3 locus relative to z~ 0 is explained by a combination of harder ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H than applies to most local galaxies, and that the position of a galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The observed nebular emission line ratios are most easily reproduced by models in which the net ionizing radiation field resembles a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to the solar value at all O/H. We critically assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities, and consider the implications of the small intrinsic scatter in the empirical relationship between excitation-sensitive line indices and stellar mass (i.e., the "mass-metallicity" relation), at z~2.3.

Metal-line absorption around z ≈ 2.4 star-forming galaxies in the Keck Baryonic Structure Survey★

We study metal absorption around 854 z ≈ 2.4 star-forming galaxies taken from the Keck Baryonic Structure Survey. The galaxies examined in this work lie in the fields of 15 hyperluminous background quasi-stellar objects, with galaxy impact parameters ranging from 35 proper kpc (pkpc) to 2 proper Mpc (pMpc). Using the pixel optical depth technique, we present the first galaxy-centred 2D maps of the median absorption by O vi, N v, C iv, C iii, and Si iv, as well as updated results for H i. At small galactocentric radii we detect a strong enhancement of the absorption relative to randomly located regions that extend out to at least 180 pkpc in the transverse direction, and ±240 km s−1 along the line of sight (LOS, ∼1 pMpc in the case of pure Hubble flow) for all ions except N v. For C iv (and H i) we detect a significant enhancement of the absorption signal out to 2 pMpc in the transverse direction, corresponding to the maximum impact parameter in our sample. After normalizing the median absorption profiles to account for variations in line strengths and detection limits, in the transverse direction we find no evidence for a sharp drop-off in metals distinct from that of H i. We argue instead that non-detection of some metal-line species in the extended circumgalactic medium is consistent with differences in the detection sensitivity. Along the LOS, the normalized profiles reveal that the enhancement in the absorption is more extended for O vi, C iv, and Si iv than for H i. We also present measurements of the scatter in the pixel optical depths, covering fractions, and equivalent widths as a function of projected galaxy distance. Limiting the sample to the 340 galaxies with redshifts measured from nebular emission lines does not decrease the extent of the enhancement along the LOS compared to that in the transverse direction. This rules out redshift errors as the source of the observed redshift-space anisotropy and thus implies that we have detected the signature of gas peculiar velocities from infall, outflows, or virial motions for H i, O vi, C iv, C iii, and Si iv.

AHST/WFC3-IR MORPHOLOGICAL SURVEY OF GALAXIES ATz= 1.5-3.6. II. THE RELATION BETWEEN MORPHOLOGY AND GAS-PHASE KINEMATICS

We analyze rest-frame optical morphologies and gas-phase kinematics as traced by rest-frame far-UV and optical spectra for a sample of 204 star forming galaxies in the redshift range z ~ 2-3 drawn from the Keck Baryonic Structure Survey (KBSS). We find that spectroscopic properties and gas-phase kinematics are closely linked to morphology: compact galaxies with semi-major axis radii r <~ 2 kpc are substantially more likely than their larger counterparts to exhibit LyA in emission. Although LyA emission strength varies widely within galaxies of a given morphological type, all but one of 19 galaxies with LyA equivalent width W_LyA > 20 Angstroms have compact and/or multiple-component morphologies with r <= 2.5 kpc. The velocity structure of absorption lines in the galactic continuum spectra also varies as a function of morphology. Galaxies of all morphological types drive similarly strong outflows (as traced by the blue wing of interstellar absorption line features), but the outflows of larger galaxies are less highly ionized and exhibit larger optical depth at the systemic redshift that may correspond to a decreasing efficiency of feedback in evacuating gas from the galaxy. This v ~ 0 km/s gas is responsible both for shifting the mean absorption line redshift and attenuating W_LyA (via a longer resonant scattering path) in galaxies with larger rest-optical half light radii. In contrast to galaxies at lower redshifts, there is no evidence for a correlation between outflow velocity and inclination, suggesting that outflows from these puffy and irregular systems may be poorly collimated. (Abbrev.)

THE HALO MASSES AND GALAXY ENVIRONMENTS OF HYPERLUMINOUS QSOs ATz≃ 2.7 IN THE KECK BARYONIC STRUCTURE SURVEY

We present an analysis of the galaxy distribution surrounding 15 of the most luminous (>10^{14} L_sun; M_1450 ~ -30) QSOs in the sky with z~2.7. Our data are drawn from the Keck Baryonic Structure Survey (KBSS). In this work, we use the positions and spectroscopic redshifts of 1558 galaxies that lie within ~3', (4.2 h^{-1} comoving Mpc; cMpc) of the hyperluminous QSO (HLQSO) sightline in one of 15 independent survey fields, together with new measurements of the HLQSO systemic redshifts. We measure the galaxy-HLQSO cross-correlation function, the galaxy-galaxy autocorrelation function, and the characteristic scale of galaxy overdensities surrounding the sites of exceedingly rare, extremely rapid, black hole accretion. On average, the HLQSOs lie within significant galaxy overdensities, characterized by a velocity dispersion sigma_v ~ 200 km s^{-1} and a transverse angular scale of ~25", (~200 physical kpc). We argue that such scales are expected for small groups with log(M_h/M_sun)~13. The galaxy-HLQSO cross-correlation function has a best-fit correlation length r_0_GQ = (7.3 \pm 1.3) h^{-1} cMpc, while the galaxy autocorrelation measured from the spectroscopic galaxy sample in the same fields has r_0_GG = (6.0 \pm 0.5) h^{-1} cMpc. Based on a comparison with simulations evaluated at z ~ 2.6, these values imply that a typical galaxy lives in a host halo with log(M_h/M_sun) = 11.9\pm0.1, while HLQSOs inhabit host halos of log(M_h/M_sun) = 12.3\pm0.5. In spite of the extremely large black hole masses implied by their observed luminosities [log(M_BH/M_sun) > 9.7], it appears that HLQSOs do not require environments very different from their much less luminous QSO counterparts. Evidently, the exceedingly low space density of HLQSOs (< 10^{-9} cMpc^{-3}) results from a one-in-a-million event on scales << 1 Mpc, and not from being hosted by rare dark matter halos.

THE GASEOUS ENVIRONMENT OF HIGH-zGALAXIES: PRECISION MEASUREMENTS OF NEUTRAL HYDROGEN IN THE CIRCUMGALACTIC MEDIUM OFz∼ 2-3 GALAXIES IN THE KECK BARYONIC STRUCTURE SURVEY

We present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey of the distant universe designed to explore the details of the connection between galaxies and intergalactic baryons within the same survey volumes, focusing particularly on scales from ∼50 kpc to a few Mpc. The KBSS is optimized for the redshift range z ∼ 2–3, combining S/N ∼100 Keck/HIRES spectra of 15 of the brightest QSOs in the sky at z ≃ 2.5–2.9 with very densely sampled galaxy redshift surveys within a few arcmin of each QSO sightline. In this paper, we present quantitative results on the distribution, column density, kinematics, and absorber line widths of neutral hydrogen (H i) surrounding a subset of 886 KBSS star-forming galaxies with 2.0 ≲ z ≲ 2.8 and with projected distances ⩽3 physical Mpc from a QSO sightline. Using Voigt profile decompositions of the full Lyα forest region of all 15 QSO spectra, we compiled a catalog of ∼6000 individual absorbers in the redshift range of interest, with 12 ⩽ log (NH i) ⩽21. These are used to measure H i absorption statistics near the redshifts of foreground galaxies as a function of projected galactocentric distance from the QSO sightline and for randomly chosen locations in the intergalactic medium (IGM) within the survey volume. We find that NH i and the multiplicity of velocity-associated H i components increase rapidly with decreasing galactocentric impact parameter and as the systemic redshift of the galaxy is approached. The strongest H i absorbers within ≃ 100 physical kpc of galaxies have NH i ∼3 orders of magnitude higher than those near random locations in the IGM. The circumgalactic zone of most significantly enhanced H i absorption is found within transverse distances of ≲ 300 kpc and within ±300 km s−1 of galaxy systemic redshifts. Taking this region as the defining bounds of the circumgalactic medium (CGM), nearly half of absorbers with log(NH i) > 15.5 are found within the CGM of galaxies meeting our photometric selection criteria, while their CGM occupy only 1.5% of the cosmic volume. The spatial covering fraction, multiplicity of absorption components, and characteristic NH i remain significantly elevated to transverse distances of ∼2 physical Mpc from galaxies in our sample. Absorbers with NH i >1014.5 cm−2 are tightly correlated with the positions of galaxies, while absorbers with lower NH i are correlated with galaxy positions only on ≳Mpc scales. Redshift anisotropies on these larger scales indicate coherent infall toward galaxy locations, while on scales of ∼100 physical kpc peculiar velocities of Δv ≃ ±260 km s−1 with respect to the galaxies are indicated. The median Doppler widths of individual absorbers within 1–3 rvir of galaxies are larger by ≃ 50% than randomly chosen absorbers of the same NH i, suggesting higher gas temperatures and/or increased turbulence likely caused by some combination of accretion shocks and galactic winds around galaxies with Mhalo ≃ 1012 M☉ at z ∼ 2–3.