The halo masses and galaxy environments of hyperluminous QSOs at z~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.Comment: 15 pages, 6 figures. Accepted for publication in Ap

Wallace L. W. Sargent (1935–2012)

By any measure, Professor Wallace L. W. Sargent (known to colleagues and friends as “Wal”) was one of the most influential and consistently productive astronomers of the last 50 years; he authored or coauthored more than 320 refereed journal articles, which have received more than 25,000 citations. He never rested on past laurels—65 of his papers (with >5000 citations) have appeared since the year 2000. Wal remained active as a teacher and researcher until just weeks before the end of his life (he officially retired from the Caltech faculty on 2012 October 1). His impact on the field of astrophysics was remarkably broad as well as seminal; among many other awards, he was elected in 1981 as a Fellow of the Royal Society (U.K.) and as a member of the National Academy of Sciences in 2005, his first year of eligibility after becoming a naturalized U.S. citizen

Galaxies at high redshift: progress and prospects

There has been considerable progress made in the discovery, observation, and understanding of high redshift galaxies in the last few years; most of this progress is attributable to greatly improved spectroscopy throughput made possible by state-of-the-art instruments on the new generation of 8-10m telescopes. Here we review a few of the areas in which substantial progress has been made, and discuss the future of high redshift galaxy work in the context of the observational facilities that are either in operation or soon to come

Filamentary Large-scale Structure Traced by Six Lyα Blobs at z = 2.3

Extended nebulae of Lyα emission ("Lyα blobs") are known to be associated with overdense regions at high redshift. Here we present six large Lyα blobs in a previously known protocluster with galaxy overdensity δ ~ 7 at z = 2.3; this is the richest field of giant Lyα blobs detected to date. The blobs have linear sizes of ≳ 100 kpc and Lyα luminosities of ~10^(43) erg s^(–1). The positions of the blobs define two linear filaments with an extent of at least 12 comoving Mpc; these filaments intersect at the center of one of the blobs. Measurement of the position angles of the blobs indicates that five of the six are aligned with these filaments to within ~10°, suggesting a connection between the physical processes powering extended Lyα emission and those driving structure on larger scales

Substructure within the SSA22 protocluster at z≈3.09z\approx3.09

We present the results of a densely sampled spectroscopic survey of the SSA22 protocluster at $z\approx 3.09$. Our sample with Keck/LRIS spectroscopy includes 106 Ly$\alpha$ Emitters (LAEs) and 40 Lyman Break Galaxies (LBGs) at $z=3.05-3.12$. These galaxies are contained within the $9'\times9'$ region in which the protocluster was discovered, which also hosts the maximum galaxy overdensity in the SSA22 region. The redshift histogram of our spectroscopic sample reveals two distinct peaks, at $z=3.069$ (blue, 43 galaxies) and $z=3.095$ (red, 103 galaxies). Furthermore, objects in the blue and red peaks are segregated on the sky, with galaxies in the blue peak concentrating towards the western half of the field. These results suggest that the blue and red redshift peaks represent two distinct structures in physical space. Although the double-peaked redshift histogram is traced in the same manner by LBGs and LAEs, and brighter and fainter galaxies, we find that nine out of 10 X-ray AGNs in SSA22, and all seven spectroscopically-confirmed giant Ly$\alpha$ "blobs," reside in the red peak. We combine our dataset with sparsely sampled spectroscopy from the literature over a significantly wider area, finding preliminary evidence that the double-peaked structure in redshift space extends beyond the region of our dense spectroscopic sampling. In order to fully characterize the three-dimensional structure, dynamics, and evolution of large-scale structure in the SSA22 overdensity, we require the measurement of large samples of LAE and LBG redshifts over a significantly wider area, as well as detailed comparisons with cosmological simulations of massive cluster formation.Comment: 6 pages, 4 figures, Accepted to ApJ Letter

Calibrating Galaxy Redshifts Using Absorption by the Surrounding Intergalactic Medium

Rest-frame UV spectral lines of star-forming galaxies are systematically offset from the galaxies' systemic redshifts, probably because of large-scale outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by utilizing the fact that the mean HI Ly-alpha absorption profiles around the galaxies, as seen in spectra of background objects, must be symmetric with respect to the true galaxy redshifts if the galaxies are oriented randomly with respect to the lines of sight to the background objects. We use 15 QSOs at z~2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at z~1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the background QSOs. We find that LyA emission and ISM absorption redshifts require systematic shifts of v_LyA=-295(+35)(-35) km/s and v_ISM=145(+70)(-35) km/s. Assuming a Gaussian distribution, we put 1-sigma upper limits on possible random redshift offsets of <220 km/s for LyA and <420 km/s for ISM redshifts. For the small subset (<10%) of galaxies for which near-IR spectra have been obtained, we can compare our results to direct measurements based on nebular emission lines which we confirm to mark the systemic redshifts. While our v_ISM agrees with the direct measurements, our v_LyA is significantly smaller. However, when we apply our method to the near-IR subsample which is characterized by slightly different selection effects, the best-fit velocity offset comes into agreement with the direct measurement. This confirms the validity of our approach, and implies that no single number appropriately describes the whole population of galaxies, in line with the observation that the line offset depends on galaxy spectral morphology. This method provides accurate redshift calibrations and will enable studies of circumgalactic matter around galaxies for which rest-frame optical observations are not available.Comment: 7 pages, 3 figures, accepted for publication in MNRA

Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve at z~3

We present the first measurements of the shape of the far-ultraviolet (far-UV; lambda=950-1500 A) dust attenuation curve at high redshift (z~3). Our analysis employs rest-frame UV spectra of 933 galaxies at z~3, 121 of which have very deep spectroscopic observations (>7 hrs) at lambda=850-1300 A, with the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an iterative approach in which we calculate the ratios of composite spectra in different bins of continuum color excess, E(B-V), we derive a dust curve that implies a lower attenuation in the far-UV for a given E(B-V) than those obtained with standard attenuation curves. We demonstrate that the UV composite spectra of z~3 galaxies can be modeled well by assuming our new attenuation curve, a high covering fraction of HI, and absorption from the Lyman-Werner bands of H2 with a small (<20%) covering fraction. The low covering fraction of H2 relative to that of the HI and dust suggests that most of the dust in the ISM of typical galaxies at z~3 is unrelated to the catalysis of H2, and is associated with other phases of the ISM (i.e., the ionized and neutral gas). The far-UV dust curve implies a factor of ~2 lower dust attenuation of Lyman continuum (ionizing) photons relative to those inferred from the most commonly assumed attenuation curves for L* galaxies at z~3. Our results may be utilized to assess the degree to which ionizing photons are attenuated in HII regions or, more generally, in the ionized or low column density (N(HI)<10^17.2 cm^-2) neutral ISM of high-redshift galaxies.Comment: 12 pages, 1 table, 8 figures, accepted to the Astrophysical Journa

Physical Properties of a Pilot Sample of Spectroscopic Close Pair Galaxies at z ~ 2

We use Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) rest-frame optical imaging to select a pilot sample of star-forming galaxies in the redshift range z = 2.00-2.65 whose multi-component morphologies are consistent with expectations for major mergers. We follow up this sample of major merger candidates with Keck/NIRSPEC longslit spectroscopy obtained in excellent seeing conditions (FWHM ~ 0.5 arcsec) to obtain Halpha-based redshifts of each of the morphological components in order to distinguish spectroscopic pairs from false pairs created by projection along the line of sight. Of six pair candidates observed, companions (estimated mass ratios 5:1 and 7:1) are detected for two galaxies down to a 3sigma limiting emission-line flux of ~ 10^{-17} erg/s/cm2. This detection rate is consistent with a ~ 50% false pair fraction at such angular separations (1-2 arcsec), and with recent claims that the star-formation rate (SFR) can differ by an order of magnitude between the components in such mergers. The two spectroscopic pairs identified have total SFR, SFR surface densities, and stellar masses consistent on average with the overall z ~ 2 star forming galaxy population.Comment: 11 pages, 5 figures. Accepted for publication in Ap

Halo gas cross sections and covering fractions of MgII absorption selected galaxies

We examine halo gas cross sections and covering fractions, fc, of intermediate-redshift Mg II absorption selected galaxies. We computed statistical absorber halo radii, Rx, using current values of dN/dz and Schechter luminosity function parameters, and have compared these values to the distribution of impact parameters and luminosities from a sample of 37 galaxies. For equivalent widths Wr(2796) ≥ 0.3 Å, we find 43 ≤ Rx ≤ 88 kpc, depending on the lower luminosity cutoff and the slope, β, of the Holmberg-like luminosity scaling, R ∝ α L^β . The observed distribution of impact parameters, D, are such that several absorbing galaxies lie at D > Rx and several non-absorbing galaxies lie at D ~ 0.5 for our sample. Moreover, the data suggest that halo radii of Mg II absorbing galaxies do not follow a luminosity scaling with β in the range of 0.2–0.28, if fc = 1 as previously reported. However, provided fc ~ 0.5, we find that halo radii can remain consistent with a Holmberg-like luminosity relation with β ≃ 0.2 and R∗ = Rx/√(fc) ~ 110 kpc. No luminosity scaling (β = 0) is also consistent with the observed distribution of impact parameters if fc ≤ 0.37. The data support a scenario in which gaseous halos are patchy and likely have non-symmetric geometric distributions about the galaxies. We suggest that halo gas distributions may not be governed primarily by galaxy mass/luminosity but also by stochastic processes local to the galaxy

The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift

We use a large sample of galaxies at z~3 to establish a relationship between reddening, neutral gas covering fraction (fcov(HI)), and the escape of ionizing photons at high redshift. Our sample includes 933 galaxies at z~3, 121 of which have very deep spectroscopic observations (>7 hrs) in the rest-UV (lambda=850-1300 A) with Keck/LRIS. Based on the high covering fraction of outflowing optically-thick HI indicated by the composite spectra of these galaxies, we conclude that photoelectric absorption, rather than dust attenuation, dominates the depletion of ionizing photons. By modeling the composite spectra as the combination of an unattenuated stellar spectrum including nebular continuum emission with one that is absorbed by HI and reddened by a line-of-sight extinction, we derive an empirical relationship between E(B-V) and fcov(HI). Galaxies with redder UV continua have larger covering fractions of HI characterized by higher line-of-sight extinctions. Our results are consistent with the escape of Lya through gas-free lines-of-sight. Covering fractions based on low-ionization interstellar absorption lines systematically underpredict those deduced from the HI lines, suggesting that much of the outflowing gas may be metal-poor. We develop a model which connects the ionizing escape fraction with E(B-V), and which may be used to estimate the escape fraction for an ensemble of high-redshift galaxies. Alternatively, direct measurements of the escape fraction for our data allow us to constrain the intrinsic 900-to-1500 A flux density ratio to be >0.20, a value that favors stellar population models that include weaker stellar winds, a flatter initial mass function, and/or binary evolution. Lastly, we demonstrate how the framework discussed here may be used to assess the pathways by which ionizing radiation escapes from high-redshift galaxies. [Abridged]Comment: 22 pages, 3 tables, 14 figures, accepted to the Astrophysical Journa