Milan Bogosavljevic

THE GASEOUS ENVIRONMENT OF HIGH-z GALAXIES: PRECISION MEASUREMENTS OF NEUTRAL HYDROGEN IN THE CIRCUMGALACTIC MEDIUM OF z ∼ 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(N_(HI)) ≤ 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 intergalacticmedium (IGM) within the survey volume. We find that NHI 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 N_(HI) ∼ 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(N_(HI)) > 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 NHI remain significantly elevated to transverse distances of ∼2 physical Mpc from galaxies in our sample. Absorbers with N_(HI) > 10^(14.5) cm^(-2) are tightly correlated with the positions of galaxies, while absorbers with lower N_(HI) 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 r_(vir) of galaxies are larger by ≃ 50% than randomly chosen absorbers of the same NHI, suggesting higher gas temperatures and/or increased turbulence likely caused by some combination of accretion shocks and galactic winds around galaxies with M_(halo) ≃ 10^(12) M_⊙ at z ∼ 2-3.

The Relationship between Stellar Populations and Lyα Emission in Lyman Break Galaxies

We present the results of a photometric and spectroscopic survey of 321 Lyman break galaxies (LBGs) at z ~ 3 to investigate systematically the relationship between Lyα emission and stellar populations. Lyα equivalent widths (W_(Lyα)) were calculated from rest-frame UV spectroscopy and optical/near-infrared/Spitzer photometry was used in population synthesis modeling to derive the key properties of age, dust extinction, star formation rate (SFR), and stellar mass. We directly compare the stellar populations of LBGs with and without strong Lyα emission, where we designate the former group (W_(Lyα) ≥ 20 A) as Lyα emitters (LAEs) and the latter group (W_(Lyα) < 20 A) as non-LAEs. This controlled method of comparing objects from the same UV luminosity distribution represents an improvement over previous studies in which the stellar populations of LBGs and narrowband-selected LAEs were contrasted, where the latter were often intrinsically fainter in broadband filters by an order of magnitude simply due to different selection criteria. Using a variety of statistical tests, we find that Lyα equivalent width and age, SFR, and dust extinction, respectively, are significantly correlated in the sense that objects with strong Lyα emission also tend to be older, lower in SFR, and less dusty than objects with weak Lyα emission, or the line in absorption. We accordingly conclude that, within the LBG sample, objects with strong Lyα emission represent a later stage of galaxy evolution in which supernovae-induced outflows have reduced the dust covering fraction. We also examined the hypothesis that the attenuation of Lyα photons is lower than that of the continuum, as proposed by some, but found no evidence to support this picture.

The Lyα Properties of Faint Galaxies at z ~ 2-3 with Systemic Redshifts and Velocity Dispersions from Keck-MOSFIRE

We study the Lya profiles of 36 spectroscopically-detected Lya-emitters (LAEs) at z2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The sample has a median optical magnitude R=26.0, and ranges from R23 to R>27, corresponding to rest-frame UV absolute magnitudes M_UV-22 to M_UV>-18.2. Dynamical masses range from M_dyn 3 sigma significance: brighter galaxies with larger velocity dispersions tend to have larger values of dv_Lya. We also make use of a comparison sample of 122 UV-color-selected R<25.5 galaxies at z2, all with Lya emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that dv_Lya is anti-correlated with the Lya equivalent width with 7 sigma significance. Our results are consistent with a scenario in which the Lya profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lya photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.

The Faint End of the Quasar Luminosity Function at z~4

The evolution of the quasar luminosity function (QLF) is one of the basic cosmological measures providing insight into structure formation and mass assembly in the universe. We have conducted a spectroscopic survey to find faint quasars (–26.0 < M_(1450) < –22.0) at redshifts z = 3.8-5.2 in order to measure the faint end of the QLF at these early times. Using available optical imaging data from portions of the NOAO Deep Wide-Field Survey and the Deep Lens Survey, we have color-selected quasar candidates in a total area of 3.76 deg^2. Thirty candidates have R ≤ 23 mag. We conducted spectroscopic follow-up for 28 of our candidates and found 23 QSOs, 21 of which are reported here for the first time, in the 3.74 < z < 5.06 redshift range. We estimate our survey completeness through detailed Monte Carlo simulations and derive the first measurement of the density of quasars in this magnitude and redshift interval. We find that the binned luminosity function (LF) is somewhat affected by the K-correction used to compute the rest-frame absolute magnitude at 1450 A. Considering only our R ≤ 23 sample, the best-fit single power law (Φ ∝ L^β) gives a faint-end slope β = –1.6 ± 0.2. If we consider our larger, but highly incomplete sample going 1 mag fainter, we measure a steeper faint-end slope –2 < β < –2.5. In all cases, we consistently find faint-end slopes that are steeper than expected based on measurements at z ~ 3. We combine our sample with bright quasars from the Sloan Digital Sky Survey to derive parameters for a double-power-law LF. Our best fit finds a bright-end slope, α = –2.4 ± 0.2, and faint-end slope, β = –2.3 ± 0.2, without a well-constrained break luminosity. This is effectively a single power law, with β = –2.7 ± 0.1. We use these results to place limits on the amount of ultraviolet radiation produced by quasars and find that quasars are able to ionize the intergalactic medium at these redshifts.

Excess AGN activity in the z = 2.30 Protocluster in HS 1700+64

We present the results of spectroscopic, narrow-band and X-ray observations of a z= 2.30 protocluster in the field of the QSO HS 1700+643. Using a sample of BX/MD galaxies, which are selected to be at z ~ 2.2–2.7 by their rest-frame ultraviolet colours, we find that there are five protocluster AGN which have been identified by characteristic emission-lines in their optical/near-IR spectra; this represents an enhancement over the field significant at >98.5 per cent confidence. Using a ~200-ks Chandra/ACIS-I observation of this field we detect a total of 161 X-ray point sources to a Poissonian false-probability limit of 4 × 10^(−6) and identify eight of these with BX/MD galaxies. Two of these are spectroscopically confirmed protocluster members and are also classified as emission-line AGN. When compared to a similarly selected field sample, the analysis indicates this is also evidence for an enhancement of X-ray selected BX/MD AGN over the field, significant at >99 per cent confidence. Deep Lyα narrow-band imaging reveals that a total of 4/123 Lyα emitters (LAEs) are found to be associated with X-ray sources, with two of these confirmed protocluster members and one highly likely member. We do not find a significant enhancement of AGN activity in this LAE sample over that of the field (result is significant at only 87 per cent confidence). The X-ray emitting AGN fractions for the BX/MD and LAE samples are found to be 6.9^(+9.2)_(−4.4) and 2.9^(+2.9)_(−1.6) per cent, respectively, for protocluster AGN with L_(2-10 keV)≥ 4.6 × 10^(43) erg s^(−1) at z= 2.30. These findings are similar to results from the z= 3.09 protocluster in the SSA 22 field found by Lehmer et al. (2009), in that both suggest AGN activity is favoured in dense environments at z > 2.

Quasars as probes of late reionization and early structure formation

Observations of QSOs at z ~ 5.7–6.4 show the appearance of Gunn–Peterson troughs around z ~ 6, and a change in the slope of the IGM optical depth τ(z) near z ~ 5.5. These results are interpreted as a signature of the end of the reionization era, which probably started at considerably higher redshifts. However, there also appears to be a substantial cosmic variance in the transmission of the IGM, both along some lines of sight, and among different lines of sight, in this intriguing redshift regime. We suggest that this is indicative of a spatially uneven reionization, possibly caused by the bias-driven primordial clustering of the reionization sources. There is also some independent evidence for a strong clustering of QSOs at z ~ 4–5 and galaxies around them, supporting the idea of the strong biasing of the first luminous sources at these redshifts. Larger samples of high-z QSOs are needed in order to provide improved, statistically significant constraints for the models of these phenomena. We expect that the Palomar-Quest (PQ) survey will soon provide a new set of QSOs to be used as cosmological probes in this redshift regime.

Discovery of two spectroscopically peculiar, low-luminosity quasars at z ~ 4

We report the discovery of two low-luminosity quasars at z ~ 4, both of which show prominent N iv] λ1486 emission. This line is extremely rare in quasar spectra at any redshift; detecting it in two of a sample of 23 objects (i.e., ~9% of the sample) is intriguing and is likely due to the low-luminosity, high-redshift quasar sample we are studying. This is still a poorly explored regime, where contributions from associated, early starbursts may be significant. One interpretation of this line posits photoionization by very massive young stars. Seeing N iv] λ1486 emission in a high-redshift quasar may thus be understood in the context of coformation and early coevolution of galaxies and their supermassive black holes. Alternatively, we may be seeing a phenomenon related to the early evolution of quasar broad emission line regions. The nondetection (and possibly even broad absorption) of N v λ1240 line in the spectrum of one of these quasars may support that interpretation. These two objects may signal a new faint quasar population or an early AGN evolutionary stage at high redshifts.