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

The Spectroscopic Properties of Ly{\alpha}-Emitters at z ≈\approx 2.7: Escaping Gas and Photons from Faint Galaxies

We present a spectroscopic survey of 318 faint $(R\sim 27$, $L\sim0.1L_*)$, Ly{\alpha}-emission-selected galaxies (LAEs) at 2.5<z<3. A sample of 32 LAEs with rest-frame optical spectra from Keck/MOSFIRE are used to interpret the LAE spectra in the context of their systemic redshifts. We find that the Ly{\alpha} emission of LAEs is typically less spectrally extended than among samples of more luminous continuum-selected galaxies (LBGs) at similar redshifts. Using the MOSFIRE subsample, we find that the peak of the Ly{\alpha} line is shifted by +200 km/s with respect to systemic across a diverse set of galaxies including both LAEs and LBGs. We also find a small number of objects with significantly blueshifted Ly{\alpha} emission, a potential indicator of accreting gas. The Ly{\alpha}-to-H{\alpha} line ratios suggest that the LAEs have Ly{\alpha} escape fractions $f_{\rm esc,Ly{\alpha}} \approx 30$%, significantly higher than typical LBG samples. Using redshifts calibrated by our MOSFIRE sample, we construct composite LAE spectra, finding the first evidence for metal-enriched outflows in such intrinsically-faint high-redshift galaxies. These outflows have smaller continuum covering fractions $(f_c \approx 0.3)$ and velocities $(v_{\rm ave} \approx 100-200$ km/s, $v_{\rm max} \approx 500$ km/s$)$ than those associated with typical LBGs, suggesting that gas covering fraction is a likely driver of the high Ly{\alpha} and Ly-continuum escape fractions of LAEs. Our results suggest a similar scaling of outflow velocity with star formation rate as is observed at lower redshifts $(v_{\rm outflow} \sim {\rm SFR}^{0.25})$ and indicate that a substantial fraction of gas is ejected with $v > v_{esc}$

A High-Resolution Hubble Space Telescope Study of Apparent Lyman Continuum Leakers at z∼3z\sim3

We present $U_{336}V_{606}J_{125}H_{160}$ follow-up $HST$ observations of 16 $z\sim3$ candidate LyC emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\ flux-density ratio of $(F_{UV}/F_{LyC})_{obs}=4.0\pm2.0$, compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of $f_{esc,rel}^{MD5b}=75-100$% ($f_{esc,abs}^{MD5b}=14-19$%). The stellar population fit to MD5b indicates an age of $\lesssim50$Myr, which is in the youngest 10% of the $HST$ sample and the youngest third of typical $z\sim3$ Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at $z=2.85$, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from $z\sim3$ star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.Comment: 31 pages, 5 tables, 19 figures. Accepted to ApJ. Version with full-resolution figures is available at: http://www.astro.ucla.edu/~aes/Mostardi_HST_LyC.pd

The Rest-frame Optical Spectroscopic Properties of Lyα-emitters at z~2.5: The Physical Origins of Strong Lyα Emission

We present the rest-frame optical spectroscopic properties of 60 faint (R_(AB) ~ 27; L ~ 0.1 L_*) Lyα-selected galaxies (LAEs) at z ≈ 2.56. These LAEs also have rest-UV spectra of their Lyα emission line morphologies, which trace the effects of interstellar and circumgalactic gas on the escape of Lyα photons. We find that the LAEs have diverse rest-optical spectra, but their average spectroscopic properties are broadly consistent with the extreme low-metallicity end of the populations of continuum-selected galaxies selected at z ≈ 2–3. In particular, the LAEs have extremely high [O iii] λ5008/Hβ ratios (log([O iii]/Hβ) ~ 0.8) and low [N ii] λ6585/Hα ratios (log([N ii]/Hα) < 1.15). Coupled with a detection of the [O iii] λ4364 auroral line, these measurements indicate that the star-forming regions in faint LAEs are characterized by high electron temperatures (T_e ≈ 1.8 × 10^4 K), low oxygen abundances (12 + log(O/H) ≈ 8.04, Z_(neb) ≈ 0.22Z_⊙), and high excitations with respect to their more luminous continuum-selected analogs. Several of our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) ≈ 6.9–7.4 (Z_(neb) ≈ 0.02–0.05Z_⊙). We interpret these observations in light of new models of stellar evolution (including binary interactions) that have been shown to produce long-lived populations of hot, massive stars at low metallicities. We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate the physical trends accompanying Lyα emission across the largest current sample of combined Lyα and rest-optical galaxy spectroscopy, including both the 60 KBSS-Lyα LAEs and 368 more luminous galaxies at similar redshifts. We find that the net Lyα emissivity (parameterized by the Lyα equivalent width) is strongly correlated with nebular excitation and ionization properties and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content

A High Fraction of Ly-alpha-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2

Star-forming galaxies form a sequence in the [OIII]/H-beta vs. [NII]/H-alpha diagnostic diagram, with low metallicity, highly ionized galaxies falling in the upper left corner. Drawing from a large sample of UV-selected star-forming galaxies at z~2 with rest-frame optical nebular emission line measurements from Keck-MOSFIRE, we select the extreme ~5% of the galaxies lying in this upper left corner, requiring log([NII]/H-alpha) = 0.75. These cuts identify galaxies with 12 + log(O/H) <~ 8.0, when oxygen abundances are measured via the O3N2 diagnostic. We study the Ly-alpha properties of the resulting sample of 14 galaxies. The mean (median) rest-frame Ly-alpha equivalent width is 39 (36) A, and 11 of the 14 objects (79%) are Ly-alpha-emitters (LAEs) with W_Lya > 20 A. We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at 2.0<z<2.6 identified without regard to their optical line ratios; this sample has mean (median) Ly-alpha equivalent width -1 (-4) A, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at Ly-alpha than those in the comparison sample, and have ~50% lower median oxygen abundances. Both factors are likely to facilitate the escape of Ly-alpha: in less dusty galaxies Ly-alpha photons are less likely to be absorbed during multiple scatterings, while the harder ionizing spectrum and higher ionization parameter associated with strong, low metallicity star formation may reduce the covering fraction or column density of neutral hydrogen, further easing Ly-alpha escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to Ly-alpha photons across a range of redshifts.Comment: 12 pages, 5 figures, 2 tables. Accepted for publication in Ap

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

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

Imaging Spectroscopy of Ionized Gaseous Nebulae around Optically Faint AGNs at Redshift z ∼ 2

We present Keck/OSIRIS laser guide-star assisted adaptive optics (LGSAO) integral-field spectroscopy of [O III] λ5007 nebular emission from 12 galaxies hosting optically faint (R = 20–25; ν L_ν ~ 10^(44) – 10^(46) erg s^(−1)) active galactic nuclei (AGNs) at redshift z ~ 2–3. In combination with deep Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) rest-frame optical imaging, Keck/MOSFIRE rest-optical spectroscopy, and Keck/KCWI rest-UV integral-field spectroscopy, we demonstrate that both the continuum and emission-line structures of these sources exhibit a wide range of morphologies, from compact, isolated point sources to double-AGN merging systems with extensive ~50 kpc tidal tails. One of the 12 galaxies previously known to exhibit a proximate damped Lyα system coincident in redshift with the galaxy shows evidence for both an extended [O III] narrow-line emission region and spatially offset Lyα emission (with morphologically distinct blueshifted and redshifted components) indicative of large-scale gas flows photoionized by the central AGN. We do not find widespread evidence of star formation in the host galaxies surrounding these AGNs; the [O III] velocity dispersions tend to be high (σ = 100–500 km s^(-1)), the continuum morphologies are much more compact than a mass-matched star-forming comparison sample, and the diagnostic nebular emission-line ratios are dominated by an AGN-like ionizing spectrum. The sample is most consistent with a population of AGNs that radiate at approximately their Eddington limit and photoionize extended [O III] nebulae whose characteristic sizes scale approximately as the square root of the AGN luminosity

Predicting Lyα Emission from Galaxies via Empirical Markers of Production and Escape in the KBSS

Lyα emission is widely used to detect and confirm high-redshift galaxies and characterize the evolution of the intergalactic medium (IGM). However, many galaxies do not display Lyα emission in typical spectroscopic observations, and intrinsic Lyα emitters represent a potentially biased set of high-redshift galaxies. In this work, we analyze a set of 703 galaxies at 2 ≾ z ≾ 3 with both Lyα spectroscopy and measurements of other rest-frame ultraviolet and optical properties in order to develop an empirical model for Lyα emission from galaxies and understand how the probability of Lyα emission depends on other observables. We consider several empirical proxies for the efficiency of Lyα photon production, as well as the subsequent escape of these photons through their local interstellar medium. We find that the equivalent width of metal-line absorption and the O3 ratio of rest-frame optical nebular lines are advantageous empirical proxies for Lyα escape and production, respectively. We develop a new quantity, X_(LIS)^(O3), that combines these two properties into a single predictor of net Lyα emission, which we find describes ~90% of the observed variance in Lyα equivalent width when accounting for our observational uncertainties. We also construct conditional probability distributions demonstrating that galaxy selection based on measurements of galaxy properties yield samples of galaxies with widely varying probabilities of net Lyα emission. The application of the empirical models and probability distributions described here may be used to infer the selection biases of current galaxy surveys and evaluate the significance of high-redshift Lyα (non)detections in studies of reionization and the IGM

A NEWLY FORMING COLD FLOW PROTOGALACTIC DISK, A SIGNATURE of COLD ACCRETION from the COSMIC WEB

How galaxies form from, and are fueled by, gas from the intergalactic medium (IGM) remains one of the major unsolved problems in galaxy formation. While the classical Cold Dark Matter paradigm posits galaxies forming from cooling virialized gas, recent theory and numerical simulations have highlighted the importance of cold accretion flows - relatively cool (T ∼ few × 104 K) unshocked gas streaming along filaments into dark matter halos, including hot, massive, high-redshift halos. These flows are thought to deposit gas and angular momentum into the circumgalactic medium resulting in disk- or ring-like structures, eventually coalescing into galaxies forming at filamentary intersections. We earlier reported a bright, Lyα emitting filament near the QSO HS1549+19 at redshift z = 2.843 discovered with the Palomar Cosmic Web Imager. We now report that the bright part of this filament is an enormous (R > 100 kpc) rotating structure of hydrogen gas with a disk-like velocity profile consistent with a 4 × 1012 M o halo. The orbital time of the outer part of the what we term a "protodisk" is comparable to the virialization time and the age of the universe at this redshift. We propose that this protodisk can only have recently formed from cold gas flowing directly from the cosmic we

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

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