A Refined Estimate of the Ionizing Emissivity from Galaxies at z ≃ 3: Spectroscopic Follow-up in the SSA22a Field

We investigate the contribution of star-forming galaxies to the ionizing background at z ~ 3, building on previous work based on narrowband (NB3640) imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman break galaxies (LBGs) and narrowband-selected Lyα emitters (LAEs) to measure redshifts for 16 LBGs and 87 LAEs at z > 3.055, such that our NB3640 imaging probes the Lyman-continuum (LyC) region. When we include the existing set of spectroscopically confirmed LBGs, our total sample with z > 3.055 consists of 41 LBGs and 91 LAEs, of which 9 LBGs and 20 LAEs are detected in our NB3640 image. With our combined imaging and spectroscopic data sets, we critically investigate the origin of NB3640 emission for detected LBGs and LAEs. We remove from our samples three LBGs and three LAEs with spectroscopic evidence of contamination of their NB3640 flux by foreground galaxies and statistically model the effects of additional, unidentified foreground contaminants. The resulting contamination and LyC-detection rates, respectively, are 62% ± 13% and 8% ± 3% for our LBG sample, and 47% ± 10% and 12% ± 2% for our LAE sample. The corresponding ratios of non-ionizing UV to LyC flux density, corrected for intergalactic medium (IGM) attenuation, are 18.0^(+34.8)_(–7.4) for LBGs and 3.7^(+2.5)_(–1.1) for LAEs. We use these ratios to estimate the total contribution of star-forming galaxies to the ionizing background and the hydrogen photoionization rate in the IGM, finding values larger than, but consistent with, those measured in the Lyα forest. Finally, the measured UV to LyC flux-density ratios imply model-dependent LyC escape fractions of f^(LyC)_(esc) ~ 5%-7% for our LBG sample and f^(LyC)_(esc) ~ 10%-30% for our fainter LAE sample

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 Å) as Lyα emitters (LAEs) and the latter group (W_(Lyα) < 20 Å) 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

Rest-Frame Optical Spectra of Three Strongly Lensed Galaxies at z~2

We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest-frame 3600 to 6800AA, including robust detections of fainter lines such as H-gamma, [SII]6717,6732, and in one instance [NeII]3869. SDSS J090122.37+181432.3 shows evidence for AGN activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties, including star-formation rate (SFR), metallicity, dynamical mass, and dust extinction. In all respects, the lensed objects appear fairly typical of UV-selected star-forming galaxies at z~2. The Clone occupies a position on the emission-line diagnostic diagram of [OIII]/H-beta vs. [NII]/H-alpha that is offset from the locations of z~0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [SII] line ratio, high electron densities (~1000 cm^(-3)) are inferred compared to local galaxies, and [OIII]/[OII] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z~2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts (abridged).Comment: 16 pages, 8 figures. Accepted for publication in the Astrophysical Journa

A Young Super Star Cluster in the Nuclear Region of NGC 253

We present observations of a massive star cluster near the nuclear region of the nearby starburst galaxy NGC 253. The peak of near-infrared emission, which is spatially separated by 4" from the kinematic center of the galaxy, is coincident with a super star cluster whose properties we examine with low-resolution (R ~ 1,200) infrared CTIO spectroscopy and optical/near-infrared HST imaging. Extinction, measured from [FeII] lines, is estimated at Av = 17.7 +/- 2.6. The age of the cluster is estimated at 5.7 Myr, based on Bry equivalent width for an instantaneous burst using Starburst99 modeling. However, a complex star formation history is inferred from the presence of both recombination emission and photospheric CO absorption. The ionizing photon flux has a lower limit of 7.3 +/- 2.5 x 10^53 inverse seconds, corrected for extinction. Assuming a Kroupa IMF, we estimate a cluster mass of 1.4 +/- 0.4 x 10^7 solar masses. We observe a strong Wolf-Rayet signature at 2.06 microns and report a weak feature at 2.19 microns which may be due to a massive stellar population, consistent with the derived mass and age of this cluster.Comment: 8 pages, 5 figures, accepted for publication in Ap

Demographics and Physical Properties of Gas Out/Inflows at 0.4 < z < 1.4

We present Keck/LRIS spectra of over 200 galaxies with well-determined redshifts between 0.4 and 1.4. We combine new measurements of near-ultraviolet, low-ionization absorption lines with previously measured masses, luminosities, colors, and star formation rates to describe the demographics and properties of galactic flows. Among star-forming galaxies with blue colors, we find a net blueshift of the FeII absorption greater than 200 km/s (100 km/s) towards 2.5% (20%) of the galaxies. The fraction of blueshifted spectra does not vary significantly with stellar mass, color, or luminosity but does decline at specific star formation rates less than roughly 0.8 Gyr^{-1}. The insensitivity of the blueshifted fraction to galaxy properties requires collimated outflows at these redshifts, while the decline in outflow fraction with increasing blueshift might reflect the angular dependence of the outflow velocity. The low detection rate of infalling gas, 3 to 6% of the spectra, suggests an origin in (enriched) streams favorably aligned with our sightline. We find 4 of these 9 infalling streams have projected velocities commensurate with the kinematics of an extended disk or satellite galaxy. The strength of the MgII absorption increases with stellar mass, B-band luminosity, and U-B color, trends arising from a combination of more interstellar absorption at the systemic velocity and less emission filling in more massive galaxies. Our results provides a new quantitative understanding of gas flows between galaxies and the circumgalactic medium over a critical period in galaxy evolution.Comment: Accepted version in 2-column format with embedded figure