Observations of reionization-era analogs at z ∼ 3 are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 z ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum region. We present a method of removing foreground contamination from our sample using high-resolution, multi-band Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually-detected Lyman continuum sources and 107 individually-undetected sources, including a sample-averaged absolute escape fraction of fesc, abs = 0.06 ± 0.01 and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of 〈f900/f1500〉out = 0.040 ± 0.006, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between 〈f900/f1500〉out and Lyα equivalent width (Wλ(Ly$\rm \alpha$)) and a negative correlation between 〈f900/f1500〉out and UV luminosity. As in previous work, we interpret the relationship between 〈f900/f1500〉out and Wλ(Ly$\rm \alpha$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a Wλ(Ly$\rm \alpha$)-weighted 〈f900/f1500〉out to estimate an ionizing emissivity from star-forming galaxies at z ∼ 3 as εLyC ≃ 5.5 × 1024 erg s−1 Hz−1 Mpc−3. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at z ∼ 3 is comparable to that of active galactic nuclei.

To date, no direct detection of Lyman continuum emission has been measured for intermediate-redshift (z 1) star-forming galaxies. We combine Hubble Space Telescope grism spectroscopy with GALEX UV and groundbased optical imaging to extend the search for escaping Lyman continuum to a large (∼600) sample ofz 1 low-, moderately star-forming (Y  10  M yr −1 ) galaxies selected initially on Hα emission. The characteristic escape fraction of LyC from star-forming galaxies (SFGs) that populate this parameter space remains weakly constrained by previous surveys, but these faint (sub-L å ) SFGs are assumed to play a significant role in the reionization of neutral hydrogen in the intergalactic medium (IGM) at high redshift > z 6. We do not make an unambiguous detection of escaping LyC radiation from thisz 1 sample, individual non-detections to constrain the absolute Lyman continuum escape fraction, f esc < 2.1% (3σ). We measure an upper limit of f esc < 9.6% from a sample of SFGs selected on high Hα equivalent width (EW > 200 Å), which are thought to be close analogs of high redshift sources of reionization. For reference, we also present an emissivity-weighted escape fraction that is useful for measuring the general contribution SFGs to the ionizing UV background. In the discussion, we consider the implications of these intermediate redshift constraints for the reionization of hydrogen in the IGM at high ( > z 6) redshift. If we assume ourz 1 SFGs, for which we measure this emissivity-weighted f esc , are analogs to the high redshift sources of reionization, we find it is difficult to reconcile reionization by faint (  -M 13 UV ) SFGs with a low escape fraction ( f esc < 3%), with constraints from independent high redshift observations. If f esc evolves with redshift, reionization by SFGs may be consistent with observations from Planck.

We present a new measurement of the gas-phase mass-metallicity relation (MZR) and its dependence on star formation rates (SFRs) at 1.3 < z < 2.3. Our sample comprises 1056 galaxies with a mean redshift of z = 1.9, identified from the Hubble Space Telescope Wide Field Camera 3 (WFC3) grism spectroscopy in the Cosmic Assembly Near-infrared Deep Extragalactic Survey and the WFC3 Infrared Spectroscopic Parallel Survey. This sample is four times larger than previous metallicity surveys at z ∼ 2 and reaches an order of magnitude lower in stellar mass (10 8 M e ). Using stacked spectra, we find that the MZR evolves by 0.3 dex relative to z ∼ 0.1. Additionally, we identify a subset of 49 galaxies with high signal-to-noise (S/N) spectra and redshifts between 1.3 < z < 1.5, where Hα emission is observed along with [O III] and [O II]. With accurate measurements of SFR in these objects, we confirm the existence of a mass-metallicity-SFR (M-Z-SFR) relation at high redshifts. These galaxies show systematic differences from the local M-Z-SFR relation, which vary depending on the adopted measurement of the local relation. However, it remains difficult to ascertain whether these differences could be due to redshift evolution, as the local M-Z-SFR relation is poorly constrained at the masses and SFRs of our sample. Lastly, we reproduced our sample selection in the IllustrisTNG hydrodynamical simulation, demonstrating that our line flux limit lowers the normalization of the simulated MZR by 0.2 dex. We show that the M-Z-SFR relation in IllustrisTNG has an SFR dependence that is too steep by a factor of around 3.

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z ∼ 5. We combine new z ∼ 5 measurements of HI Lyα and low-and high-ionization interstellar metal absorption features with comparable measurements at z ∼ 2-4. This redshift range covers the peak epoch of star formation in the Universe and extends back towards the epoch of reionization. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Lyα equivalent width, and perform corrections to the strength of Lyα based on a model for the transmission of the intergalactic medium. We find a strong trend of decreasing low-ionization line strength with increasing Lyα emission strength over the redshift range z ∼ 2-5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Lyα equivalent widths, we see evolution with increasing redshift towards greater Lyα emission strength at fixed low-ionization absorption strength. This evolution suggests a higher intrinsic production rate of Lyα photons at z ∼ 5 than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Lyα emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star-formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Lyα emission evolve towards higher redshift. We conclude that increasing intrinsic Lyα photon production and strong detection of nebular C IV emission (signaling lower metallicity) at z ∼ 5 indicate an elevated ionized photon production efficiency (ξ ion ).

We present the first statistical analysis of kinematically-resolved, spatially-extended Lyα 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 (z med = 2.29) comprises the subset with typical KCWI integration times of ∼ 5 hours 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 Lyα 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 Lyα 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 Lyα 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 Lyα emission along the galaxy principal axes are statistically indistinguishable, with residual asymmetry of ≤ 2% (∼ 2σ) of the integrated Lyα emission. The symmetry implies that the Lyα scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Lyα emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Lyα emission. We speculate that this weak directional dependence of Lyα emission becomes discernible only when the Lyα escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Lyα radiative transfer modeling.