We investigate the evolution of galaxy gas-phase metallicity (O/H) over the range z = 0 − 3.3 using samples of ∼ 300 galaxies at z ∼ 2.3 and ∼ 150 galaxies at z ∼ 3.3 from the MOSDEF survey. This analysis crucially utilizes different metallicity calibrations at z ∼ 0 and z > 1 to account for evolving ISM conditions. We find significant correlations between O/H and stellar mass (M * ) at z ∼ 2.3 and z ∼ 3.3. The low-mass power law slope of the mass-metallicity relation is remarkably invariant over z = 0 − 3.3, such that O/H∝M 0.30 * at all redshifts in this range. At fixed M * , O/H decreases with increasing redshift as dlog(O/H)/dz = −0.11 ± 0.02. We find no evidence that the fundamental metallicity relation between M * , O/H, and star-formation rate (SFR) evolves out to z ∼ 3.3. We employ analytic chemical evolution models to place constraints on the mass and metal loading factors of galactic outflows. The efficiency of metal removal increases toward lower M * at fixed redshift, and toward higher redshift at fixed M * . These models suggest that the slope of the mass-metallicity relation is primarily set by the scaling of the outflow metal loading factor with M * , not by the change in gas fraction as a function of M * . The evolution toward lower O/H at fixed M * with increasing redshift is driven by both higher gas fraction (leading to stronger dilution of ISM metals) and higher metal removal efficiency. These results suggest that the processes governing the smooth baryonic growth of galaxies via gas flows and star formation hold in the same form over at least the past 12 Gyr.

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z /2. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at z > 6 in recent years. We present HST /COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼ 10 2 Gyr −1 ). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii] equivalent widths comparable to those seen in systems at z ∼ 6 − 7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H 8.0 (Z/Z 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He + -ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.

We present and discuss measurements of the gas-phase metallicity gradient in gravitationally lensed galaxies at z = 2.0 − 2.4 based on adaptive optics-assisted imaging spectroscopy with the Keck II telescope. Through deep exposures we have secured high signal to noise data for four galaxies with well-understood kinematic properties. Three galaxies with well-ordered rotation reveal metallicity gradients in the sense of having lower gas-phase metallicities at larger galactocentric radii. Two of these display gradients much steeper than found locally, while a third has one similar to that seen in local disk galaxies. The fourth galaxy exhibits complex kinematics indicative of an ongoing merger and reveals an "inverted" gradient with lower metallicity in the central regions. By comparing our sample to similar data in the literature for lower redshift galaxies, we determine that, on average, metallicity gradients must flatten by a factor of 2.6 ± 0.9 between z = 2.2 and the present epoch. This factor is in rough agreement with the size growth of massive galaxies suggesting that inside-out growth can account for the evolution of metallicity gradients. Since the addition of our new data provides the first indication of a coherent picture of this evolution, we develop a simple model of chemical evolution to explain the collective data. We find that metallicity gradients and their evolution can be explained by the inward radial migration of gas together with a radial variation in the mass loading factor governing the ratio of outflowing gas to the local star formation rate. Average mass loading factors of ∼ < 2 are inferred from our model in good agreement with direct measurements of outflowing gas in z ≃ 2 galaxies.

We present adaptive optics-assisted integral field spectroscopy around the Hα or Hβ lines of 12 gravitationally lensed galaxies obtained with VLT/SINFONI, Keck/OSIRIS and Gemini/NIFS. We combine these data with previous observations and investigate the dynamics and star formation properties of 17 lensed galaxies at 1 < z < 4. Thanks to gravitational magnification of 1.4 − 90× by foreground clusters, effective spatial resolutions of 40−700 pc are achieved. The magnification also allows us to probe lower star formation rates and stellar masses than unlensed samples; our target galaxies feature dust-corrected SFRs derived from Hα or Hβ emission of ∼ 0.8 − 40M ⊙ yr −1 , and stellar masses M * ∼ 4 × 10 8 − 6 × 10 10 M ⊙ . All of the galaxies have velocity gradients, with 59% consistent with being rotating discs and a likely merger fraction of 29%, with the remaining 12% classed as 'undetermined.' We extract 50 star-forming clumps with sizes in the range 60 pc -1 kpc from the Hα (or Hβ) maps, and find that their surface brightnesses, Σ clump and their characteristic luminosities, L 0 , evolve to higher luminosities with redshift. We show that this evolution can be described by fragmentation on larger scales in gas-rich discs, and is likely to be driven by evolving gas fractions.