We investigate the bursty star formation histories (SFHs) of dwarf galaxies using the distribution of log(L Hα /L U V ) of 185 local galaxies. We expand on the work of Weisz et al. (2012) to consider a wider range of SFHs and stellar metallicities, and show that there are large degeneracies in a periodic, top-hat burst model. We argue that all galaxies of a given mass have similar SFHs and we can therefore include the L Hα distributions (subtracting the median trend with stellar mass, referred to as ∆log(L Hα )) in our analyses. ∆log(L Hα ) traces the amplitude of the bursts, and log(L Hα /L U V ) is a function of timescale, amplitude, and shape of the bursts. We examine the 2-dimensional distribution of these two indicators to constrain the SFHs. We use exponentially rising/falling bursts to determine timescales (e-folding time, τ ). We find that galaxies below 10 7.5 M undergo large (maximum amplitudes of ∼ 100) and rapid (τ < 30 Myr) bursts, while galaxies above 10 8.5 M experience smaller (maximum amplitudes ∼ 10), slower (τ 300 Myr) bursts. We compare to the FIRE-2 hydrodynamical simulations and find that the burst amplitudes agree with observations, but they are too rapid in intermediate-mass galaxies (M * > 10 8 M ). Finally, we confirm that stochastic sampling of the stellar mass function can not reproduce the observed distributions unless the standard assumptions of cluster and stellar mass functions are changed. With the next generation of telescopes, measurements of L U V and L Hα will become available for dwarf galaxies at high-redshift, enabling similar analyses of galaxies in the early universe.

The redshift range z = 4 − 6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 z ∼ 4.5 main-sequence galaxies at log(M/M ) > 9.7 by comparing their ultraviolet (UV) continuum, Hα luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6 µm]−[4.5 µm] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between Hα and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in Hα compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between Hα EW and stellar mass, ranging from 1000Å at log(M/M ) < 10 to below 100Å at log(M/M ) > 11. Consulting models suggests that most z ∼ 4.5 galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z = 4. We identify differential dust attenuation (f ) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing Hα EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z > 4 consistent with the extrapolation of measurements out to z = 2.

We measure the ionizing photon production efficiency (ξ ion) of low-mass galaxies (10 7.8-10 9.8 M e) at 1.4<z<2.7 to better understand the contribution of dwarf galaxies to the ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of Hα-to-UV luminosity ratios (L Hα /L UV) of galaxies to determine the standard log(ξ ion). Second, we take the logarithm of the total L Hα over the total L UV. We prefer the latter, as it provides the total ionizing UV luminosity density of galaxies when multiplied by the nonionizing UV luminosity density. log(ξ ion) calculated from the second method is ∼0.2 dex higher than the first method. We do not find any strong dependence between log(ξ ion) and stellar mass, far-UV magnitude (M UV), or UV spectral slope (β). We report a value of log(ξ ion) ∼ 25.47±0.09 for our UV-complete sample (-< <-M 22 17.3 UV) and ∼25.37±0.11 for our mass-complete sample (7.8<log(M *)<9.8). These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log(ξ ion) is 0.2-0.3 dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity or age. We also find a correlation between log(ξ ion) and the equivalent widths of Hα and [O III] λ5007 fluxes, confirming that these equivalent widths can be used to estimate ξ ion .

We present Keck/MOSFIRE and Keck/LRIS spectroscopy of A1689-217, a lensed (magnification ∼ 7.9), star-forming (SFR ∼ 16 M yr −1 ), dwarf (log(M * /M ) = 8.07−8.59) Lyα-emitter (EW 0 ∼ 138Å) at z = 2.5918. Dwarf galaxies similar to A1689-217 are common at high redshift and likely responsible for reionization, yet few have been studied with detailed spectroscopy. We report a 4.2σ detection of the electron-temperature-sensitive [O III]λ4363 emission line, the second such detection at z > 2, and use this line to directly measure an oxygen abundance of 12+log(O/H) = 8.06 ± 0.12 (∼ 1/4 Z ). Using the rest-optical emission lines, we measure A1689-217's other nebular conditions including electron temperature (T e ([O III]) ∼ 14,000 K), electron density (n e ∼ 220 cm −3 ) and reddening (E(B −V ) ∼ 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O III]λ4363 detections at z ∼ 0 − 3.1, showing that the locally-calibrated, oxygenbased, strong-line relations are consistent from z ∼ 0 − 3.1. We also show additional evidence that the O 32 vs. R 23 excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to z ∼ 3.1. From this excitation diagram and the strong-line ratio − metallicity plots, we observe that the ionization parameter at fixed O/H is consistent with no redshift evolution. Although A1689-217 is metal-rich for its M * and SFR, we find it to be consistent within the large scatter of the low-mass end of the Fundamental Metallicity Relation.

We use a sample of 78,340 star-forming galaxies at z ≃ 0.04 − 0.1 from the SDSS DR8 survey to calculate the average nebular dust attenuation curve and its variation with the physical properties of galaxies. Using the first four low-order Balmer emission lines (H$α$, H$β$, H$γ$, H$δ$) detected in the composite spectrum of all galaxies in the sample, we derive a nebular attenuation curve in the range of 0.41 μm to 0.66 μm that has a similar shape and normalization to that of the Galactic extinction curve (Milky Way curve), the SMC curve and the nebular attenuation curve derived recently for typical star-forming galaxies at z ∼ 2. We divide the galaxies into bins of stellar mass, gas-phase metallicity, and specific star-formation rate, and derive the nebular attenuation curve in each of these bins. This analysis indicates that there is very little variation in the shape of the nebular dust attenuation curve with the properties used to bin the galaxies, and suggests a near universal shape of the nebular dust attenuation curve at least among the galaxies and the range of properties considered in our sample.