We present the rest-1500 Å UV luminosity functions (LF) for star-forming galaxies during the cosmic high noonthe peak of cosmic star formation rate at z 1.5 3 < < . We use deep NUV imaging data obtained as part of the Hubble Ultra-Violet Ultra Deep Field (UVUDF) program, along with existing deep optical and NIR coverage on the HUDF. We select F225W, F275W, and F336W dropout samples using the Lyman break technique, along with samples in the corresponding redshift ranges selected using photometric redshifts, and measure the rest-frame UV LF at z 1.7, 2.2, 3.0 , respectively, using the modified maximum likelihood estimator. We perform simulations to quantify the survey and sample incompleteness for the UVUDF samples to correct the effective volume calculations for the LF. We select galaxies down to M 15.9, 16.3, 16.8 UV = ---and fit a faint-end slope of

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 .

SPitzer InfraRed Intensive Transients Survey-SPIRITS-is an ongoing survey of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We present the discovery and follow-up observations of one of our most luminous (M . Pre-discovery ground-based imaging revealed an associated, shorter-duration transient in the optical and near-IR (NIR). NIR spectroscopy showed a broad (≈8400 km s −1 ), double-peaked emission line of HeI at 1.083 μm, indicating an explosive origin. The NIR spectrum of SPIRITS 15c is similar to that of the Type IIb SN2011dh at a phase of ≈200days. Assuming an A V =2.2 mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The NIR light curves, however, show some minor discrepancies when compared with SN2011dh, and the extreme [3.6]-[4.5] color has not been previously observed for any SNIIb. Another luminous (M 4.5 = −16.1 ± 0.4 mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting 80days, and we suggest a scenario similar to the low-luminosity TypeIIP SN2005cs obscured by A V ≈1.5 mag. Other classes of IR-luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest that 18% of nearby core-collapse SNe are missed by currently operating optical surveys.

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 present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲ z ≲ 2.6 (z mean = 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of log ( M * / M ⊙ ) med = 8.29 − 0.43 + 0.51 and a median star formation rate of S F R H α m e d = 2.25 − 1.26 + 2.15 M ⊙ y r − 1 . We measure the faint electron-temperature-sensitive [O iii] λ4363 emission line at 2.5σ (4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of 12 + log ( O / H ) direct = 7.88 − 0.22 + 0.25 ( 0.15 − 0.06 + 0.12 Z ⊙ ). We investigate the applicability at high z of locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixed M *, our composite is well represented by the z ∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories ( log ( M * / M ⊙ ) med = 8.92 − 0.22 + 0.31 ) , we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed M * and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [O ii] λ3729/[O ii] λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of n e = 1 − 0 + 215 cm − 3 ( n e = 1 − 0 + 74 cm − 3 ) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at z ∼ 2.