We present results on the dust attenuation curve of z ∼ 2 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Our sample consists of 224 star-forming galaxies with nebular spectroscopic redshifts in the range z spec = 1.36 − 2.59 and high S/N measurements of, or upper limits on, the Hα and Hβ emission lines obtained with the MOSFIRE spectrograph on the Keck I telescope. Using deep multi-wavelength photometry, we construct composite spectral energy distributions (SEDs) of galaxies in bins of specific star-formation rate (SFR/M * ) and Balmer optical depth. These composites are used to directly constrain the shape and normalization of the dust attenuation curve over the full wavelength range from the UV through near-IR for typical star-forming galaxies at high redshift (z 1.4). Our results imply an attenuation curve that is very similar in shape and normalization to the SMC extinction curve at wavelengths λ 2500Å. At shorter wavelengths, the shape of the curve is identical to that of the Calzetti et al. (2000) starburst attenuation relation, but with a lower normalization (R V ), implying less attenuation at a fixed wavelength for a given SED shape. Hence, the new attenuation curve results in SFRs that are ≈ 20% lower, and stellar masses that are ∆ log(M * /M ) 0.16 dex lower, than those obtained with the starburst attenuation curve. We find marginal evidence for excess absorption at 2175Å. Moreover, we find that the difference in the reddening-and the total attenuation-of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs 20 M yr −1 , assuming a Chabrier (2003) IMF, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV-and SED-based SFRs may underestimate the total SFR at even modest levels of ≈ 20 M yr −1 .

Using observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we investigate the physical conditions of star-forming regions in z ∼ 2.3 galaxies, specifically the electron density and ionization state. From measurements of the [O ii]λλ3726,3729 and [S ii]λλ6716,6731 doublets, we find a median electron density of ∼ 250 cm −3 at z ∼ 2.3, an increase of an order of magnitude compared to measurements of galaxies at z ∼ 0. While z ∼ 2.3 galaxies are offset towards significantly higher O 32 values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O 32 vs. R 23 and O3N2 diagrams. Based on these results, we propose that z ∼ 2.3 star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and z ∼ 2.3 galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from z ∼ 0 to z ∼ 2.3. We find that z ∼ 2.3 galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving [N ii]λ6584. We conclude that the offset of z ∼ 2.3 galaxies from the local star-forming sequence in the [N ii] BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at z ∼ 0 and z ∼ 2.

In this paper we present the MOSFIRE Deep Evolution Field (MOSDEF) survey. The MOSDEF survey aims to obtain moderate-resolution (R = 3000 − 3650) rest-frame optical spectra (∼ 3700 − 7000Å) for ∼1500 galaxies at 1.37 ≤ z ≤ 3.80 in three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are selected in three redshift intervals: 1.37 ≤ z ≤ 1.70, 2.09 ≤ z ≤ 2.61, and 2.95 ≤ z ≤ 3.80, down to fixed H AB (F160W) magnitudes of 24.0, 24.5 and 25.0, respectively, using the photometric and spectroscopic catalogs from the 3D-HST survey. We target both strong nebular emission lines (e.g.,, 6585, and [S ii] λλ6718, 6733) and stellar continuum and absorption features (e.g., Balmer lines, Ca-ii H and K, Mgb, 4000Å break). Here we present an overview of our survey, the observational strategy, the data reduction and analysis, and the sample characteristics based on spectra obtained during the first 24 nights. To date, we have completed 21 masks, obtaining spectra for 591 galaxies. For ∼80% of the targets we derive a robust redshift from either emission or absorption lines. In addition, we confirm 55 additional galaxies, which were serendipitously detected. The MOSDEF galaxy sample includes unobscured star-forming, dusty star-forming, and quiescent galaxies and spans a wide range in stellar mass (∼ 10 9 − 10 11.5 M ⊙ ) and star formation rate (∼ 10 0 − 10 3 M ⊙ yr −1 ). The spectroscopically confirmed sample is roughly representative of an H-band limited galaxy sample at these redshifts. With its large sample size, broad diversity in galaxy properties, and wealth of available ancillary data, MOSDEF will transform our understanding of the stellar, gaseous, metal, dust, and black hole content of galaxies during the time when the universe was most active.

We present results on the excitation properties of z ∼ 2.3 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) Survey. With its coverage of the full suite of strong restframe optical emission lines, MOSDEF provides an unprecedented view of the rest-frame optical spectra of a representative sample of distant star-forming galaxies. We investigate the locations of z ∼ 2.3 MOSDEF galaxies in multiple emission-line diagnostic diagrams. These include the [OIII]λ5007/Hβ vs.[NII]/Hα and [OIII]λ5007/Hβ vs.[SII]λλ6717, 6731/Hα "BPT" diagrams, as well as the O 32 vs. R 23 excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the [OIII]λ5007/Hβ vs.[NII]/Hα BPT diagram relative to Sloan Digital Sky Survey starforming galaxies. However, the shift for our rest-frame optically selected sample is less significant than for rest-frame-UV selected and emission-line selected galaxies at z ∼ 2. Furthermore, we find that the offset is mass-dependent, only appearing within the low-mass half of the z ∼ 2.3 MOSDEF sample, where galaxies are shifted towards higher [NII]/Hα at fixed [OIII]/Hβ. Within the [OIII]λ5007/Hβ vs.[SII]/Hα and O 32 vs. R 23 diagrams, we find that z ∼ 2.3 galaxies are distributed like local ones, and therefore attribute the shift in the [OIII]λ5007/Hβ vs.[NII]/Hα BPT diagram to elevated N/O abundance ratios among lower-mass (M * < 10 10 M ⊙ ) high-redshift galaxies. The variation in N/O ratios calls into question the use at high redshift of oxygen abundance indicators based on nitrogen lines, but the apparent invariance with redshift of the excitation sequence in the O 32 vs. R 23 diagram paves the way for using the combination of O 32 and R 23 as an unbiased metallicity indicator over a wide range in redshift. This indicator will allow for an accurate characterization of the shape and normalization of the mass-metallicity relationship over more than 10 Gyr.

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.