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Abstract: We use the large spectroscopic data set of the MOSFIRE Deep Evolution Field survey to investigate some of the key factors responsible for the elevated ionization parameters (U) inferred for high-redshift galaxies, focusing in particular on the role of star-formation-rate surface density (ΣSFR). Using a sample of 317 galaxies with spectroscopic redshifts z spec ≃ 1.9–3.7, we construct composite rest-frame optical spectra in bins of ΣSFR and infer electron densities, n e, using the ratio of the [O ii] λ λ3727, 3730 doublet. Our analysis suggests a significant (≃3σ) correlation between n e and ΣSFR. We further find significant correlations between U and ΣSFR for composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences of U. The increase in n e—and possibly also the volume filling factor of dense clumps in H ii regions—with ΣSFR appear to be important factors in explaining the relationship between U and ΣSFR. Further, the increase in n e and SFR with redshift at a fixed stellar mass can account for most of the redshift evolution of U. These results suggest that the gas density, which sets n e and the overall level of star formation activity, may play a more important role than metallicity evolution in explaining the elevated ionization parameters of high-redshift galaxies.

Abstract: We investigate dust attenuation and its dependence on viewing angle for 308 star-forming galaxies at 1.3 ≤ z ≤ 2.6 from the MOSFIRE Deep Evolution Field survey. We divide galaxies with a detected Hα emission line and coverage of Hβ into eight groups by stellar mass, star formation rate (SFR), and inclination (i.e., axis ratio), and we then stack their spectra. From each stack, we measure the Balmer decrement and gas-phase metallicity, and then we compute the median A V and UV continuum spectral slope (β). First, we find that none of the dust properties (Balmer decrement, A V, or β) varies with the axis ratio. Second, both stellar and nebular attenuation increase with increasing galaxy mass, showing little residual dependence on SFR or metallicity. Third, nebular emission is more attenuated than stellar emission, and this difference grows even larger at higher galaxy masses and SFRs. Based on these results, we propose a three-component dust model in which attenuation predominantly occurs in star-forming regions and large, dusty star-forming clumps, with minimal attenuation in the diffuse ISM. In this model, nebular attenuation primarily originates in clumps, while stellar attenuation is dominated by star-forming regions. Clumps become larger and more common with increasing galaxy mass, creating the above mass trends. Finally, we argue that a fixed metal yield naturally leads to mass regulating dust attenuation. Infall of low-metallicity gas increases the SFR and lowers the metallicity, but leaves the dust column density mostly unchanged. We quantify this idea using the Kennicutt–Schmidt and fundamental metallicity relations, showing that galaxy mass is indeed the primary driver of dust attenuation.

We present spatially resolved Hubble Space Telescope grism spectroscopy of 15 galaxies at z ∼ 0.8 drawn from the DEEP2 survey. We analyze Hα+[N II], [S II], and [S III] emission on kiloparsec scales to explore which mechanisms are powering emission lines at high redshifts, testing which processes may be responsible for the well-known offset of high-redshift galaxies from the z ∼ 0 locus in the [O III]/Hβ versus [N II]/Hα Baldwin - Phillips - Terlevich (BPT) excitation diagram. We study spatially resolved emission-line maps to examine evidence for active galactic nuclei (AGN), shocks, diffuse ionized gas (DIG), or escaping ionizing radiation, all of which may contribute to the BPT offsets observed in our sample. We do not find significant evidence of AGN in our sample and quantify that, on average, AGN would need to contribute ∼25% of the Hα flux in the central resolution element in order to cause the observed BPT offsets. We find weak (2σ) evidence of DIG emission at low surface brightnesses, yielding an implied total DIG emission fraction of ∼20%, which is not significant enough to be the dominant emission line driver in our sample. In general we find that the observed emission is dominated by star-forming H II regions. We discuss trends with demographic properties and the possible role of α-enhanced abundance patterns in the emission spectra of high-redshift galaxies. Our results indicate that photoionization modeling with stellar population synthesis inputs is a valid tool to explore the specific star formation properties which may cause BPT offsets, to be explored in future work.

We present ionized gas kinematics for 681 galaxies at z &tild; 1.4-3.8 from the MOSFIRE Deep Evolution Field survey, measured using models that account for random galaxy-slit misalignments together with structural parameters derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes are used to derive dynamical masses. Baryonic masses are estimated from stellar masses and inferred gas masses from dust-corrected star formation rates (SFRs) and the Kennicutt-Schmidt relation. We measure resolved rotation for 105 galaxies. For the remaining 576 galaxies we use models based on HST imaging structural parameters together with integrated velocity dispersions and baryonic masses to statistically constrain the median ratio of intrinsic ordered to disordered motion,. We find that increases with increasing stellar mass and decreasing specific SFR (sSFR). These trends may reflect marginal disk stability, where systems with higher gas fractions have thicker disks. For galaxies with detected rotation we assess trends between their kinematics and mass, sSFR, and baryon surface density (Σbar,e). Intrinsic dispersion correlates most with Σbar,e, and velocity correlates most with mass. By comparing dynamical and baryonic masses, we find that galaxies at z &tild; 1.4-3.8 are baryon dominated within their effective radii (RE), with Mdyn/Mbaryon increasing over time. The inferred baryon fractions within RE, fbar, decrease over time, even at fixed mass, size, or surface density. At fixed redshift, fbar does not appear to vary with stellar mass but increases with decreasing RE and increasing Σbar,e. For galaxies at z ≥ 2, the median inferred baryon fractions generally exceed 100%. We discuss possible explanations and future avenues to resolve this tension.

Strong [O iii]λλ4959,5007 + Hβ emission appears to be typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic reionization, these galaxies and the physical conditions within them are of great interest. At z > 6.5, where Lyα is greatly attenuated by the intergalactic medium, rest-UV metal emission lines provide an alternative measure of redshift and constraints on the physical properties of star-forming regions and massive stars. We present the first statistical sample of rest-UV line measurements in z ∼ 2 galaxies selected as analogs of those in the reionization era based on [O iii]λλ4959,5007 equivalent width (EW) or rest-frame U - B color. Our sample is drawn from the 3D-HST Survey and spans the redshift range 1.36 ≤ z ≤ 2.49. We find that the median Lyα and C iii]λλ1907,1909 EWs of our sample are significantly greater than those of z ∼ 2 UV-continuum-selected star-forming galaxies. Measurements from both individual and composite spectra indicate a monotonic, positive correlation between C iii] and [O iii], while a lack of trend is observed between Lyα and [O iii] at EW[O iii] ≲ 1000 Å. At higher EW[O iii], extreme Lyα emission starts to emerge. Using stacked spectra, we find that Lyα and C iii] are significantly enhanced in galaxies with lower metallicity. Two objects in our sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV metal line emission. These objects have significant C iv λλ1548,1550, He ii λ1640, and O iii]λλ1661,1665 emission in addition to intense Lyα or C iii]. Detailed characterization of these lower-redshift analogs provides unique insights into the physical conditions in z > 6.5 star-forming regions, motivating future observations of reionization-era analogs at lower redshifts.

We present measurements of [S III]λλ9069,9531 for a sample of z ∼ 1.5 star-forming galaxies, the first representative sample with measurements of these lines at z ≿ 0.1. We employ the line ratio S32 ≡ [S III]λλ9069,9531/[S II]λλ6716,6731 as a novel probe of evolving interstellar medium (ISM) conditions. Since this ratio includes the low-ionization line [S II], it is crucial that the effects of diffuse ionized gas (DIG) on emission-line ratios be accounted for in z ∼ 0 galaxy spectra, or else that comparisons be made to samples of local H II regions in which DIG emission is not present. We find that S32 decreases with increasing stellar mass at both z ∼ 1.5 and z ∼ 0, but with a shallow slope suggesting S32 has a weak dependence on metallicity, in contrast with [O III]/[O II] that displays a strong metallicity dependence. As a result, S32 only mildly evolves with redshift at fixed stellar mass. The z ∼ 1.5 sample is systematically offset toward lower S32 and higher [S II]/Hα at fixed [O III]/Hβ relative to z = 0 H II regions. We find that such trends can be explained by a scenario in which the ionizing spectrum is harder at fixed O/H with increasing redshift, but are inconsistent with an increase in ionization parameter at fixed O/H. This analysis demonstrates the advantages of expanding beyond the strongest rest-optical lines for evolutionary studies, and the particular utility of [S III] for characterizing evolving ISM conditions and stellar compositions. These measurements provide a basis for estimating [S III] line strengths for high-redshift galaxies, a line that the James Webb Space Telescope will measure out to z ∼ 5.5.

We present measurements of [S III]λλ9069,9531 for a sample of z ∼ 1.5 star-forming galaxies, the first representative sample with measurements of these lines at z ≿ 0.1. We employ the line ratio S32 ≡ [S III]λλ9069,9531/[S II]λλ6716,6731 as a novel probe of evolving interstellar medium (ISM) conditions. Since this ratio includes the low-ionization line [S II], it is crucial that the effects of diffuse ionized gas (DIG) on emission-line ratios be accounted for in z ∼ 0 galaxy spectra, or else that comparisons be made to samples of local H II regions in which DIG emission is not present. We find that S32 decreases with increasing stellar mass at both z ∼ 1.5 and z ∼ 0, but with a shallow slope suggesting S32 has a weak dependence on metallicity, in contrast with [O III]/[O II] that displays a strong metallicity dependence. As a result, S32 only mildly evolves with redshift at fixed stellar mass. The z ∼ 1.5 sample is systematically offset toward lower S32 and higher [S II]/Hα at fixed [O III]/Hβ relative to z = 0 H II regions. We find that such trends can be explained by a scenario in which the ionizing spectrum is harder at fixed O/H with increasing redshift, but are inconsistent with an increase in ionization parameter at fixed O/H. This analysis demonstrates the advantages of expanding beyond the strongest rest-optical lines for evolutionary studies, and the particular utility of [S III] for characterizing evolving ISM conditions and stellar compositions. These measurements provide a basis for estimating [S III] line strengths for high-redshift galaxies, a line that the James Webb Space Telescope will measure out to z ∼ 5.5.

Using data from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we present a census of active galactic nucleus (AGN)-driven ionized outflows in a sample of 159 AGNs at 1.4 ≤ z ≤ 3.8. The sample spans AGN bolometric luminosities of 1044-47 erg s-1 and includes both quiescent and star-forming galaxies extending across 3 orders of magnitude in stellar mass. We identify and characterize outflows from the Hβ, [O iii], H, and [N ii] emission line spectra. We detect outflows in 17% of the AGNs, seven times more often than in a mass-matched sample of inactive galaxies in MOSDEF. The outflows are fast and galaxy-wide, with velocities of ∼400-3500 km s-1 and spatial extents of 0.3-11.0 kpc. The incidence of outflows among AGNs is independent of the stellar mass of the host galaxy, with outflows detected in both star-forming and quiescent galaxies. This suggests that outflows exist across different phases in galaxy evolution. We investigate relations between outflow kinematic, spatial, and energetic properties and both AGN and host galaxy properties. Our results show that AGN-driven outflows are widespread in galaxies along the star-forming main sequence. The mass-loading factors of the outflows are typically 0.1-1 and increase with AGN luminosity, capable of exceeding unity at at LAGN≳ 1046 erg s-1. In these more luminous sources, the ionized outflow alone is likely sufficient to regulate star formation and, when combined with outflowing neutral and molecular gas, may be able to quench star formation in their host galaxies.

Population synthesis models predict that high-mass X-ray binary (HMXB) populations produced in low-metallicity environments should be more X-ray luminous, a trend supported by studies of nearby galaxies. This trend may be responsible for the observed increase of the X-ray luminosity (L X) per star formation rate (SFR) with redshift due to the decrease of metallicity (Z) at fixed stellar mass as a function of redshift. To test this hypothesis, we use a sample of 79 z ∼ 2 star-forming galaxies with oxygen abundance measurements from the MOSDEF survey, which obtained rest-frame optical spectra for ∼1500 galaxies in the CANDELS fields at 1.37 < z < 3.80. Using Chandra data from the Chandra AEGIS-X Deep, Chandra Deep Field North, and Chandra Deep Field South surveys, we stack the X-ray data at the galaxy locations in bins of redshift and Z because the galaxies are too faint to be individually detected. In agreement with previous studies, the average L X/SFR of our z ∼ 2 galaxy sample is enhanced by ≈0.4-0.8 dex relative to local HMXB L X-SFR scaling relations. Splitting our sample by Z, we find that L X/SFR and Z are anticorrelated with 97% confidence. This observed Z dependence for HMXB-dominated galaxies is consistent with both the local L X-SFR-Z relation and a subset of population synthesis models. Although the statistical significance of the observed trends is weak owing to the low X-ray statistics, these results constitute the first direct evidence connecting the redshift evolution of L X/SFR and the Z dependence of HMXBs.

We present measurements of [SIII]$\lambda\lambda$9069,9531 for a sample of$z\sim1.5$ star-forming galaxies, the first sample with measurements of theselines at z>0.1. We employ the line ratioS$_{32}$$\equiv$[SIII]$\lambda\lambda$9069,9531/[SII]$\lambda\lambda$6716,6731as a novel probe of evolving ISM conditions. Since this ratio includes thelow-ionization line [SII], it is crucial that the effects of diffuse ionizedgas (DIG) on emission-line ratios be accounted for in $z\sim0$ integratedgalaxy spectra, or else that comparisons be made to samples of local HIIregions in which DIG emission is not present. We find that S$_{32}$ decreaseswith increasing stellar mass at both $z\sim1.5$ and $z\sim0$, but that thedependence is weak suggesting S$_{32}$ has a very shallow anticorrelation withmetallicity, in contrast with O$_{32}$ that displays a strong metallicitydependence. As a result, S$_{32}$ only mildly evolves with redshift at fixedstellar mass. The $z\sim1.5$ sample is systematicallty offset towards lowerS$_{32}$ and higher [SII]/H$\alpha$ at fixed [OIII]/H$\beta$ relative to $z=0$HII regions. By comparing to photoionization model grids, we find that suchtrends can be explained by a scenario in which the ionizing spectrum is harderat fixed O/H with increasing redshift, but are inconsistent with an increase inionization parameter at fixed O/H. This analysis demonstrates the advantages ofexpanding beyond the strongest rest-optical lines for evolutionary studies, andthe particular utility of [SIII] for characterizing evolving ISM conditions andstellar compositions. These measurements provide a basis for estimating [SIII]line strengths for high-redshift galaxies, a line that the James Webb SpaceTelescope will measure out to z~5.5.

We present results on the emission-line properties of 1.3≤z≤2.7 galaxies drawn from the complete the MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [O iii]λ 5007/Hβ versus [S ii]λλ6717,6731/Hα, i.e., the "[S ii] BPT diagram," to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset toward lower [S ii]λλ6717,6731/Hα at fixed [O iii]λ5007/Hβ, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O iii]λ5007/Hβ, local SDSS galaxies follow a trend of decreasing [S ii]λλ6717,6731/Hα as the surface density of star formation (ΣSFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f DIG) as ΣSFR increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H ii-region emission. The z∼0 relationship between f DIG and ΣSFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical ΣSFR. When an appropriate low-redshift benchmark with zero or minimal f DIG is used, high-redshift MOSDEF galaxies appear offset toward higher [S ii]λλ6717,6731/Hα and/or [O iii]λ 5007/Hβ. The joint shifts of high-redshift galaxies in the [S ii] and [N ii] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of H ii regions and diffuse ionized gas is an essential ingredient of our description of the interstellar medium over cosmic time.

We study the properties of 30 spectroscopically identified pairs of galaxies observed during the peak epoch of star formation in the universe. These systems are drawn from the MOSFIRE Deep Evolution Field (MOSDEF) Survey at 1.4 ≤ z ≤ 3.8, and are interpreted as early-stage galaxy mergers. Galaxy pairs in our sample are identified as two objects whose spectra were collected on the same Keck/MOSFIRE spectroscopic slit. Accordingly, all pairs in the sample have projected separations R proj ≤ 60 kpc. The velocity separation for pairs was required to be Δv ≤ 500 km s -1 , which is a standard threshold for defining interacting galaxy pairs at low redshift. Stellar mass ratios in our sample range from 1.1 to 550, with 12 ratios closer than or equal to 3:1, the common definition of a "major merger." Studies of merging pairs in the local universe indicate an enhancement in star formation activity and deficit in gas-phase oxygen abundance relative to isolated galaxies of the same mass. We compare the MOSDEF pairs sample to a control sample of isolated galaxies at the same redshift, finding no measurable SFR enhancement or metallicity deficit at fixed stellar mass for the pairs sample. The lack of significant difference between the average properties of pairs and control samples appears in contrast to results from low-redshift studies, although the small sample size and lower signal-to-noise of the high-redshift data limit definitive conclusions on redshift evolution. These results are consistent with some theoretical works, suggesting a reduced differential effect of precoalescence mergers on galaxy properties at high redshift - specifically that precoalescence mergers do not drive strong starbursts.

Studying the formation and evolution of galaxies at the earliest cosmictimes, and their role in reionization, requires the deepest imaging possible.Ultra-deep surveys like the HUDF and HFF have pushed to mag \mAB$\,\sim\,$30,revealing galaxies at the faint end of the LF to $z$$\,\sim\,$9$\,-\,$11 andconstraining their role in reionization. However, a key limitation of thesefields is their size, only a few arcminutes (less than a Mpc at theseredshifts), too small to probe large-scale environments or clusteringproperties of these galaxies, crucial for advancing our understanding ofreionization. Achieving HUDF-quality depth over areas $\sim$100 times largerbecomes possible with a mission like the Wide Field Infrared Survey Telescope(WFIRST), a 2.4-m telescope with similar optical properties to HST, with afield of view of $\sim$1000 arcmin$^2$, $\sim$100$\times$ the area of theHST/ACS HUDF. This whitepaper motivates an Ultra-Deep Field survey with WFIRST, covering$\sim$100$\,-\,$300$\times$ the area of the HUDF, or up to $\sim$1 deg$^2$, to\mAB$\,\sim\,$30, potentially revealing thousands of galaxies and AGN at thefaint end of the LF, at or beyond $z$\,$\sim$\,9$\,-\,$10 in the epoch ofreionization, and tracing their LSS environments, dramatically increasing thediscovery potential at these redshifts. (Note: This paper is a somewhat expanded version of one that was submitted asinput to the Astro2020 Decadal Survey, with this version including an Appendix(which exceeded the Astro2020 page limits), describing how the science driversfor a WFIRST Ultra Deep Field might map into a notional observing program,including the filters used and exposure times needed to achieve these depths.)

Galaxies evolve under the influence of gas flows between their interstellarmedium and their surrounding gaseous halos known as the circumgalactic medium(CGM). The CGM is a major reservoir of galactic baryons and metals, and plays akey role in the long cycles of accretion, feedback, and recycling of gas thatdrive star formation. In order to fully understand the physical processes atwork within galaxies, it is therefore essential to have a firm understanding ofthe composition, structure, kinematics, thermodynamics, and evolution of theCGM. In this white paper we outline connections between the CGM and galacticstar formation histories, internal kinematics, chemical evolution, quenching,satellite evolution, dark matter halo occupation, and the reionization of thelarger-scale intergalactic medium in light of the advances that will be made onthese topics in the 2020s. We argue that, in the next decade, fundamentalprogress on all of these major issues depends critically on improved empiricalcharacterization and theoretical understanding of the CGM. In particular, wediscuss how future advances in spatially-resolved CGM observations at highspectral resolution, broader characterization of the CGM across galaxy mass andredshift, and expected breakthroughs in cosmological hydrodynamic simulationswill help resolve these major problems in galaxy evolution.

The rest-frame UV emission from massive stars contains a wealth ofinformation about the physical nature and conditions of star formation ingalaxies. Using studies of the rest-frame UV, the past decade has witnessed thebeginning of knowledge about the existence and properties of galaxies duringthe first few billion years after the Big Bang. This period of historycorresponds to the formation of the first stars, the rapid formation of galaxystellar populations, the reionization of the IGM, the production anddissemination of heavy elements, and the formation of the first black holes.Massive stars in these galaxies drive all of these events, and their lightdominates the spectral energy distributions of galaxies. As we look to the2020s, fundamental questions remain about the nature of these stellarpopulations and their evolution, from just before the peak of the cosmic starformation density (z~3), up to the epoch of reionization (z > 6). This nextdecade will provide transformative gains both in our ability to identifystar-forming galaxies and accreting supermassive black holes at these earlyepochs with imaging surveys in the rest-frame UV (e.g., LSST, WFIRST).Ground-based, rest-frame UV spectroscopy on >20 m-class telescopes (e.g.,GMT/TMT) offers the ability to investigate the astrophysical conditions ingalaxies at the earliest cosmic times. This includes studies of the evolutionin galaxy stellar populations, gas ionization (temperature, pressure),metallicity, and interstellar (and circumgalactic) gas kinematics and coveringfractions. In this white paper, we describe the scientific prospects and therequirements for research in this area.

We present results from the MOSFIRE Deep Evolution Field survey on broad flux from the nebular emission lines Hα, [N ii], [O iii], Hβ, and [S ii]. The sample consists of 127 star-forming galaxies at 1.37

We use extensive spectroscopy from the MOSFIRE Deep Evolution Field survey to investigate the relationships between rest-frame optical emission line equivalent widths (W) and a number of galaxy and interstellar medium (ISM) characteristics for a sample of 1134 star-forming galaxies at redshifts 1.4 ≲ z ≲ 3.8. We examine how the equivalent widths of , , λλ4960, 5008, + Hβ, , and , depend on stellar mass, UV slope, age, star formation rate (SFR) and specific SFR (sSFR), ionization parameter and excitation conditions (O32 and /Hβ), gas-phase metallicity, and ionizing photon production efficiency (ξ ion). The trend of increasing W with decreasing stellar mass is strongest for (and +Hβ). More generally, the equivalent widths of all the lines increase with redshift at a fixed stellar mass or fixed gas-phase metallicity, suggesting that high equivalent width galaxies are common at high redshift. This redshift evolution in equivalent widths can be explained by the increase in SFR and decrease in metallicity with redshift at a fixed stellar mass. Consequently, the dependence of W on sSFR is largely invariant with redshift, particularly when examined for galaxies of a given metallicity. Our results show that high equivalent width galaxies, specifically those with high , have low stellar masses, blue UV slopes, young ages, high sSFRs, ISM line ratios indicative of high ionization parameters, high ξ ion, and low metallicities. As these characteristics are often attributed to galaxies with high ionizing escape fractions, galaxies with high W are likely candidates for the population that dominates cosmic reionization.

Using the MOSFIRE Deep Evolution Field (MOSDEF) rest-frame optical spectroscopic survey, we investigate the star formation histories (SFHs) of different galaxy types, ranging from actively star-forming to quiescent at 1.4 ≤ z ≤ 2.6. SFHs are constrained utilizing stellar continuum spectroscopy, specifically through a combination of Balmer absorption lines, the 4000 Å break, and the equivalent width of the Hα emission line. To attain a sufficiently high signal-to-noise ratio (S/N) to conduct these measurements we stack spectra of galaxies with similar spectral types, as determined from their rest-frame U - V and V - J colors. We bin the MOSDEF sample into five spectral types, subdividing the quiescent and star-forming bins to better explore galaxies transitioning between the two. We constrain the average SFHs for each type, finding that quiescent and transitional galaxies in the MOSDEF sample are dominated by an SFH with an average star formation timescale of τ ∼ 0.1-0.2 Gyr. These findings contrast with measurements from the low-redshift Universe where, on average, galaxies form their stars over a more extended time period (τ > 1 Gyr). Furthermore, our spectral index measurements correlate with mass surface density for all spectral types. Finally, we compare the average properties of the galaxies in our transitional bins to investigate possible paths to quiescence, and speculate on the viability of a dusty post-starburst phase.

We present an analysis using the MOSFIRE Deep Evolution Field (MOSDEF) survey on the nature of "MIR-excess" galaxies, which have star formation rates (SFRs) inferred from mid-infrared (MIR) data that are substantially elevated relative to those estimated from dust-corrected UV data. We use a sample of ∼200 galaxies and active galactic nuclei (AGNs) at 1.40 < z < 2.61 with 24 μm detections (rest-frame 8 μm) from MIPS/Spitzer. We find that the identification of MIR-excess galaxies strongly depends on the methodologies used to estimate IR luminosity (L1R) and to correct the UV light for dust attenuation. We find that extrapolations of the SFR from the observed 24 μm flux, using luminosity-dependent templates based on local galaxies, substantially overestimate in z ∼ 2 galaxies. By including Herschel observations and using a stellar-mass-dependent, luminosity-independent L1R, we obtain more reliable estimates of the SFR and a lower fraction of MIR-excess galaxies. Once stellar-mass selection biases are taken into account, we identify ∼24% of our galaxies as MIR excess. However, is not elevated in MIR-excess galaxies compared to MIR-normal galaxies, indicating that the intrinsic fraction of MIR excess may be lower. Using X-ray-, IR-, and optically selected AGNs in MOSDEF, we do not find a higher prevalence for AGNs in MIR-excess galaxies relative to MIR-normal galaxies. A stacking analysis of X-ray-undetected galaxies does not reveal a harder spectrum in MIR-excess galaxies relative to MIR-normal galaxies. Our analysis indicates that AGN activity does not contribute substantially to the MIR excess and instead implies that it is likely due to the enhanced polycyclic aromatic hydrocarbon emission.

We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ∼ 2-4. We match samples at different redshifts in UV luminosity and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Lyα equivalent width (EW) and various galaxy properties, we study the evolutionary trends among Lyα, low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Lyα and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Lyα, LIS lines, and dust extinction are redshift independent, as is the decoupling of the Lyα and HIS line strengths, and the bulk outflow kinematics as traced by the LIS lines. Stronger Lyα emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in the average Lyα strength with redshift, and the variation in Lyα strength at fixed redshift, can be explained in terms of variations in the neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Lyα and C iii] emission strengths, we additionally find evidence for variations in the intrinsic production rate of Lyα photons at the highest Lyα EWs. The challenge now is to understand the observed evolution of the neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6.

We investigate the nature of the relation among stellar mass, star formation rate, and gas-phase metallicity (the M∗-SFR-Z relation) at high redshifts using a sample of 260 star-forming galaxies at z∼2.3 from the MOSDEF survey. We present an analysis of the high-redshift M∗-SFR-Z relation based on several emission-line ratios for the first time. We show that a M∗-SFR-Z relation clearly exists at z∼2.3. The strength of this relation is similar to predictions from cosmological hydrodynamical simulations. By performing a direct comparison of stacks of z∼0 and z∼2.3 galaxies, we find that z∼2.3 galaxies have ∼0.1 dex lower metallicity at fixed M∗ and SFR. In the context of chemical evolution models, this evolution of the M∗-SFR-Z relation suggests an increase with redshift of the mass-loading factor at fixed M∗, as well as a decrease in the metallicity of infalling gas that is likely due to a lower importance of gas recycling relative to accretion from the intergalactic medium at high redshifts. Performing this analysis simultaneously with multiple metallicity-sensitive line ratios allows us to rule out the evolution in physical conditions (e.g., N/O ratio, ionization parameter, and hardness of the ionizing spectrum) at fixed metallicity as the source of the observed trends with redshift and with SFR at fixed M∗ at z∼2.3. While this study highlights the promise of performing high-order tests of chemical evolution models at high redshifts, detailed quantitative comparisons ultimately await a full understanding of the evolution of metallicity calibrations with redshift.

We combine Hα and Hβ spectroscopic measurements and UV photometry for a sample of 673 galaxies from the MOSDEF survey to constrain hydrogen-ionizing photon production efficiencies (ζion) at z = 1.4-2.6. We find 〈log(ζion/[s-1erg s-1Hz-1])〉=25.06(25.34), assuming the Calzetti (SMC) curve for the UV dust correction and a scatter of 0.28 dex in the ζiondistribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in ζionis likely dominated by galaxy-to-galaxy variations in stellar populations, including the slope and upper-mass cutoff of the initial mass function, stellar metallicity, star formation burstiness, and stellar evolution (e.g., single/binary star evolution). Moreover, ζionis elevated in galaxies with high ionization states (high [O iii]/[O ii]) and low oxygen abundances (low [N ii]/Hα and high [O iii]/Hβ) in the ionized ISM. However, ζiondoes not correlate with the offset from the z ∼ 0 star-forming locus in the BPT diagram, suggesting no change in the hardness of the ionizing radiation accompanying the offset from the z ∼ 0 sequence. We also find that galaxies with blue UV spectral slopes (〈β〉=-2.1) have ζionelevated by a factor of ∼2 relative to the average ζionof the sample (〈β〉=-1.4). If these blue galaxies are similar to those at z > 6, our results suggest that a lower Lyman-continuum escape fraction is required for galaxies to maintain reionization, compared to the canonical predictions from stellar population models. Furthermore, we demonstrate that even with robustly dust-corrected Hα, the UV dust attenuation can cause on average a ∼0.3 dex systematic uncertainty in calculations.

We combine Hα and Hβ spectroscopic measurements and UV photometry for a sample of 673 galaxies from the MOSDEF survey to constrain hydrogen-ionizing photon production efficiencies (ζion) at z = 1.4-2.6. We find 〈log(ζion/[s-1erg s-1Hz-1])〉=25.06(25.34), assuming the Calzetti (SMC) curve for the UV dust correction and a scatter of 0.28 dex in the ζiondistribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in ζionis likely dominated by galaxy-to-galaxy variations in stellar populations, including the slope and upper-mass cutoff of the initial mass function, stellar metallicity, star formation burstiness, and stellar evolution (e.g., single/binary star evolution). Moreover, ζionis elevated in galaxies with high ionization states (high [O iii]/[O ii]) and low oxygen abundances (low [N ii]/Hα and high [O iii]/Hβ) in the ionized ISM. However, ζiondoes not correlate with the offset from the z ∼ 0 star-forming locus in the BPT diagram, suggesting no change in the hardness of the ionizing radiation accompanying the offset from the z ∼ 0 sequence. We also find that galaxies with blue UV spectral slopes (〈β〉=-2.1) have ζionelevated by a factor of ∼2 relative to the average ζionof the sample (〈β〉=-1.4). If these blue galaxies are similar to those at z > 6, our results suggest that a lower Lyman-continuum escape fraction is required for galaxies to maintain reionization, compared to the canonical predictions from stellar population models. Furthermore, we demonstrate that even with robustly dust-corrected Hα, the UV dust attenuation can cause on average a ∼0.3 dex systematic uncertainty in calculations.

Using observations from the first 2 yr of the MOSFIRE Deep Evolution Field (MOSDEF) survey, we study 13 active galactic nucleus (AGN) driven outflows detected from a sample of 67 X-ray, IR, and/or optically selected AGNs at. The AGNs have bolometric luminosities of, including both quasars and moderate-luminosity AGNs. We detect blueshifted, ionized gas outflows in the Hβ, [O iii], Hα, and/or [N ii] emission lines of 19% of the AGNs, while only 1.8% of the MOSDEF galaxies have similarly detected outflows. The outflow velocities span ∼300 to 1000 km s-1. Eight of the 13 outflows are spatially extended on similar scales to the host galaxies, with spatial extents of 2.5-11.0 kpc. Outflows are detected uniformly across the star-forming main sequence, showing little trend with the host galaxy star formation rate. Line ratio diagnostics indicate that the outflowing gas is photoionized by the AGNs. We do not find evidence for positive AGN feedback, in either our small MOSDEF sample or a much larger Sloan Digital Sky Survey sample, using the BPT diagram. Given that a galaxy with an AGN is 10 times more likely to have a detected outflow, the outflowing gas is photoionized by the AGNs, and estimates of the mass and energy outflow rates indicate that stellar feedback is insufficient to drive at least some of these outflows; they are very likely to be AGN driven. The outflows have mass-loading factors of the order of unity, suggesting that they help regulate star formation in their host galaxies, though they may be insufficient to fully quench it.

We present the first spectroscopic measurement of multiple rest-frame optical emission lines at z > 4. During the MOSFIRE Deep Evolution Field survey, we observed the galaxy GOODSN-17940 with the Keck I/MOSFIRE spectrograph. The K-band spectrum of GOODSN-17940 includes significant detections of the [O ii]λλ3726,3729, [Ne iii]λ3869, and Hγ emission lines and a tentative detection of Hδ, indicating z spec = 4.4121. GOODSN-17940 is an actively star-forming z > 4 galaxy based on its K-band spectrum and broadband spectral energy distribution. A significant excess relative to the surrounding continuum is present in the Spitzer/IRAC channel 1 photometry of GOODSN-17940, due primarily to strong Hα emission with a rest-frame equivalent width of EW(Hα) = 1200. Based on the assumption of 0.5 Z o models and the Calzetti attenuation curve, GOODSN-17940 is characterized by . The Balmer decrement inferred from Hα/Hγ is used to dust correct the Hα emission, yielding . These M ∗ and star formation rate (SFR) values place GOODSN-17940 an order of magnitude in SFR above the z ∼ 4 star-forming "main sequence." Finally, we use the observed ratio of [Ne iii]/[O ii] to estimate the nebular oxygen abundance in GOODSN-17940, finding O/H ∼ 0.2 (O/H)o. Combining our new [Ne iii]/[O ii] measurement with those from stacked spectra at z ∼ 0, 2, and 3, we show that GOODSN-17940 represents an extension to z > 4 of the evolution toward higher [Ne iii]/[O ii] (i.e., lower O/H) at fixed stellar mass. It will be possible to perform the measurements presented here out to z ∼ 10 using the James Webb Space Telescope.

The C iii]λλ1907, 1909 rest-frame UV emission doublet has recently been detected in galaxies during the epoch of reionization (z > 6), with a high equivalent width (EW; 10 Å, rest frame). Currently, it is possible to obtain much more detailed information for star-forming galaxies at significantly lower redshift. Accordingly, studies of their far-UV spectra are useful for understanding the factors modulating the strength of C iii] emission. We present the first statistical sample of C iii] emission measurements in star-forming galaxies at z ∼ 1. Our sample is drawn from the DEEP2 survey and spans the redshifts 0.64 z 1.35 ((z) = 1.08). We find that the median EW of individual C iii] detections in our sample (1.30 Å) is much smaller than the typical value observed thus far at z > 6. Furthermore, out of 184 galaxies with coverage of C iii], only 40 have significant detections. Galaxies with individual C iii] detections have bluer colors and lower luminosities on average than those without, implying that strong C iii] emitters are in general young and low-mass galaxies without significant dust extinction. Using stacked spectra, we further investigate how C iii] strength correlates with multiple galaxy properties (M B, U - B, M ∗, star formation rate, specific star formation rate) and rest-frame near-UV (Fe ii∗ and Mg ii) and optical ([O iii] and Hβ) emission line strengths. These results provide a detailed picture of the physical environment in star-forming galaxies at z ∼ 1, and motivate future observations of strong C iii] emitters at similar redshifts.

We present results on the variation of 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission in galaxies spanning a wide range in metallicity at z ∼ 2. For this analysis, we use rest-frame optical spectra of 476 galaxies at 1.37 ≤ z ≤ 2.61 from the MOSFIRE Deep Evolution Field (MOSDEF) survey to infer metallicities and ionization states. Spitzer/MIPS 24 μm and Herschel/PACS 100 and 160 μm observations are used to derive rest-frame 7.7 μm luminosities (L7.7) and total IR luminosities (LIR), respectively. We find significant trends between the ratio of L7.7 to LIR (and to dust-corrected star formation rate [SFR]) and both metallicity and [O iii]/[O ii] (O32) emission line ratio. The latter is an empirical proxy for the ionization parameter. These trends indicate a paucity of PAH emission in low-metallicity environments with harder and more intense radiation fields. Additionally L7.7, is significantly lower in the youngest quartile of our sample (ages of ≲500 Myr) compared to older galaxies, which may be a result of the delayed production of PAHs by AGB stars. The relative strength of L7.7 to LIR is also lower by a factor of ∼2 for galaxies with masses M∗ ∼ 109.6-10.0, compared to the more massive ones. We demonstrate that commonly used conversions of (or 24 μm flux density, f 24) to underestimate the IR luminosity by more than a factor of 2 at . We adopt a mass-dependent conversion of to with = 0.09 and 0.22 for M∗ ≤ 1010 and > 1010M⊙, respectively. Based on the new scaling, the SFR-M ∗ relation has a shallower slope than previously derived. Our results also suggest a higher IR luminosity density at z ∼ 2 than previously measured, corresponding to a ∼30% increase in the SFR density.

We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on the identification, selection biases, and host galaxy properties of 55 X-ray, IR, and optically selected active galactic nuclei (AGNs) at 1.4 < z < 3.8. We obtain rest-frame optical spectra of galaxies and AGNs and use the BPT diagram to identify optical AGNs. We examine the uniqueness and overlap of the AGNs identified at different wavelengths. There is a strong bias against identifying AGNs at any wavelength in low-mass galaxies, and an additional bias against identifying IR AGNs in the most massive galaxies. AGN hosts span a wide range of star formation rates (SFRs), similar to inactive galaxies once stellar mass selection effects are accounted for. However, we find (at ∼ 2-3σ significance) that IR AGNs are in less dusty galaxies with relatively higher SFR and optical AGNs in dusty galaxies with relatively lower SFR. X-ray AGN selection does not display a bias with host galaxy SFR. These results are consistent with those from larger studies at lower redshifts. Within star-forming galaxies, once selection biases are accounted for, we find AGNs in galaxies with similar physical properties as inactive galaxies, with no evidence for AGN activity in particular types of galaxies. This is consistent with AGNs being fueled stochastically in any star-forming host galaxy. We do not detect a significant correlation between SFR and AGN luminosity for individual AGN hosts, which may indicate the timescale difference between the growth of galaxies and their supermassive black holes.

Unlike spiral galaxies such as the Milky Way, the majority of the stars in massive elliptical galaxies were formed in a short period early in the history of the Universe. The duration of this formation period can be measured using the ratio of magnesium to iron abundance ([Mg/Fe]) in spectra, which reflects the relative enrichment by core-collapse and type Ia supernovae. For local galaxies, [Mg/Fe] probes the combined formation history of all stars currently in the galaxy, including younger and metal-poor stars that were added during late-time mergers. Therefore, to directly constrain the initial star-formation period, we must study galaxies at earlier epochs. The most distant galaxy for which [Mg/Fe] had previously been measured is at a redshift of z ≈ 1.4, with [Mg/Fe] = . A slightly earlier epoch (z ≈ 1.6) was probed by combining the spectra of 24 massive quiescent galaxies, yielding an average [Mg/Fe] = 0.31 ± 0.12 (ref. 7). However, the relatively low signal-to-noise ratio of the data and the use of index analysis techniques for both of these studies resulted in measurement errors that are too large to allow us to form strong conclusions. Deeper spectra at even earlier epochs in combination with analysis techniques based on full spectral fitting are required to precisely measure the abundance pattern shortly after the major star-forming phase (z > 2). Here we report a measurement of [Mg/Fe] for a massive quiescent galaxy at a redshift of z = 2.1, when the Universe was three billion years old. With [Mg/Fe] = 0.59 ± 0.11, this galaxy is the most Mg-enhanced massive galaxy found so far, having twice the Mg enhancement of similar-mass galaxies today. The abundance pattern of the galaxy is consistent with enrichment exclusively by core-collapse supernovae and with a star-formation timescale of 0.1 to 0.5 billion years-characteristics that are similar to population II stars in the Milky Way. With an average past star-formation rate of 600 to 3,000 solar masses per year, this galaxy was among the most vigorous star-forming galaxies in the Universe.

We present the first statistical sample of rest-frame far-UV spectra of star-forming galaxies at z ∼ 1. These spectra are unique in that they cover the high-ionization C iv λλ1548, 1550 doublet. We also detect low-ionization features such as Si ii λ1526, Fe ii λ1608, Al ii λ1670, Ni ii λλ1741, 1751, and Si ii λ1808, and intermediate-ionization features from Al iii λλ1854, 1862. Comparing the properties of absorption lines of lower- and higher-ionization states provides a window into the multiphase nature of circumgalactic gas. Our sample is drawn from the DEEP2 survey and spans the redshift range 1.01 ≤ z ≤ 1.35 (〈z〉 = 1.25). By isolating the interstellar C iv absorption from the stellar P Cygni wind profile, we find that 69% of the C iv profiles are blueshifted with respect to the systemic velocity. Furthermore, C iv shows a small but significant blueshift relative to Fe ii (offset of the best-fit linear regression -76 ± 26 km s-1). At the same time, the C iv blueshift is on average comparable to that of Mg ii λλ2796, 2803. At this point, in explaining the larger blueshift of C iv absorption at the ∼3σ level, we cannot distinguish between the faster motion of highly ionized gas relative to gas traced by Fe ii and filling in on the red side from resonant C iv emission. We investigate how far-UV interstellar absorption kinematics correlate with other galaxy properties using stacked spectra. These stacking results show a direct link between C iv absorption and the current star formation rate, though we only observe small velocity differences among different ionization states tracing the outflowing interstellar medium.

We present measurements of the electron-temperature-based oxygen abundance for a highly star-forming galaxy at z = 3.08, COSMOS-1908. This is the highest redshift at which [O iii]λ4363 has been detected and the first time that this line has been measured at z > 2. We estimate an oxygen abundance of 12 + log(O/H) = 8.00-0.14+0.13. This galaxy is a low-mass (109.3 M⊙), highly star-forming (∼50M⊙ yr-1) system that hosts a young stellar population (∼160 Myr). We investigate the physical conditions of the ionized gas in COSMOS-1908 and find that this galaxy has a high ionization parameter, little nebular reddening (E(B-V)gas < 0.14), and a high electron density (ne ∼ 500 cm-3). We compare the ratios of strong oxygen, neon, and hydrogen lines to the direct-method oxygen abundance for COSMOS-1908 and additional star-forming galaxies at z=0-1.8 with [O iii]λ4363 measurements and show that galaxies at z ∼ 1-3 follow the same strong-line correlations as galaxies in the local universe. This agreement suggests that the relationship between ionization parameter and O/H is similar for z∼0 and high-redshift galaxies. These results imply that metallicity calibrations based on lines of oxygen, neon, and hydrogen do not strongly evolve with redshift and can reliably estimate abundances out to Z ∼ 3, paving the way for robust measurements of the evolution of the mass-metallicity relation to high redshift.

We present the first direct comparison between Balmer line and panchromatic spectral energy distribution (SED)-based star formation rates (SFRs) for z ∼ 2 galaxies. For this comparison, we used 17 star-forming galaxies selected from the MOSFIRE Deep Evolution Field (MOSDEF) survey, with 3σ detections for Hα and at least two IR bands (Spitzer/MIPS 24 μm and Herschel/PACS 100 and 160 μm, and in some cases Herschel/SPIRE 250, 350, and 500 μm). The galaxies have total IR (8-1000 μm) luminosities of ∼ 1011.4-1012.4 L⊙ and SFRs of ∼30-250 M⊙ yr-1. We fit the UV-to-far-IR SEDs with flexible stellar population synthesis (FSPS) models - which include both stellar and dust emission - and compare the inferred SFRs with the SFR(Hα, Hβ) values corrected for dust attenuation using Balmer decrements. The two SFRs agree with a scatter of 0.17 dex. Our results imply that the Balmer decrement accurately predicts the obscuration of the nebular lines and can be used to robustly calculate SFRs for star-forming galaxies at z ∼ 2 with SFRs up to ∼ 200 M⊙ yr-1. We also use our data to assess SFR indicators based on modeling the UV-to-mid-IR SEDs or by adding SFR(UV) and SFR(IR), for which the latter is based on the mid-IR only or on the full IR SED. All these SFRs show a poorer agreement with SFR(Hα, Hβ) and in some cases large systematic biases are observed. Finally, we show that the SFR and dust attenuation derived from the UV-to-near-IR SED alone are unbiased when assuming a delayed exponentially declining star formation history.

We present Ha gas kinematics for 178 star-forming galaxies at z ∼ 2 from the MOSFIRE Deep Evolution Field survey. We have developed models to interpret the kinematic measurements from fixed-angle multi-object spectroscopy, using structural parameters derived from Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey Hubble Space Telescope/F160W imaging. For 35 galaxies, we measure resolved rotation with a median of (V/σV,0 )RE 2.1 ( s ) = . We derive dynamical masses from the kinematics and sizes and compare them to baryonic masses, with gas masses estimated from dust-corrected Ha star formation rates (SFRs) and the Kennicutt-Schmidt relation. When assuming that galaxies with and without observed rotation have the same median (V/σV,0 )RE, we find good agreement between the dynamical and baryonic masses, with a scatter of σrms = 0.34 dex and a median offset of log10 M 0.04 dex D = . This comparison implies a low dark matter fraction (8% within an effective radius) for a Chabrier initial mass function (IMF), and disfavors a Salpeter IMF. Moreover, the requirement that Mdyn/Mbaryon should be independent of inclination yields a median value of (V/σV,0 )RE = 2.1 for galaxies without observed rotation. If, instead, we treat the galaxies without detected rotation as early-type galaxies, the masses are also in reasonable agreement (δlog10 M = 0.07 dex D = - , σrms = 0.37 dex). The inclusion of gas masses is critical in this comparison; if gas masses are excluded, there is an increasing trend of Mdyn/M∗ with higher specific SFR (SSFR). Furthermore, we find indications that V s decreases with increasing Ha SSFR for our full sample, which may reflect disk settling. We also study the TullyFisher relation and find that at fixed stellar mass S0.5 0.5V2.2 + σ V2,0 2 )1/2 was higher at earlier times. At fixed baryonic mass, we observe the opposite trend. Finally, the baryonic and dynamical masses of the active galactic nuclei in our sample are also in excellent agreement, suggesting that the kinematics trace the host galaxies.

Using observations from the MOSFIRE Deep Evolution Field 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 toward significantly higher O32 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 O32 versus R23 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.

We present results on the star formation rate (SFR) versus stellar mass (M∗) relation (i.e., the "main sequence") among star-forming galaxies at 1.37 ≤ z ≤ 2.61 using the MOSFIRE Deep Evolution Field (MOSDEF) survey. Based on a sample of 261 galaxies with Hα and Hβ spectroscopy, we have estimated robust dust-corrected instantaneous SFRs over a large range in M∗ (∼109.5-1011.5 Mo). We find a correlation between log(SFR(Hα)) and log(M∗) with a slope of 0.65 ± 0.08 (0.58 ± 0.10) at 1.4 < z < 2.6 (2.1 < z < 2.6). We find that different assumptions for the dust correction, such as using the color excess of the stellar continuum to correct the nebular lines, sample selection biases against red star-forming galaxies, and not accounting for Balmer absorption, can yield steeper slopes of the log(SFR)-log(M∗) relation. Our sample is immune from these biases as it is rest-frame optically selected, Hα and Hβ are corrected for Balmer absorption, and the Hα luminosity is dust corrected using the nebular color excess computed from the Balmer decrement. The scatter of the log(SFR(Hα))-log(M∗) relation, after accounting for the measurement uncertainties, is 0.31 dex at 2.1 < z < 2.6, which is 0.05 dex larger than the scatter in log(SFR(UV))-log(M∗). Based on comparisons to a simulated SFR-M∗ relation with some intrinsic scatter, we argue that in the absence of direct measurements of galaxy-to-galaxy variations in the attenuation/extinction curves and the initial mass function, one cannot use the difference in the scatter of the SFR(Hα)- and SFR(UV)-M∗ relations to constrain the stochasticity of star formation in high-redshift galaxies.

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 (∼3700-7000 ) for ∼1500 galaxies at in three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are selected in three redshift intervals:, down to fixed (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., [O ii], Hβ, [O iii], H, [N ii], and [S ii]) and stellar continuum and absorption features (e.g., Balmer lines, Ca-ii H and K, Mgb, 4000 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 () and star formation rate. 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 dust attenuation curve of z ∼ 2 galaxies using early observations from the MOSFIRE Deep Evolution Field survey. Our sample consists of 224 star-forming galaxies with zspec = 1.36-2.59 and high signal-to-noise ratio measurements of Hα and Hβ obtained with Keck/MOSFIRE. We construct composite spectral energy distributions (SEDs) of galaxies in bins of Balmer decrement to measure the attenuation curve. We find a curve that is similar to the SMC extinction curve at λ ≳ 2500 Å. At shorter wavelengths, the shape is identical to that of the Calzetti et al. relation, but with a lower normalization. Hence, the new attenuation curve results in star formation rates (SFRs) that are ≈20% lower, and stellar masses that are Δ log(M∗/M⊙) ≃ 0.16 dex lower, than those obtained with the Calzetti relation. We find that the difference in the total attenuation of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs ≳ 20 Mȯ yr-1, assuming a Chabrier initial mass function, 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 Mȯ yr-1.

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 rest-frame 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 [O iii]λ5007/Hβ vs. [N ii]/Hα and [O iii]λ5007/Hβ vs. [S ii]λλ6717, 6731/Hα "BPT" diagrams, as well as the O32 vs. R23 excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the [O iii]λ5007/Hβ vs. [N ii]/Hα BPT diagram relative to Sloan Digital Sky Survey star-forming 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 toward higher [N ii]/Hα at fixed [O iii]/Hβ. Within the [O iii]λ5007/Hβ vs. [S ii]/Hα and O32 vs. R23 diagrams, we find that z ∼ 2.3 galaxies are distributed like local ones, and therefore attribute the shift in the [O iii]λ5007/Hβ vs. [N ii]/Hα BPT diagram to elevated N/O abundance ratios among lower-mass () 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 O32 vs. R23 diagram paves the way for using the combination of O32 and R23 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 present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on rest-frame optical active galactic nucleus (AGN) identification and completeness at z ∼ 2.3. With our sample of 50 galaxies and 10 X-ray and IR-selected AGNs with measured Hβ, [O III], Hα, and N II emission lines, we investigate the location of AGNs in the BPT, MEx (mass-excitation), and CEx (color-excitation) diagrams. We find that th BPT diagram works well to identify AGNs at z ∼ 2.3 and that the z ∼ 0 AGN/star-forming galaxy classifications do not need to shift substantially at z ∼ 2.3 to robustly separate these populations. However, the MEx diagram fails to identify all of the AGN identified in the BPT diagram, and the CEx diagram is substantially contaminated at high redshift. We further show that AGN samples selected using the BPT diagram have selection biases in terms of both host stellar mass and stellar population, in that AGNs in low mass and/or high specific star formation rate galaxies are difficult to identify using the BPT diagram. These selection biases become increasingly severe at high redshift, such that optically selected AGN samples at high redshift will necessarily be incomplete. We also find that the gas in the narrow-line region appears to be more enriched than gas in the host galaxy for at least some MOSDEF AGNs. However, AGNs at z ∼ 2 are generally less enriched than local AGNs with the same host stellar mass.

We present results on the z ∼ 2.3 mass-metallicity relation (MZR) using early observations from the MOSFIRE Deep Evolution Field survey. We use an initial sample of 87 star-forming galaxies with spectroscopic coverage of Hβ, [OIII] λ5007, Hα, and [NII] λ6584 rest-frame optical emission lines, and estimate the gas-phase oxygen abundance based on the N2 and O3N2 strong-line indicators. We find a positive correlation between stellar mass and metallicity among individual z ∼ 2.3 galaxies using both the N2 and O3N2 indicators. We also measure the emission-line ratios and corresponding oxygen abundances for composite spectra in bins of stellar mass. Among composite spectra, we find a monotonic increase in metallicity with increasing stellar mass, offset ∼0.15-0.3 dex below the local MZR. When the sample is divided at the median star-formation rate (SFR), we do not observe significant SFR dependence of the z ∼ 2.3 MZR among either individual galaxies or composite spectra. We furthermore find that z ∼ 2.3 galaxies have metallicities ∼0.1 dex lower at a given stellar mass and SFR than is observed locally. This offset suggests that high-redshift galaxies do not fall on the local "fundamental metallicity relation" among stellar mass, metallicity, and SFR, and may provide evidence of a phase of galaxy growth in which the gas reservoir is built up due to inflow rates that are higher than star-formation and outflow rates. However, robust conclusions regarding the gas-phase oxygen abundances of high-redshift galaxies await a systematic reappraisal of the application of locally calibrated metallicity indicators at high redshift.

We present a study of the prevalence, strength, and kinematics of ultraviolet Fe II and Mg II emission lines in 212 star-forming galaxies at z 1 selected from the DEEP2 survey. We find Fe II* emission in composite spectra assembled on the basis of different galaxy properties, indicating that Fe II* emission is common at z 1. In these composites, Fe II* emission is observed at roughly the systemic velocity. At z 1, we find that the strength of Fe II* emission is most strongly modulated by dust attenuation, and is additionally correlated with redshift, star formation rate, and [O II] equivalent width, such that systems at higher redshifts with lower dust levels, lower star formation rates, and larger [O II] equivalent widths show stronger Fe II* emission. We detect Mg II emission in at least 15% of the individual spectra and we find that objects showing stronger Mg II emission have higher specific star formation rates, smaller [O II] linewidths, larger [O II] equivalent widths, lower dust attenuations, and lower stellar masses than the sample as a whole. Mg II emission strength exhibits the strongest correlation with specific star formation rate, although we find evidence that dust attenuation and stellar mass also play roles in the regulation of Mg II emission. Future integral field unit observations of the spatial extent of Fe II* and Mg II emission in galaxies with high specific star formation rates, low dust attenuations, and low stellar masses will be important for probing the morphology of circumgalactic gas. © 2013. The American Astronomical Society. All rights reserved..

We report matched resolution imaging spectroscopy of the CO 3-2 e (with the IRAM Plateau de Bure millimeter interferometer) and of the Hα e (with LUCI at the Large Binocular Telescope) in the massive z = 1.53 main-sequence galaxy EGS 13011166, as part of the "Plateau de Bure high-z, blue-sequence survey" (PHIBSS: Tacconi et al.). We combine these data with Hubble Space Telescope V-I-J-H-band maps to derive spatially resolved distributions of stellar surface density, star formation rate, molecular gas surface density, optical extinction, and gas kinematics. The spatial distribution and kinematics of the ionized and molecular gas are remarkably similar and are well modeled by a turbulent, globally Toomre unstable, rotating disk. The stellar surface density distribution is smoother than the clumpy rest-frame UV/optical light distribution and peaks in an obscured, star-forming massive bulge near the dynamical center. The molecular gas surface density and the effective optical screen extinction track each other and are well modeled by a "mixed" extinction model. The inferred slope of the spatially resolved molecular gas to star formation rate relation, N = dlogΣstar form/dlogΣmol gas, depends strongly on the adopted extinction model, and can vary from 0.8 to 1.7. For the preferred mixed dust-gas model, we find N = 1.14 ± 0.1. © 2013. The American Astronomical Society. All rights reserved.

The Walras core of an economy is the set of allocations that are attainable for the consumers when their trades are constrained to be based on some agreed set of prices, and such that no alternative price system exists for any sub-coalition that allows all members to trade to something better. As compared with the Edgeworth core, both coalitional improvements and being a candidate allocation for the Walras core become harder. The Walras core may even contain allocations that violate the usual Pareto effciency. Nevertheless, the competitive allocations are the same under the two theories, and the equal-treatment Walras core allocations converge under general conditions to the competitive allocations in the process of replication.

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of approximately 1.7, the farthest SN observed to date. The supernova, SN 1997, was discovered in a repeat observation by the Hubble Space Telescope (HST) of the Hubble Deep Field{North (HDF-N), and serendipitously monitored with NICMOS on HST throughout the Thompson et al. GTO campaign. The SN type can be determined from the host galaxy type: an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse supernovae. The classification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photometric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z = 1:7 plus or minus 0:1, is in excellent agreement with the redshift estimate of z = 1:65 plus or minus 0:15 derived from the U_300 B_450 V_-606 I_814 J_110 J_125 H_160 H_165 K_s photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M approximately equal to 1/3; Omega_Lambda approximately equal to 2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astrophysical effects which could mimic previous evidence for an accelerating Universe from SNe Ia at z approximately equal to 0:5. We consider several sources of potential systematic error including gravitational lensing, supernova misclassification, sample selection bias, and luminosity calibration errors. Currently, none of these effects alone appears likely to challenge our conclusions. Additional SNe Ia at z > 1 will be required to test more exotic alternatives to the accelerating Universe hypothesis and to probe the nature of dark energy.