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Abstract We present Keck Cosmic Web Imager integral field observations of extended Lyαemission in the circumgalactic medium of 27 typical star-forming galaxies atz∼ 2, drawn from the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) Deep Evolution Field (MOSDEF) survey. Using composite spectra in two bins of star formation rate (SFR), star formation rate surface density (Σ_SFR), and other galactic properties, we measure spatial variations in the Lyαprofile across three regions in the Lyαhalo. We find single-peaked, redshifted profiles are ubiquitous within a central 7 kpc radius region. Further out in the halo (7–14 and 14–21 kpc), the Lyαprofile of the resonantly scattered emission exhibits more diversity, either transitioning to a double-peaked profile or remaining single peaked across the halo. We find a shorter scale length of the Lyαhalo surface brightness profile for composite halos with faster winds. The composites have a similar average inclination, suggesting those with faster winds clear channels in the interstellar medium (ISM), reducing the fraction of Lyαphotons resonantly scattered to large radii. A uniform expanding shell radiative transfer model reproduces the shape but not the normalization of the observed double-peaked Lyαprofiles. Models that adopt a more realistic, clumpy ISM are likely needed to reproduce both the shape and normalization of the Lyαprofiles. 

Abstract We present Keck Cosmic Web Imager integral-field unit observations around extended Lyαhalos of 27 typical star-forming galaxies with redshifts 2.0 <z< 3.2 drawn from the MOSFIRE Deep Evolution Field survey. We examine the average Lyαsurface brightness profiles in bins of star formation rate (SFR), stellar mass (M_*), age, stellar continuum reddening, SFR surface density (Σ_SFR), and Σ_SFRnormalized by stellar mass (Σ_sSFR). The scale lengths of the halos correlate with stellar mass, age, and stellar continuum reddening and anticorrelate with SFR, Σ_SFR, and Σ_sSFR. These results are consistent with a scenario in which the down-the-barrel fraction of Lyαemission is modulated by the low-column-density channels in the interstellar medium, and in which the neutral gas covering fraction is related to the physical properties of the galaxies. Specifically, we find that this covering fraction increases with stellar mass, age, andE(B−V) and decreases with SFR, Σ_SFR, and Σ_sSFR. We also find that the resonantly scattered Lyαemission suffers greater attenuation than the (nonresonant) stellar continuum emission, and that the difference in attenuation increases with stellar mass, age, and stellar continuum reddening, and decreases with Σ_sSFR. These results imply that more reddened galaxies have more dust in their circumgalactic medium. 

ABSTRACT We use the large spectroscopic data set of the MOSFIRE Deep Evolution Field survey to investigate the kinematics and energetics of ionized gas outflows. Using a sample of 598 star-forming galaxies at redshift 1.4 < z < 3.8, we decompose [O iii] and $$\rm {H}\,\alpha$$ emission lines into narrow and broad components, finding significant detections of broad components in 10 per cent of the sample. The ionized outflow velocity from individual galaxies appears independent of galaxy properties, such as stellar mass, star formation rate (SFR), and SFR surface density (ΣSFR). Adopting a simple outflow model, we estimate the mass-, energy-, and momentum-loading factors of the ionized outflows, finding modest values with averages of 0.33, 0.04, and 0.22, respectively. The larger momentum- than energy-loading factors, for the adopted physical parameters, imply that these ionized outflows are primarily momentum driven. We further find a marginal correlation (2.5σ) between the mass-loading factor and stellar mass in agreement with predictions by simulations, scaling as ηm$$\propto M_{\star }^{-0.45}$$. This shallow scaling relation is consistent with these ionized outflows being driven by a combination of mechanical energy generated by supernovae explosions and radiation pressure acting on dusty material. In a majority of galaxies, the outflowing material does not appear to have sufficient velocity to escape the gravitational potential of their host, likely recycling back at later times. Together, these results suggest that the ionized outflows traced by nebular emission lines are negligible, with the bulk of mass and energy carried out in other gaseous phases. 

ABSTRACT We present determinations of the gas-phase and stellar metallicities of a sample of 65 star-forming galaxies at $$z \simeq 3.5$$ using rest-frame far-ultraviolet (FUV) spectroscopy from the VANDELS survey in combination with follow-up rest-frame optical spectroscopy from VLT/KMOS and Keck/MOSFIRE. We infer gas-phase oxygen abundances ($$Z_{\mathrm{g}}$$; tracing O/H) via strong optical nebular lines and stellar iron abundances ($$Z_{\star }$$; tracing Fe/H) from full spectral fitting to the FUV continuum. Our sample spans the stellar mass range $$8.5 \lt \mathrm{log}(M_{\star }/\mathrm{M}_{\odot }) \lt 10.5$$ and shows clear evidence for both a stellar and gas-phase mass-metallicity relation (MZR). We find that our O and Fe abundance estimates both exhibit a similar mass-dependence, such that $$\mathrm{Fe/H}\propto M_{\star }^{0.30\pm 0.11}$$ and $$\mathrm{O/H}\propto M_{\star }^{0.32\pm 0.09}$$. At fixed $$M_{\star }$$ we find that, relative to their solar values, O abundances are systematically larger than Fe abundances (i.e. α-enhancement). We estimate an average enhancement of $$\mathrm{(O/Fe)} = 2.65 \pm 0.16 \times \mathrm{(O/Fe)_\odot }$$ which appears to be independent of $$M_{\star }$$. We employ analytic chemical evolution models to place a constraint on the strength of galactic-level outflows via the mass-outflow factor ($$\eta$$). We show that outflow efficiencies that scale as $$\eta \propto M_{\star }^{-0.32}$$ can simultaneously explain the functional form of of the stellar and gas-phase MZR, as well as the degree of α-enhancement at fixed Fe/H. Our results add further evidence to support a picture in which α-enhanced abundance ratios are ubiquitous in high-redshift star-forming galaxies, as expected for young systems whose interstellar medium is primarily enriched by core-collapse supernovae. 

ABSTRACT We report on the discovery of cool gas inflows towards three star-forming galaxies at <z> ∼ 2.30. Analysis of Keck Low-Resolution Imaging Spectrometer spectroscopy reveals redshifted low-ionization interstellar (LIS) metal absorption lines with centroid velocities of 60–130 km s−1. These inflows represent some of the most robust detections of inflowing gas into isolated, star-forming galaxies at high redshift. Our analysis suggests that the inflows are due to recycling metal-enriched gas from previous ejections. Comparisons between the galaxies with inflows and a larger parent sample of 131 objects indicate that galaxies with detected inflows may have higher specific star formation rates (sSFRs) and star-formation-rate surface densities (ΣSFR). However, when additional galaxies without robustly detected inflows based on centroid velocity but whose LIS absorption line profiles indicate large red-wing velocities are considered, galaxies with inflows do not show unique properties relative to those lacking inflows. Additionally, we calculate the covering fraction of cool inflowing gas as a function of red-wing inflow velocity, finding an enhancement in high-sSFR binned galaxies, likely due to an increase in the amount of recycling gas. Together, these results suggest that the low detection rate of galaxies with cool inflows is primarily related to the viewing angle rather than the physical properties of the galaxies. 

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 (Σ_SFR). Using a sample of 317 galaxies with spectroscopic redshiftsz_spec≃ 1.9–3.7, we construct composite rest-frame optical spectra in bins of Σ_SFRand infer electron densities,n_e, using the ratio of the [Oii]λλ3727, 3730 doublet. Our analysis suggests a significant (≃3σ) correlation betweenn_eand Σ_SFR. We further find significant correlations betweenUand Σ_SFRfor composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences ofU. The increase inn_e—and possibly also the volume filling factor of dense clumps in Hiiregions—with Σ_SFRappear to be important factors in explaining the relationship betweenUand Σ_SFR. Further, the increase inn_eand SFR with redshift at a fixed stellar mass can account for most of the redshift evolution ofU. These results suggest that the gas density, which setsn_eand 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 Hαemission line and coverage of Hβinto 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 medianA_Vand UV continuum spectral slope (β). First, we find that none of the dust properties (Balmer decrement,A_V, orβ) 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. 

Abstract We present ultradeep Keck/MOSFIRE rest-optical spectra of two star-forming galaxies atz= 2.18 in the COSMOS field with bright emission lines, representing more than 20 hr of total integration. The fidelity of these spectra enabled the detection of more than 20 unique emission lines for each galaxy, including the first detection of the auroral [Oii]λλ7322, 7332 lines at high redshift. We use these measurements to calculate the electron temperature in the low-ionization O⁺zone of the ionized interstellar medium and derive abundance ratios of O/H, N/H, and N/O using the direct method. The N/O andα/Fe abundance patterns of these galaxies are consistent with rapid formation timescales and ongoing strong starbursts, in accord with their high specific star formation rates. These results demonstrate the feasibility of using auroral [Oii] measurements for accurate metallicity studies at high redshift in a higher-metallicity and lower-excitation regime previously unexplored with the direct method in distant galaxies. These results also highlight the difficulty in obtaining the measurements required for direct-method metallicities from the ground. We emphasize the advantages that the JWST/NIRSpec instrument will bring to high-redshift metallicity studies, where the combination of increased sensitivity and uninterrupted wavelength coverage will yield more than an order of magnitude increase in efficiency for multiplexed auroral-line surveys relative to current ground-based facilities. Consequently, the advent of JWST promises to be the beginning of a new era of precision chemical abundance studies of the early universe at a level of detail rivaling that of local galaxy studies. 

ABSTRACT We present a detailed study of a galaxy merger taking place at z = 1.89 in the GOODS-S field. Here, we analyse Keck/MOSFIRE spectroscopic observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey along with multiwavelength photometry assembled by the 3D-HST survey. The combined data set is modelled to infer the past star formation histories (SFHs) of both merging galaxies. They are found to be massive, with log10(M*/M⊙) > 11, with a close mass ratio satisfying the typical major-merger definition. Additionally, in the context of delayed-τ models, GOODS-S 43114, and GOODS-S 43683 have similar SFHs and low star formation rates (log10(SFR(SED)/$${\rm M}_{\odot }\,\rm {yr}^{-1}$$) < 1.0) compared to their past averages. The best-fitting model SEDs show elevated H δA values for both galaxies, indicating that their stellar spectra are dominated by A-type stars, and that star formation peaked ∼0.5−1 Gyr ago and has recently declined. Additionally, based on SED fitting both merging galaxies turned on and shut off star formation within a few hundred Myr of each other, suggesting that their bursts of star formation may be linked. Combining the SFHs and H δA results with recent galaxy merger simulations, we infer that these galaxies have recently completed their first pericentric passage and are moving apart. Finally, the relatively low second velocity moment of GOODS-S 43114, given its stellar mass suggests a disc-like structure. However, including the geometry of the galaxy in the modelling does not completely resolve the discrepancy between the dynamical and stellar masses. Future work is needed to resolve this inconsistency in mass. 

ABSTRACT We investigate the conditions that facilitate galactic-scale outflows using a sample of 155 typical star-forming galaxies at z ∼ 2 drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. The sample includes deep rest-frame UV spectroscopy from the Keck Low-Resolution Imaging Spectrometer (LRIS), which provides spectral coverage of several low-ionization interstellar (LIS) metal absorption lines and Lyα emission. Outflow velocities are calculated from the centroids of the LIS absorption and/or Lyα emission, as well as the highest velocity component of the outflow from the blue wings of the LIS absorption lines. Outflow velocities are found to be marginally correlated or independent of galaxy properties, such as star-formation rate (SFR) and star-formation rate surface density (ΣSFR). Outflow velocity scales with SFR as a power-law with index 0.24, which suggests that the outflows may be primarily driven by mechanical energy generated by supernovae explosions, as opposed to radiation pressure acting on dusty material. On the other hand, outflow velocity and ΣSFR are not significantly correlated, which may be due to the limited dynamic range of ΣSFR probed by our sample. The relationship between outflow velocity and ΣSFR normalized by stellar mass (ΣsSFR), as a proxy for gravitational potential, suggests that strong outflows (e.g. > 200 km s−1) become common above a threshold of log(ΣsSFR/$$\rm {yr}^{-1}\ \rm {kpc}^{-2}$$) ∼ −11.3, and that above this threshold, outflow velocity uncouples from ΣsSFR. These results highlight the need for higher resolution spectroscopic data and spatially resolved imaging to test the driving mechanisms of outflows predicted by theory.