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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. 

We investigate the multi-phase structure of gas flows in galaxies. We study 80 galaxies during the epoch of peak star formation (1.4 ≤ z ≤ 2.7) using data from Keck/LRIS and VLT/KMOS. Our analysis provides a simultaneous probe of outflows using UV emission and absorption features and Hα emission. With this unprecedented data set, we examine the properties of gas flows estimated from LRIS and KMOS in relation to other galaxy properties, such as star formation rate (SFR), star formation rate surface density (ΣSFR), stellar mass (M∗), and main sequence offset (ΔMS). We find no strong correlations between outflow velocity measured from rest-UV lines and galaxy properties. However, we find that galaxies with detected outflows show higher averages in SFR, ΣSFR, and ΔMS than those lacking outflow detections, indicating a connection between outflow and galaxy properties. Furthermore, we find a lower average outflow velocity than previously reported, suggesting greater absorption at the systemic redshift of the galaxy. Finally, we detect outflows in 49% of our LRIS sample and 30% in the KMOS sample, and find no significant correlation between outflow detection and inclination. These results may indicate that outflows are not collimated and that Hα outflows have a lower covering fraction than low-ionization interstellar absorption lines. Additionally, these tracers may be sensitive to different physical scales of outflow activity. A larger sample size with a wider dynamic range in galaxy properties is needed to further test this picture. 

Aims. We aim to quantify the relation between the dust-to-gas mass ratio (DTG) and gas-phase metallicity of z  = 2.1 − 2.5 luminous galaxies and contrast this high-redshift relation against analogous constraints at z  = 0. Methods. We present a sample of ten star-forming main-sequence galaxies in the redshift range 2.1 <  z  < 2.5 with rest-optical emission-line information available from the MOSDEF survey and with ALMA 1.2 millimetre and CO J  = 3 − 2 follow-up observations. The galaxies have stellar masses ranging from 10 10.3 to 10 10.6   M ⊙ and cover a range in star-formation rate from 35 to 145 M ⊙ yr −1 . We calculated the gas-phase oxygen abundance of these galaxies from rest-optical nebular emission lines (8.4 < 12 + log(O/H) < 8.8, corresponding to 0.5−1.25 Z ⊙ ). We estimated the dust and H 2 masses of the galaxies (using a metallicity-dependent CO-to-H 2 conversion factor) from the 1.2 mm and CO J  = 3 − 2 observations, respectively, from which we estimated a DTG. Results. We find that the galaxies in this sample follow the trends already observed between CO line luminosity and dust-continuum luminosity from z  = 0 to z  = 3, extending such trends to fainter galaxies at 2.1 <  z  < 2.5 than observed to date. We find no second-order metallicity dependence in the CO – dust-continuum luminosity relation for the galaxies presented in this work. The DTGs of main-sequence galaxies at 2.1 <  z  < 2.5 are consistent with an increase in the DTG with gas-phase metallicity. The metallicity dependence of the DTG is driven by the metallicity dependence of the CO-to-H 2 conversion factor. Galaxies at z  = 2.1 − 2.5 are furthermore consistent with the DTG-metallicity relation found at z  = 0 (i.e. with no significant evolution), providing relevant constraints for galaxy formation models. These results furthermore imply that the metallicity of galaxies should be taken into account when estimating cold-gas masses from dust-continuum emission, which is especially relevant when studying metal-poor low-mass or high-redshift 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 present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L CO ( 3 − 2 ) ′ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 2, M mol increases and the molecular gas fraction ( M mol / M * ) decreases with increasing M * , with a significant secondary dependence on SFR. Galaxies at z ∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR− M * , O/H− M * , and M mol − M * relations is correlated such that, at fixed M * , z ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium. 

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 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 define a new morphology metric called ‘patchiness’ (P) that is sensitive to deviations from the average of a resolved distribution, does not require the galaxy centre to be defined, and can be used on the spatially resolved distribution of any galaxy property. While the patchiness metric has a broad range of applications, we demonstrate its utility by investigating the distribution of dust in the interstellar medium (ISM) of 310 star-forming galaxies at spectroscopic redshifts 1.36 < z < 2.66 observed by the MOSFIRE Deep Evolution Field survey. The stellar continuum reddening distribution, derived from high-resolution multiwaveband CANDELS/3D-HST imaging, is quantified using the patchiness, Gini, and M20 coefficients. We find that the reddening maps of high-mass galaxies, which are dustier and more metal-rich on average, tend to exhibit patchier distributions (high P) with the reddest components concentrated within a single region (low M20). Our results support a picture where dust is uniformly distributed in low-mass galaxies (≲1010 M⊙), implying efficient mixing of dust throughout the ISM. On the other hand, the dust distribution is patchier in high-mass galaxies (≳1010 M⊙). Dust is concentrated near regions of active star formation and dust mixing time-scales are expected to be longer in high-mass galaxies, such that the outskirt regions of these physically larger galaxies remain relatively unenriched. This study presents direct evidence for patchy dust distributions on scales of a few kpc in high-redshift galaxies, which previously has only been suggested as a possible explanation for the observed differences between nebular and stellar continuum reddening, star formation rate indicators, and dust attenuation curves. 

Abstract We perform joint modeling of the composite rest-frame far-UV and optical spectra of redshift 1.85 ≤ z ≤ 3.49 star-forming galaxies to deduce key properties of the massive stars, ionized interstellar medium (ISM), and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Ly α photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Ly β through C iii ] λλ 1907, 1909 and [O ii ], [Ne iii ], H β , [O iii ], H α , [N ii ], and [S ii ]. Spectral and photoionization modeling indicates that the galaxies are uniformly consistent with stellar population synthesis models that include the effects of stellar binarity. Over the dynamic range of our sample, there is little variation in stellar and nebular abundance with Ly α equivalent width, W λ (Ly α ), and only a marginal anticorrelation between age and W λ (Ly α ). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W λ (Ly α ); rather, the covering fraction of optically thick H i appears to be the principal factor modulating the escape of Ly α , with most of the Ly α photons in down-the-barrel observations of galaxies escaping through low column density or ionized channels in the ISM. Our analysis shows that a high star-formation-rate surface density, Σ SFR , particularly when coupled with a low galaxy potential (i.e., low stellar mass), can aid in reducing the covering fraction and ease the escape of Ly α photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.