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 Lyα photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Lyβ through C iii] λλ1907, 1909 and [O ii], [Ne iii], Hβ, [O iii], Hα, [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 Lyα equivalent width, W λ (Lyα), and only a marginal anticorrelation between age and W λ (Lyα). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W λ (Lyα); rather, the covering fraction of optically thick H i appears to be the principal factor modulating the escape of Lyα, with most of the Lyα 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, ΣSFR, 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 Lyα photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.

We present a joint analysis of rest-UV and rest-optical spectra obtained using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at z ∼ 2.3. We divide our sample into two bins based on their location in the [OIII]5007/Hβ vs. [NII]6584/Hα BPT diagram, and perform the first differential study of the rest-UV properties of massive ionizing stars as a function of rest-optical emission-line ratios. Fitting BPASS stellar population synthesis models, including nebular continuum emission, to our rest-UV composite spectra, we find that high-redshift galaxies offset towards higher [OIII]λ5007/Hβ and [NII]λ6584/Hα have younger ages ($\log (\textrm {~Age/yr})=7.20^{+0.57}_{-0.20}$) and lower stellar metallicities ($Z_*=0.0010^{+0.0011}_{-0.0003}$) resulting in a harder ionizing spectrum, compared to the galaxies in our sample that lie on the local BPT star-forming sequence ($\log (\textrm {Age/yr})=8.57^{+0.88}_{-0.84}$, $Z_*=0.0019^{+0.0006}_{-0.0006}$). Additionally, we find that the offset galaxies have an ionization parameter of $\log (U)=-3.04^{+0.06}_{-0.11}$ and nebular metallicity of ($12+\log (\textrm {~O/H})=8.40^{+0.06}_{-0.07}$), and the non-offset galaxies have an ionization parameter of $\log (U)=-3.11^{+0.08}_{-0.08}$ and nebular metallicity of $12+\log (\textrm {~O/H})=8.30^{+0.05}_{-0.06}$. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more α-enhanced ($7.28^{+2.52}_{-2.82}\textrm {~O/Fe}_{\odot }$) compared to those consistent with the local sequence ($3.04^{+0.95}_{-0.54}\textrm {~O/Fe}_{\odot }$). However, even galaxies that are entirely consistent with the local nebular excitation sequence appear to be α-enhanced – in contrast with typical local systems. Such differences must be considered when estimating gas-phase oxygen abundances at high redshift based on strong emission-line ratios. Specifically, a similarity in the location of high-redshift and local galaxies in the BPT diagram may not be indicative of a similarity in their physical properties.

We present constraints on the massive star and ionized gas properties for a sample of 62 star-forming galaxies at z ∼ 2.3. Using BPASS stellar population models, we fit the rest-UV spectra of galaxies in our sample to estimate age and stellar metallicity which, in turn, determine the ionizing spectrum. In addition to the median properties of well-defined subsets of our sample, we derive the ages and stellar metallicities for 30 high-SNR individual galaxies – the largest sample of individual galaxies at high redshift with such measurements. Most galaxies in this high-SNR subsample have stellar metallicities of 0.001 < Z* < 0.004. We then use Cloudy+BPASS photoionization models to match observed rest-optical line ratios and infer nebular properties. Our high-SNR subsample is characterized by a median ionization parameter and oxygen abundance, respectively, of log (U)med = −2.98 ± 0.25 and 12 + log (O/H)med = 8.48 ± 0.11. Accordingly, we find that all galaxies in our sample show evidence for α-enhancement. In addition, based on inferred log (U) and 12 + log (O/H) values, we find that the local relationship between ionization parameter and metallicity applies at z ∼ 2. Finally, we find that the high-redshift galaxies most offset from the local excitation sequence in the BPT diagram are the most α-enhanced. This trend suggests that α-enhancement resulting in a harder ionizing spectrum at fixed oxygen abundance is a significant driver of the high-redshift galaxy offset on the BPT diagram relative to local systems. The ubiquity of α-enhancement among z ∼ 2.3 star-forming galaxies indicates important differences between high-redshift and local galaxies that must be accounted for in order to derive physical properties at high redshift.

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature Td remains mostly unconstrained due to the few available FIR continuum data at redshift z > 5. This introduces large uncertainties in several properties of high-z galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [C $\scriptstyle \rm II$] 158μm line and underlying dust continuum measurements, we derive Td in the continuum and [C $\scriptstyle \rm II$] detected z ≈ 7 galaxies in the ALMA Large Project REBELS sample. We find 39 K < Td < 58 K, and dust masses in the narrow range Md = (0.9 − 3.6) × 107M⊙. These results allow us to extend for the first time the reported Td(z) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing Td(z) trend with the decrease of gas depletion time, tdep = Mg/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields Td∝(1 + z)0.4. The model also explains the observed Td scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, Td only depends on the gas column density (metallicity), $T_{\rm d} \propto N_{\rm H}^{1/6}$ (Td∝Z−1/6). REBELS galaxies are on average relatively transparent, with effective gas column densities around NH ≃ (0.03 − 1) × 1021cm−2. We predict that other high-z galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated Td ≫ 60 K, are significantly obscured, low-metallicity systems. In fact Td is higher in metal-poor systems due to their smaller dust content, which for fixed LIR results in warmer temperatures.

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   z 1.36 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 starforming 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IVλλ1548,1550, HeIIλ1640, 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. Unified Astronomy Thesaurus concepts: High-redshift galaxies (734); H II regions (694); Emission line galaxies (459)