Search

Reionization is thought to be driven by faint star-forming galaxies, but characterizing this population has long remained very challenging. Here, we utilize deep nine-band JADES (JWST Advanced Deep Extragalactic Survey)/NIRCam (Near-Infrared Camera) imaging to study the star-forming and ionizing properties of 756 |$z\sim 6-9$| galaxies, including hundreds of very ultraviolet (UV)-faint objects (⁠|$M_\mathrm{UV}\gt -18$|⁠). The faintest (⁠|$m\sim 30$|⁠) galaxies in our sample typically have stellar masses of |$M_\ast \sim (1-3)\times 10^7\ \mathrm{ M}_\odot$| and young light-weighted ages (⁠|$\sim$| 50 Myr), though some show strong Balmer breaks implying much older ages (⁠|$\sim$| 500 Myr). We find no evidence for extremely massive galaxies (⁠|$\gt 3\times 10^{10}\ \mathrm{ M}_\odot$|⁠) in our sample. We infer a strong (factor |$\gt $| 2) decline in the typical [O  iii ] |$+$| H  |$\beta$| equivalent widths (EWs) towards very faint |$z\sim 6-9$| galaxies, yet a weak UV luminosity dependence on the H  |$\alpha$| EWs at |$z\sim 6$|⁠. We demonstrate that these EW trends can be explained if fainter galaxies have systematically lower metallicities as well as more recently declining star formation histories relative to the most UV-luminous galaxies. Our data provide evidence that the brightest galaxies are frequently experiencing a recent strong upturn in star formation rate. We also discuss how the EW trends may be influenced by a strong correlation between |$M_\mathrm{UV}$| and Lyman continuum escape fraction. This alternative explanation has dramatically different implications for the contribution of galaxies along the luminosity function to cosmic reionization. Finally, we quantify the photometric overdensities around two |$z\,\gt\,7$| strong Ly  |$\alpha$| emitters. One Ly  |$\alpha$| emitter lies close to a strong photometric overdensity, while the other shows no significant nearby overdensity, perhaps implying that not all strong |$z\,\gt\, 7$| Ly  |$\alpha$| emitters reside in large ionized bubbles. [ABSTRACT FROM AUTHOR]

JWST has recently sparked a new era of Lyα spectroscopy, delivering the first measurements of the Lyα escape fraction and velocity profile in typical galaxies at z ≃ 6−10. These observations offer new prospects for insight into the earliest stages of reionization. But to realize this potential, we need robust models of Lyα properties in galaxies at z ≃ 5−6 when the IGM is mostly ionized. Here, we use new JWST observations from the JADES and FRESCO surveys combined with VLT/MUSE and Keck/DEIMOS data to characterize statistical distributions of Lyα velocity offsets, escape fractions, and EWs in z ≃ 5−6 galaxies. We find that galaxies with large Lyα escape fractions (>0.2) are common at z ≃ 5−6, comprising 30 per cent of Lyman break selected samples. Comparing to literature studies, our census suggests that Lyα becomes more prevalent in the galaxy population towards higher redshift from z ∼ 3 to z ∼ 6, although we find that this evolution slows considerably between z ∼ 5 and z ∼ 6, consistent with modest attenuation from residual H i in the mostly ionized IGM at z ≃ 5−6. We find significant evolution in Lyα velocity profiles between z ≃ 2−3 and z ≃ 5−6, likely reflecting the influence of resonant scattering from residual intergalactic H i on the escape of Lyα emission near line centre. This effect will make it challenging to use Lyα peak offsets as a probe of Lyman continuum leakage at z ≃ 5−6. We use our z ≃ 5−6 Lyα distributions to make predictions for typical Lyα properties at z ≳ 8 and discuss implications of a recently discovered Lyα emitter at z ≃ 8.5 with a small peak velocity offset (156 km s−1). [ABSTRACT FROM AUTHOR]

The power-law slope of the rest-ultraviolet (UV) continuum (f λ ∝ λβ) is a key metric of early star-forming galaxies, providing one of our only windows into the stellar populations and physical conditions of z ≳ 10 galaxies. Expanding upon previous studies with limited sample sizes, we leverage deep imaging from the JWST Advanced Deep Extragalactic Survey (JADES) to investigate the UV slopes of 179 z ≳ 9 galaxies with apparent magnitudes of m F 200 W ≃ 26–31, which display a median UV slope of β = −2.4. We compare to a statistical sample of z ≃ 5–9 galaxies, finding a shift towards bluer rest-UV colours at all |$M_{\rm UV}$|⁠. The most UV-luminous z ≳ 9 galaxies are significantly bluer than their lower redshift counterparts, representing a dearth of moderately red galaxies within the first 500 Myr. At yet earlier times, the z ≳ 11 galaxy population exhibits very blue UV slopes, implying very low impact from dust attenuation. We identify a robust sample of 44 galaxies with β ≲ −2.8, which have spectral energy distributions requiring models of density-bounded H  ii regions and median ionizing photon escape fractions of 0.51 to reproduce. Their rest-optical colours imply that this sample has weaker emission lines (median m F 356 W − m F 444 W  = 0.19 mag) than typical galaxies (median m F 356 W − m F 444 W  = 0.39 mag), consistent with the inferred escape fractions. This sample consists of relatively low stellar masses (median |$\log (M/{\rm M}_{\odot })=7.5\pm 0.2$|⁠), and specific star formation rates (sSFRs; median  |$=79 \, \rm Gyr^{-1}$|⁠) nearly twice that of our full galaxy sample (median sSFRs  |$=44 \, \rm Gyr^{-1}$|⁠), suggesting these objects are more common among systems experiencing a recent upturn in star formation. We demonstrate that the shutoff of star formation provides an alternative solution for modelling of extremely blue UV colours, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures. [ABSTRACT FROM AUTHOR]

Nearly a decade ago, we began to see indications that reionization-era galaxies power hard radiation fields rarely seen at lower redshift. Most striking were detections of nebular C  iv emission in what appeared to be typical low-mass galaxies, requiring an ample supply of 48 eV photons to triply ionize carbon. We have obtained deep JWST/NIRSpec R = 1000 spectroscopy of the two z > 6 C  iv -emitting galaxies known prior to JWST. Here, we present a rest-UV to optical spectrum of one of these two systems, the multiply-imaged z = 6.1 lensed galaxy RXCJ2248-ID. NIRCam imaging reveals two compact (<22 pc) clumps separated by 220 pc, with one comprising a dense concentration of massive stars (>10 400 M⊙ yr−1 kpc−2) formed in a recent burst. We stack spectra of 3 images of the galaxy (J = 24.8–25.9), yielding a very deep spectrum providing a high-S/N template of strong emission line sources at z > 6. The spectrum reveals narrow high-ionization lines (He  ii , C  iv , N  iv ]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([O  iii ] EW = 2800 Å), albeit with weak emission from low-ionization transitions ([O  iii ]/[O  ii ] = 184). The electron density is found to be very high (6.4–31.0 × 104 cm−3) based on three UV transitions. The ionized gas is metal poor (⁠|$12+\log (\rm O/H)=7.43^{+0.17}_{-0.09}$|⁠), yet highly enriched in nitrogen (⁠|$\log (\rm N/O)=-0.39^{+0.11}_{-0.10}$|⁠). The spectrum appears broadly similar to that of GNz11 at z = 10.6, without showing the same AGN signatures. We suggest that the hard radiation field and rapid nitrogen enrichment may be a short-lived phase that many z > 6 galaxies go through as they undergo strong bursts of star formation. We comment on the potential link of such spectra to globular cluster formation. [ABSTRACT FROM AUTHOR]

Several studies have detected Lyman-alpha (Ly  α) from bright (⁠|$M_{\small UV}\lesssim -21.5$|⁠) galaxies during the early stages of reionization despite the significantly neutral intergalactic medium. To explain these detections, it has been suggested that z > 7 Ly  α emitters (LAEs) inhabit physical Mpc (pMpc)-scale ionized regions powered by overdensities of faint galaxies; however, systematic searches for these overdensities near LAEs have been challenging. Here, we use Cosmic Evolution Early Release Science JWST /Near Infrared Camera imaging to search for large-scale galaxy overdensities near two very ultraviolet (UV)-bright, z  = 8.7 LAEs in the Extended Groth Strip (EGS) field. We colour select 27 z  = 8.4–9.1 candidates, including the one LAE in the footprint (EGSY8p7). From spectral energy distribution models, we infer moderately faint UV luminosities (⁠|$-21.2\lesssim {M_{\small UV}}\lesssim -19.1$|⁠) and stellar masses of M * ≈ 107.5–8.8 M⊙. All are efficient ionizing agents (⁠|$\xi _{\text{ion}}^{*}\approx 10^{25.5-26.0}$| Hz erg−1) and are generally morphologically simple with only one compact (re ≲ 140 to ∼650 pc) star-forming component. 13 candidates lie within 5 arcmin of EGSY8p7, leading to a factor-of-four galaxy overdensity at ≲5 arcmin (∼1.4 projected pMpc at z ∼ 8.7) separations from EGSY8p7. Separations of 10–15 arcmin (∼2.7–4.1 projected pMpc) are consistent with an average field. The spatial distribution of our sample may qualitatively suggest an R ≥ 2 pMpc ionized bubble encompassing both LAEs in EGS, which is theoretically unexpected but may be possible for a galaxy population four times more numerous than the average to create with moderate escape fractions (f esc ≳ 0.15) over long times (≳ 200 Myr). Upcoming spectroscopic follow-up will characterize the size of any ionized bubble that may exist and the properties of the galaxies powering such a bubble. [ABSTRACT FROM AUTHOR]

We describe JWST /NIRSpec prism measurements of Ly α emission in z ≳ 5 galaxies. We identify Ly α detections in 10 out of 69 galaxies with robust rest-optical emission-line redshift measurements at 5 ≤ z < 7 in the Cosmic Evolution Early Release Science (CEERS) and DDT-2750 observations of the Extended Groth Strip field. Galaxies at z ≃ 6 with faint continuum (F 150 W =27–29 mag) are found with extremely large rest-frame Ly α equivalent widths (EWs; ranging up to 286 Å). Likely Ly α detections are also seen in two new z > 7 galaxies (z  = 7.49 and 7.17) from the second epoch of CEERS observations, both showing large Ly α EWs that likely indicate significant transmission through the intergalactic medium (IGM). We measure high Ly α escape fractions in the 12 Ly α emitters in our sample (median 0.28), two of which show |$f_{\rm esc}^{ {\rm Ly}\alpha }$| near unity (>0.80). We find that |$50_{-11}^{+11}$|  per cent of z ≃ 6 galaxies with [O  iii ] + H β EW>1000 Å have |$f_{\rm esc}^{ {\rm Ly}\alpha }$| >0.2, consistent with the fractions found in lower redshift samples with matched [O  iii ] + H β EWs. While uncertainties are still significant, we find that only |$10_{-5}^{+9}$|  per cent of z > 7 galaxies with similarly strong rest optical emission lines show such large |$f_{\rm esc}^{ {\rm Ly}\alpha }$|⁠ , as may be expected if IGM attenuation of Ly α increases towards higher redshifts. We identify photometric galaxy overdensities near the z ≳ 7 Ly α emitters, potentially providing the ionizing flux necessary to create large ionized sightlines that facilitate Ly α transmission. Finally, we investigate the absence of Ly α emission in a comparable (and spectroscopically confirmed) galaxy overdensity at z  = 7.88 in the Abell 2744 field, discussing new prism spectra of the field obtained with the UNCOVER program. [ABSTRACT FROM AUTHOR]

We present new ALMA Band 8 (rest-frame |$90 \ \mu$| m) continuum observations of three massive (⁠|$M_\star \approx 10^{10}\ \mathrm{ M}_\odot$|⁠) galaxies at z ≈ 7 previously detected in [C  ii ] |$158 \ \mu$| m and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect dust emission from two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through optically thin modified blackbody fitting, we determine dust temperatures of |$T_\mathrm{dust}\approx 30-35\,$| K in both of the dual-band detected targets, indicating they are colder than most known galaxies at z ∼ 7. Moreover, their inferred dust masses are large (⁠|$M_\mathrm{dust}\approx 10^{8}\ \mathrm{ M}_\odot$|⁠), albeit still consistent with models of high-redshift dust production. We furthermore target and detect [O  iii ] |$88 \ \mu$| m emission in both REBELS-12 and REBELS-25, and find |$L_\mathrm{[O\, III]}/L_\mathrm{[C\, II]}\approx 1-1.5$| – low compared to the |$L_\mathrm{[O\, \rm{\small III}]}/L_\mathrm{[C\, \rm{\small II}]}\gtrsim 2-10$| observed in the known z ≳ 6 population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets, although the possibility of REBELS-12 being a merger of an [O  iii ]-bright and [O  iii ]-faint component prevents the unambiguous interpretation of its [O  iii ]/[C  ii ] ratio. Nevertheless, a low burstiness forms a natural explanation for the cold dust temperatures and low |$\mathrm{[O\, \rm{\small III}]}\lambda \lambda 4959,5007+\mathrm{H} \, \beta$| equivalent widths of REBELS-25 and REBELS-38. Overall, these observations provide evidence for the existence of a massive, dust-rich galaxy population at z ≈ 7 which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner. [ABSTRACT FROM AUTHOR]

We describe new JWST /NIRSpec observations of galaxies at z ≳ 7 taken from the CEERS survey. Previous observations of this area have revealed associations of Lyα emitters at redshifts (z = 7.5, 7.7, and 8.7) where the intergalactic medium (IGM) is thought to be mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 z ≳ 7 galaxies with robust redshifts in the CEERS data set, including 10 in the Lyα associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 = 17.84 and Ne3O2 = 0.89, linear scale) and metallicity (⁠|$12+\log {(\rm {O/H})}=7.84$|⁠) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six z ≳ 7 Lyα emitters in the sample. Each has a hard ionizing spectrum indicating that their visibility is likely enhanced by efficient ionizing photon production. Lyα velocity offsets are found to be very large (≳300 km s−1), likely also contributing to their detectability. We find that Lyα in z ≳ 7 galaxies is 6–12 × weaker than in lower redshift samples with matched rest-optical spectral properties. If the bubbles around the Lyα emitters are relatively small (≲0.5–1 pMpc), we may expect such significant attenuation of Lyα in these ionized regions. We discuss several other effects that may contribute to weaker Lyα emission at z ≳ 7. Deep spectroscopy of fainter galaxies in the vicinity of the Lyα emitters will better characterize the physical scale of the ionized bubbles in this field. [ABSTRACT FROM AUTHOR]

The |$\rm {H}\alpha$| -to-UV luminosity ratio (⁠|$L(\text{H}\alpha)/L(\rm UV)$|⁠) is often used to probe bursty star formation histories (SFHs) of star-forming galaxies and it is important to validate it against other proxies for burstiness. To address this issue, we present a statistical analysis of the resolved distribution of star formation rate surface density (ΣSFR) as well as stellar age and their correlations with the globally measured |$L(\text{H}\alpha)/L(\rm UV)$| for a sample of 310 star-forming galaxies in two redshift bins of 1.37 < z < 1.70 and 2.09 < z < 2.61 observed by the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use the multiwaveband CANDELS/3D-HST imaging of MOSDEF galaxies to construct ΣSFR and stellar age maps. We analyse the composite rest-frame far-ultraviolet spectra of a subsample of MOSFIRE Deep Evolution Field (MOSDEF) targets obtained by the Keck Low Resolution Imager and Spectrometer (LRIS), which includes 124 star-forming galaxies (MOSDEF-LRIS) at redshifts 1.4 < z < 2.6, to examine the average stellar population properties, and the strength of age-sensitive far-ultraviolet spectral features in bins of |$L(\text{H}\alpha)/L(\rm UV)$|⁠. Our results show no significant evidence that individual galaxies with higher |$L(\text{H}\alpha)/L(\rm UV)$| are undergoing a burst of star formation based on the resolved distribution of ΣSFR of individual star-forming galaxies. We segregate the sample into subsets with low and high |$L(\text{H}\alpha)/L(\rm UV)$|⁠. The high- |$L(\text{H}\alpha)/L(\rm UV)$| subset exhibits, on average, an age of |$\log [\rm {Age/yr}]$|  = 8.0, compared to |$\log [\rm {Age/yr}]$| = 8.4 for the low- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies, though the difference in age is significant at only the 2σ level. Furthermore, we find no variation in the strengths of Si  iv λλ1393, 1402 and C  iv λλ1548, 1550 P-Cygni features from massive stars between the two subsamples, suggesting that the high- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies are not preferentially undergoing a burst compared to galaxies with lower |$L(\text{H}\alpha)/L(\rm UV)$|⁠. On the other hand, we find that the high- |$L(\text{H}\alpha)/L(\rm UV)$| galaxies exhibit, on average, more intense He  ii λ1640 emission, which may possibly suggest the presence of a higher abundance of high-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

Spitzer /Infrared Array Camera (IRAC) imaging has revealed that the brightest z ∼ 7−8 galaxies often exhibit young ages and strong nebular line emission, hinting at high ionizing efficiency among early galaxies. However, IRAC's limited sensitivity has long hindered efforts to study the fainter, more numerous population often thought largely responsible for reionization. Here, we use Cosmic Evolution Early Release Science (CEERS) JWST /NIRCam data to characterize 116 ultraviolet (UV)-faint (median M UV = −19.5) z ∼ 6.5−8 galaxies. The spectral energy distributions are typically dominated by young (∼10–50 Myr), low-mass (M * ∼ 108 M⊙) stellar populations, and we find no need for extremely high stellar masses (∼1011 M⊙). Considering previous studies of UV-bright (M UV ∼ −22) z ∼ 7−8 galaxies, we find evidence for a strong (5–10 times) increase in specific star formation rate (sSFR) toward lower luminosities (median sSFR = 103 Gyr−1 in CEERS). The larger sSFRs imply a more dominant contribution from OB stars in the relatively numerous UV-faint population, perhaps suggesting that these galaxies are very efficient ionizing agents (median ξion = 1025.7 erg−1 Hz). In spite of the much larger sSFRs, we find little increase in [O  iii ] + H  β equivalent widths towards fainter M UV (median ≈780  |$\mathrm{\mathring{A}}$|⁠). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities (≲3 per cent Z ⊙) where [O  iii ] emission is suppressed. Alternatively, high ionizing photon escape fractions or bursty star formation histories can also weaken the nebular lines in a subset of our sample. While the majority of galaxies in our sample are very blue (median β  = −2.0), we identify a significant tail of very dusty galaxies (β ∼ −1) at ≈0.5 |$L_\mathrm{UV}^\ast$| which may contribute significantly to the z ∼ 7−8 star formation rate density. [ABSTRACT FROM AUTHOR]

With JWST , new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of z ≳ 7 galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual z ≳ 8 objects could be detected with Spitzer , making it difficult to robustly constrain activity at z ≳ 10. Here, we leverage the greatly improved rest-optical sensitivity of JWST at z ≳ 8 to constrain the ages of seven UV-bright (⁠|$M_{\rm uv}\lesssim -19.5$|⁠) galaxies selected to lie at z ∼ 8.5–11, then investigate implications for z ≳ 15 star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright z ∼ 8.5–11 galaxies. We find a median age of ∼20 Myr, younger than that inferred at z ∼ 7 with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only ∼3 per cent of bright z ∼ 8.5–11 galaxies would be similarly luminous at z ≳ 15, implying that the number density of bright galaxies declines by at least an order of magnitude between z ∼ 8.5–11 and |$z \sim 15$|⁠. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright z ∼ 15 galaxies are observed during a burst of star formation. [ABSTRACT FROM AUTHOR]

The onset of the JWST -era provides a much-improved opportunity to characterize the resolved structure of early star-forming systems. Previous Spitzer observations of z ≳ 6 galaxies revealed the presence of old stars and luminous H  ii regions (via [O  iii ]+H β emission), but the poor resolution stunted our ability to map their locations with respect to the star-forming regions identified in the rest-UV. In this paper, we investigate the internal structure of 12 of the most luminous z ≃ 6–8 galaxies in the EGS field observed with recent JWST /NIRCam imaging. The systems appear clumpy in the rest-UV, with more than half of the light coming from ≃ 107 to 109 M⊙ star-forming complexes that are ≃150–480 pc in size. The clumps tend to be dominated by young stars (median = 36 Myr), but we also find large variations in clump ages within individual galaxies. The [O  iii ]+H β EW varies significantly across individual galaxies (reflecting differences in stellar and gas properties), but the H  ii regions largely track the UV-bright complexes. Perhaps surprisingly, the rest-optical continuum is just as clumpy as the UV, and we do not find older (and redder) nuclear stellar components that were previously undetected or faint in the UV. The majority of the stellar mass in bright 6 < z < 8 galaxies appears to be contained in the ≳150 pc-scale clumpy star-forming complexes, reflecting the very active phase of assembly that is common in reionization-era galaxies. [ABSTRACT FROM AUTHOR]

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimetre wavelengths to resolve. Here, we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [C  ii ] and dust emission in UV-selected galaxies at z ∼ 7, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation f obs = SFRIR/SFRUV+IR depends on stellar mass, similar to what is observed at lower redshift, and ranges from f obs ≈ 0.3 − 0.6 for galaxies with log10(M ⋆/M⊙) = 9.4–10.4. We further adopt the z ∼ 7 stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between 3 ≲ z ≲ 7, with dust-obscured star formation still contributing |${\sim}30{{\ \rm per\ cent}}$| at z ∼ 7. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization. [ABSTRACT FROM AUTHOR]

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly α transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly α profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at z ∼ 7 remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [C  ii ]-based redshifts from ALMA. Here, we report MMT/Binospec Ly α spectroscopy of eight UV-bright (MUV ∼ −22) galaxies at z ≃ 7 selected from the ALMA REBELS survey. We detect Ly α in four of eight galaxies and use the [C  ii ] systemic redshifts to investigate the Ly α velocity profiles. The Ly α lines are significantly redshifted from systemic (average velocity offset = 223 km s–1) and broad (FWHM ≈ 300–650 km s−1), with two sources showing emission extending to ≈750 km s−1. We find that the broadest Ly α profiles are associated with the largest [C  ii ] line widths, suggesting a potential link between the Ly α FWHM and the dynamical mass. Since Ly α photons at high velocities transmit efficiently through the z  = 7 IGM, our data suggest that velocity profiles play a significant role in boosting the Ly α visibility of the most UV-luminous reionization-era galaxies. [ABSTRACT FROM AUTHOR]

We present specific star formation rates (sSFRs) for 40 ultraviolet (UV)-bright galaxies at z ∼ 7–8 observed as part of the Reionization Era Bright Emission Line Survey (REBELS) Atacama Large Millimeter/submillimeter Array (ALMA) large programme. The sSFRs are derived using improved star formation rate (SFR) calibrations and spectral energy distribution (SED)-based stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [C  ii ]-based spectroscopic redshifts. The median sSFR of the sample is |$18_{-5}^{+7}$|  Gyr−1, significantly larger than literature measurements lacking constraints in the FIR, reflecting the larger obscured SFRs derived from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly dependent on the assumed star formation history in reionization-era galaxies. When large sSFR galaxies (a population that is common at z > 6) are modelled with non-parametric star formation histories, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [C  ii ] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses capable of accommodating an old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties among derived parameters, we find that sSFRs increase rapidly toward higher redshifts for massive galaxies (9.6 < log (M */M⊙) < 9.8), evolving as (1 + z)1.7 ± 0.3, broadly consistent with expectations from the evolving baryon accretion rates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The combination of the MOSDEF and KBSS-MOSFIRE surveys represents the largest joint investment of Keck/MOSFIRE time to date, with ∼3000 galaxies at 1.4 ≲ z ≲ 3.8, roughly half of which are at z ∼ 2. MOSDEF is photometric- and spectroscopic-redshift selected with a rest-optical magnitude limit, while KBSS-MOSFIRE is primarily selected based on rest-UV colours and a rest-UV magnitude limit. Analysing both surveys in a uniform manner with consistent spectral-energy-distribution (SED) models, we find that the MOSDEF z ∼ 2 targeted sample has higher median M * and redder rest U − V colour than the KBSS-MOSFIRE z ∼ 2 targeted sample, and smaller median SED-based SFR and sSFR (SFR(SED) and sSFR(SED)). Specifically, MOSDEF targeted a larger population of red galaxies with U − V and V − J ≥1.25, while KBSS-MOSFIRE contains more young galaxies with intense star formation. Despite these differences in the z ∼ 2 targeted samples, the subsets of the surveys with multiple emission lines detected and analysed in previous work are much more similar. All median host-galaxy properties with the exception of stellar population age – i.e. M *, SFR(SED), sSFR(SED), A V, and UVJ colours – agree within the uncertainties. Additionally, when uniform emission-line fitting and stellar Balmer absorption correction techniques are applied, there is no significant offset between both samples in the [O  iii ]λ5008/H β versus [N  ii ]λ6585/H α diagnostic diagram, in contrast to previously reported discrepancies. We can now combine the MOSDEF and KBSS-MOSFIRE surveys to form the largest z ∼ 2 sample with moderate-resolution rest-optical spectra and construct the fundamental scaling relations of star-forming galaxies during this important epoch. [ABSTRACT FROM AUTHOR]

We analyse the rest-optical emission-line ratios of z ∼ 1.5 galaxies drawn from the Multi-Object Spectrometer for Infra-Red Exploration Deep Evolution Field (MOSDEF) survey. Using composite spectra, we investigate the mass–metallicity relation (MZR) at z  ∼ 1.5 and measure its evolution to z = 0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z  ∼ 1.5 to z = 0 depends on stellar mass, evolving by |$\Delta \rm log(\rm O/H) \sim 0.25$| dex at M *< |$10^{9.75}\, \mathrm{M}_{\odot }$| down to |$\Delta \rm log(\rm O/H) \sim 0.05$| at M * ≳ |$10^{10.5}\, \mathrm{M}_{\odot }$|⁠. In contrast, the O3N2-based MZR shows a constant offset of |$\Delta \rm log(\rm O/H) \sim 0.30$| across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z  ∼ 1.5 sample. We investigated the secondary dependence of the MZR on star formation rate (SFR) by measuring correlated scatter about the mean M *-specific SFR and M *− |$\log (\rm O3N2)$| relations. We find an anticorrelation between |$\log (\rm O/H)$| and sSFR offsets, indicating the presence of a M *−SFR−Z relation, though with limited significance. Additionally, we find that our z  ∼ 1.5 stacks lie along the z = 0 metallicity sequence at fixed μ = log (M */ M ⊙) − 0.6 × |$\log (\rm SFR / M_{\odot } \, yr^{-1})$| suggesting that the z  ∼ 1.5 stacks can be described by the z = 0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [N  ii ]/H α scales with galaxy properties is crucial to accurately describe the effects of blended [N  ii ] and H α on redshift and H α fiux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope. [ABSTRACT FROM AUTHOR]

We analyse the rest-optical emission-line spectra of z ∼ 2.3 star-forming galaxies in the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. In investigating the origin of the well-known offset between the sequences of high-redshift and local galaxies in the [O  iii ]λ5008/H β versus [N  ii ]λ6585/H α ('[N  ii ] BPT') diagram, we define two populations of z ∼ 2.3 MOSDEF galaxies. These include the high population that is offset towards higher [O  iii ]λ5008/H β and/or [N  ii ]λ6585/H α with respect to the local SDSS sequence and the low population that overlaps the SDSS sequence. These two groups are also segregated within the [O   iii ]λ5008/H β versus [S  ii ]λλ6718,6733/H α and the [O  iii ]λλ4960,5008/[O  ii  ]λλ3727,3730 (O32) versus ([O   iii ]λλ4960,5008+[O  ii ]λλ3727,3730)/H β (R23) diagrams, which suggests qualitatively that star-forming regions in the more offset galaxies are characterized by harder ionizing spectra at fixed nebular oxygen abundance. We also investigate many galaxy properties of the split sample and find that the high sample is on average smaller in size and less massive, but has higher specific star formation rate (SFR) and SFR surface density values and is slightly younger compared to the low population. From Cloudy+BPASS photoionization models, we estimate that the high population has a lower stellar metallicity (i.e. harder ionizing spectrum) but slightly higher nebular metallicity and higher ionization parameter compared to the low population. While the high population is more α -enhanced (i.e. higher α /Fe) than the low population, both samples are significantly more α -enhanced compared to local star-forming galaxies with similar rest-optical line ratios. These differences must be accounted for in all high-redshift star-forming galaxies – not only those 'offset' from local excitation sequences. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a joint analysis of rest-ultraviolet (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 [O  iii ]5007/Hβ versus [N  ii ]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 Binary Population and Spectral Synthesis (BPASS) stellar population synthesis models, including nebular continuum emission, to our rest-UV composite spectra, we find that high-redshift galaxies offset towards higher [O  iii ]λ5007/Hβ and [N  ii ]λ6584/Hα have younger ages (⁠|$\log (\rm {\,Age/yr})=7.20^{+0.57}_{-0.20}$|⁠) 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 (⁠|$12+\log (\rm {\,O/H})=8.40^{+0.06}_{-0.07}$|⁠), and the non-offset galaxies have an ionization parameter of |$\log (U)=-3.11^{+0.08}_{-0.08}$| and nebular metallicity of |$12+\log (\rm {\,O/H})=8.30^{+0.05}_{-0.06}$|⁠. The stellar and nebular metallicities derived for our sample imply that the galaxies offset from the local BPT relation are more α-enhanced (⁠|$7.28^{+2.52}_{-2.82}\rm {\,O/Fe}_{\odot }$|⁠) compared to those consistent with the local sequence (⁠|$3.04^{+0.95}_{-0.54}\rm {\,O/Fe}_{\odot }$|⁠). 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. [ABSTRACT FROM AUTHOR]

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z ∼ 5. We combine new z ∼ 5 measurements of H  i Ly α and low- and high-ionization interstellar metal absorption features with comparable measurements at z ∼ 2–4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Ly α equivalent width, performing corrections to Ly α strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Ly α emission strength over the redshift range z ∼ 2–5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Ly α equivalent widths, we observe evolution at z ∼ 5 towards greater Ly α emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly α emission strength and low-ionization line strength at z ∼ 2−4 as primarily reflecting the radiative transfer of Ly α photons, this evolution at z ∼ 5 suggests a higher intrinsic production rate of Ly α photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly α emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Ly α emission evolve towards higher redshift. We conclude that increasing intrinsic Ly α photon production and strong detection of nebular C  iv emission (signalling lower metallicity) at z ∼ 5 indicate an elevated ionized photon production efficiency (ξion). [ABSTRACT FROM AUTHOR]

To determine the factors that regulate mating opportunities of male bushy-tailed woodrats (Neotoma cinerea), we used stepwise multiple regression on measurable morphological and behavioral traits. DNA fingerprinting was used to determine the paternity of juveniles, allowing mating success (the number of females mated with), and reproductive success (the number of offspring fathered) to be quantified. Both measures of male success were significantly related to the growth rate ofmales while reproductively active. The most successful males were those that had higher growth rates indicating that there is relatively little cost (weight loss) associated with successful mating in male woodrats. Our findings demonstrate that although this species is highly sexually dimorphic, large body size does not influence mating success. In addition, it appears that male mating success cannot be predicted from morphological measures and may instead be determined by behavioral or olfactory cues. [ABSTRACT FROM AUTHOR]