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]

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 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]

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]

The variety of star formation histories (SFHs) of z ≳ 6 galaxies provides important insights into early star formation, but has been difficult to systematically quantify. Some observations suggest that many z ∼ 6–9 galaxies are dominated by ≳200 Myr stellar populations, implying significant star formation at z ≳ 9, while others find that most reionization era galaxies are ≲10 Myr, consistent with little z ≳ 9 star formation. Here, we quantify the distribution of ages of UV-bright (⁠|$-22.5\lesssim M_{\rm \small UV}\lesssim -21$|⁠) galaxies colour-selected to lie at z ≃ 6.6–6.9, an ideal redshift range to systematically study the SFHs of reionization era galaxies with ground-based observatories and Spitzer. We infer galaxy properties with two SED modelling codes and compare results, finding that stellar masses are largely insensitive to the model, but the inferred ages can vary by an order of magnitude. We infer a distribution of ages assuming a simple, parametric SFH model, finding a median age of ∼30–70 Myr depending on SED model. We quantify the fractions of ≤10 and ≥250 Myr galaxies, finding that these systems comprise ∼15–30 per cent and ∼20–25 per cent of the population, respectively. With a flexible SFH model, the shapes of the SFHs are consistent with those implied by the simple model (e.g. young galaxies have rapidly rising SFHs). However, stellar masses can differ significantly, with those of young systems sometimes being more than an order of magnitude larger with the flexible SFH. We quantify the implications of these results for z ≳ 9 stellar mass assembly and discuss improvements expected from JWST. [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]

The reionization of hydrogen is closely linked to the first structures in the Universe, so understanding the timeline of reionization promises to shed light on the nature of these early objects. In particular, transmission of Lyman alpha (Ly α) from galaxies through the intergalactic medium (IGM) is sensitive to neutral hydrogen in the IGM, so can be used to probe the reionization timeline. In this work, we implement an improved model of the galaxy UV luminosity to dark matter halo mass relation to infer the volume-averaged fraction of neutral hydrogen in the IGM from Ly α observations. Many models assume that UV-bright galaxies are hosted by massive dark matter haloes in overdense regions of the IGM, so reside in relatively large ionized regions. However, observations and N-body simulations indicate that scatter in the UV luminosity–halo mass relation is expected. Here, we model the scatter (though we assume the IGM topology is unaffected) and assess the impact on Ly α visibility during reionization. We show that UV luminosity–halo mass scatter reduces Ly α visibility compared to models without scatter, and that this is most significant for UV-bright galaxies. We then use our model with scatter to infer the neutral fraction, |$\overline{x}_{\mathrm{ H}\,{\small I}}$|⁠ , at z ∼ 7 using a sample of Lyman-break galaxies in legacy fields. We infer |$\overline{x}_{\mathrm{ H}\,{\small I}} = 0.55_{-0.13}^{+0.11}$| with scatter, compared to |$\overline{x}_{\mathrm{ H}\,{\small I}} = 0.59_{-0.14}^{+0.12}$| without scatter, a very slight decrease and consistent within the uncertainties. Finally, we place our results in the context of other constraints on the reionization timeline and discuss implications for future high-redshift galaxy studies. [ABSTRACT FROM AUTHOR]