JWST 's first glimpse of the z > 10 Universe has yielded a surprising abundance of luminous galaxy candidates. Here we present the most extreme of these systems: CEERS-1749. Based on 0.6 − 5µm photometry, this strikingly luminous (≈26 mag) galaxy appears to lie at z ≈ 17. This would make it an M UV ≈ −22, M ≈ 5 × 10 9 M system that formed a mere ∼ 220 Myrs after the Big Bang. The implied number density of this galaxy and its analogues challenges virtually every early galaxy evolution model that assumes ΛCDM cosmology. However, there is strong environmental evidence supporting a secondary redshift solution of z ≈ 5: all three of the galaxy's nearest neighbors at < 2.5 have photometric redshifts of z ≈ 5. Further, we show that CEERS-1749 may lie in a z ≈ 5 protocluster that is 5× overdense compared to the field. Intense line emission at z ≈ 5 from a quiescent galaxy harboring ionized gas, or from a dusty starburst, may provide satisfactory explanations for CEERS-1749's photometry. The emission lines at z ≈ 5 conspire to boost the > 2µm photometry, producing an apparent blue slope as well as a strong break in the SED. Such a perfectly disguised contaminant is possible only in a narrow redshift window (∆z 0.1), implying that the permitted volume for such interlopers may not be a major concern for z > 10 searches, particularly when medium-bands are deployed. If CEERS-1749 is confirmed to lie at z ≈ 5, it will be the highest-redshift quiescent galaxy, or one of the lowest mass dusty galaxies of the early Universe detected to-date (A 5500 ≈ 1.2 mag, M ≈ 5 × 10 8 M ). Both redshift solutions of this intriguing galaxy hold the potential to challenge existing models of early galaxy evolution, making spectroscopic follow-up of this source critical.

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3−7327 were recently taken as part of the Early Release Observations (EROs) program of the James Webb Space Telescope (JWST). Among these, two NIRSpec spectra of galaxies, at z = 7.7 and at z = 8.5, were obtained, revealing, for the first time, the rest-frame optical emission line spectra of galaxies in the epoch of reionization, including the detection of the important [O iii]λ4363 auroral line (see JWST PR 2022-035). We present an analysis of the emission line properties of these galaxies, finding that these galaxies have a high excitation (as indicated by high, high equivalent widths, and other properties typical of low-metallicity star-forming galaxies. Using the direct method, we determined oxygen abundances of 12 + log(O/H) ≈ 7.9 in two z = 7.7 galaxies and a lower metallicity of 12 + log(O/H) ≈ 7.4−7.5 (∼5% solar) in the z = 8.5 galaxy using different strong line methods. More accurate metallicity determinations will require better data. With stellar masses estimated from spectral energy distribution (SED) fits, we find that the three galaxies lie close to or below the z ∼ 2 mass-metallicity relation. Overall, these first galaxy spectra at z ∼ 8 show a strong resemblance in their the emission line properties of galaxies in the epoch of reionization with those of relatively rare local analogs previously studied with the SDSS. Clearly, the first JWST observations demonstrate already the incredible power of spectroscopy to reveal the properties of galaxies in the early Universe.

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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 &gt; 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 &lt; Td &lt; 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.

Over the last few years, both Atacama Large Millimeter/submillimeter Array (ALMA) and Spitzer observations have revealed a population of likely massive galaxies at z &gt; 3 that was too faint to be detected inHubble Space Telescope(HST) rest-frame ultraviolet imaging. However, due to the very limited photometry for individual galaxies, the true nature of these so-called HST-dark galaxies has remained elusive. Here, we present the first sample of such galaxies observed with very deep, high-resolution NIRCam imaging from the Early Release Science programme CEERS. 30 HST-dark sources are selected based on their red colours across 1.6–4.4 $\mu$m. Their physical properties are derived from 12-band multiwavelength photometry, including ancillary HST imaging. We find that these galaxies are generally heavily dust-obscured (AV ∼ 2 mag), massive (log (M/M⊙) ∼ 10), star-forming sources at z ∼ 2−8 with an observed surface density of ∼0.8 arcmin−2. This suggests that an important fraction of massive galaxies may have been missing from our cosmic census at z &gt; 3 all the way into the Epoch of Reionization. The HST-dark sources lie on the main sequence of galaxies and add an obscured star formation rate density of $\mathrm{3.2^{+1.8}_{-1.3} \times 10^{-3} \,{\rm M}_{\odot }\,yr^{-1}\,Mpc^{-3}}$ at z ∼ 7, showing likely presence of dust in the Epoch of Reionization. Our analysis shows the unique power of JWST to reveal this previously missing galaxy population and to provide a more complete census of galaxies at z = 2−8 based on rest-frame optical imaging.