Search

The presence of evolved stars in high-redshift galaxies can place valuable indirect constraints on the onset of star formation in the Universe. Thus, we use PEARLS GTO (Prime Extragalactic Areas for Reionization and Lensing Science Guaranteed Time Observations) and public NIRCam (Near Infrared Camera) photometric data to search for Balmer-break candidate galaxies at 7 < z < 12. We find that our Balmer-break candidates at z ∼ 10.5 tend to be older (115 Myr), have lower inferred [O  iii ] + Hβ equivalent widths (120 Å), have lower specific star formation rates (6 Gyr−1) and redder UV slopes (β = −1.8) than our control sample of galaxies. However, these trends all become less strong at z ∼ 8, where the F 444 W filter now probes the strong rest-frame optical emission lines, thus providing additional constraints on the current star formation activity of these galaxies. Indeed, the bursty nature of epoch of reionization galaxies can lead to a disconnect between their current spectral energy distribution (SED) profiles and their more extended star formation histories. We discuss how strong emission lines, the cumulative effect of weak emission lines, dusty continua, and active galactic nuclei can all contribute to the photometric excess seen in the rest-frame optical, thus mimicking the signature of a Balmer break. Additional medium-band imaging will thus be essential to more robustly identify Balmer-break galaxies. However, the Balmer break alone cannot serve as a definitive proxy for the stellar age of galaxies, being complexly dependent on the star formation history. Ultimately, deep Near Infrared Spectrograph (NIRSpec) continuum spectroscopy and Mid-Infrared Instrument (MIRI) imaging will provide the strongest indirect constraints on the formation era of the first galaxies in the Universe, thereby revealing when cosmic dawn breaks. [ABSTRACT FROM AUTHOR]

We utilize theoretical models of Population III stellar + nebular spectra to investigate the prospects of observing and accurately identifying Population III galaxies with JWST using both deep imaging and spectroscopy. We investigate a series of different colour cuts, finding that a combination of NIRCam and MIRI photometry through the F444W − F560W, F560W−F770W colours offers the most robust identifier of potential z  = 8 Pop III candidates. We calculate that NIRCam will have to reach ∼28.5–30.0 AB mag depths (1–20 h), and MIRI F560W must reach ∼27.5–29.0 AB mag depths (10–100 h) to achieve 5σ continuum detections of M * = 106 M⊙ Pop III galaxies at z  = 8. We also discuss the prospects of identifying Pop III candidates through slitless and NIRSpec spectroscopic surveys that target Lyα, Hβ, and/or He  ii λ1640. We find small differences in the Hβ rest-frame equivalent width (EW) between Pop III and non-Pop III galaxies, rendering this diagnostic likely impractical. Instead, we find that the detection of high EW He  ii λ1640 emission will serve as the definitive Pop III identifier, requiring (ultra-)deep integrations (5–150 h) with NIRSpec/G140M for M * = 106 M⊙ Pop III galaxies at z  = 8. However, MIRI F 770 W detections of Pop III galaxies will require substantial gravitational lensing (μ = 10) and/or fortuitous imaging of exceptionally massive (M * = 107 M⊙) Pop III galaxies. Thus, NIRCam medium-band imaging surveys that can search for high EW He  ii λ1640 emitters in photometry may perhaps be a viable alternative for finding Pop III candidates. [ABSTRACT FROM AUTHOR]

We analyse the photometric and spectroscopic properties of four galaxies in the epoch of reionization (EoR) within the SMACS J0723.3−7327 JWST Early Release Observations field. Given the known spectroscopic redshifts of these sources, we investigated the accuracy with which photometric redshifts can be derived using NIRCam photometry alone, finding that F115W imaging is essential to distinguish between z ∼ 8 galaxies with high equivalent width (EW) [O  iii ] λ5007 emission and z ∼ 10 Balmer break galaxies. We find that all four sources exhibit strong (≥0.6 mag) F356W−F444W colours, which sit at the extreme end of theoretical predictions from numerical simulations. We find that these galaxies deviate (by ≈0.5 dex) from the local correlation between [O  iii ] λ5007/H β and [Ne  iii ] λ3869/[O  ii ], which is consistent with the predictions from simulations of high-redshift galaxies having elevated line-excitation ratios. We measure the [O  iii ] λ5007 rest-frame EWs both directly from the spectroscopy, and indirectly as inferred from the strong F356W−F444W colours, finding large [O  iii ] λ5007 EWs of 225–1740 Å. The [O  iii ] λ5007 and H β EWs are consistent with those seen in extreme, intensely star-forming dwarf galaxies in the local Universe. Our structural analysis indicates that these galaxies are resolved, exhibiting irregular shapes with bright clumps. In line with the predictions from the FLARES hydrodynamic simulations, such intense star formation and extreme nebular conditions are likely the norm, rather than the exception, in the EoR. [ABSTRACT FROM AUTHOR]