Dr Aayush Saxena

Abstract:

We present the properties of 17 faint Lyman-α emitting galaxies (LAEs) at z > 5.8 from the JWST Advanced Deep Extragalactic Survey (JADES) spectroscopic data in the Hubble Ultra Deep Field/GOODS-S. These LAEs span a redshift range z ≈ 5.8 − 8.0 and a UV magnitude range MUV ≈ −17 to −20.6, with the Lyα equivalent width (EW) in the range ≈25 − 350 Å. The detection of other rest-optical emission lines in the spectra of these LAEs enables the determination of accurate systemic redshifts and Lyα velocity offsets, as well as the physical and chemical composition of their stars and interstellar media. These faint LAEs are consistent with metal-poor systems with high ionization parameters, similar to the general galaxy population at z > 6. We measured an average ionizing photon production efficiency, log(ξion/erg−1 Hz) ≈ 25.57 across our LAEs, which does not evolve strongly with redshift. We report an anti-correlation between the Lyα escape fraction and the velocity offset from systemic redshift, consistent with model expectations. We further find that the strength and velocity offset of Lyα are neither correlated with galaxy spectroscopic properties nor with ξion. We find a decrease in Lyα escape fractions with redshift, indicative of decreasing sizes of ionized bubbles around LAEs at high redshifts. We used a range of galaxy properties to predict Lyman continuum escape fractions for our LAEs, finding that the ionizing photon output into the intergalactic medium from our LAEs remains roughly constant across the observed Lyα EW, showing a mild increase at fainter UV magnitudes and at higher redshifts. We derived correlations between the ionizing photon output from LAEs and their UV magnitudes, Lyα strengths and redshifts, which can be used to constrain the ionizing photon contribution of LAEs at z > 6 towards cosmic reionization

Abstract:

The rest-frame UV recombination emission line Lyα can be powered by ionising photons from young massive stars in star-forming galaxies, but the fact that it can be resonantly scattered by neutral gas complicates its interpretation. For reionisation-era galaxies, a neutral intergalactic medium will scatter Lyα from the line of sight, making Lyα a useful probe of the neutral fraction evolution. Here, we explore Lyα in JWST/NIRSpec spectra from the ongoing JADES programme, which targets hundreds of galaxies in the well-studied GOODS-S and GOODS-N fields. These sources are UV-faint (−20.4 < MUV < −16.4) and thus represent a poorly explored class of galaxy. We fitted the low spectral resolution spectra (R ∼ 100) of a subset of 84 galaxies in GOODS-S with zspec > 5.6 (as derived with optical lines) with line and continuum models to search for significant line emission. Through exploration of the R100 data, we find evidence for Lyα in 17 sources. This sample allowed us to place observational constraints on the fraction of galaxies with Lyα emission in the redshift range 5.6 < z < 7.5, with a decrease from z = 6 to z = 7. We also find a positive correlation between the Lyα equivalent width and MUV, as seen in other samples. We used these results to estimate the neutral gas fraction at z ∼ 7, and our estimates are in agreement with previous results (XHI ∼ 0.5 − 0.9).

Abstract:

© 2024. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We present a catalog of 717 candidate galaxies at z > 8 selected from 125 square arcmin of NIRCam imaging as part of the JWST Advanced Deep Extragalactic Survey (JADES). We combine the full JADES imaging data set with data from the JWST Extragalactic Medium Survey and First Reionization Epoch Spectroscopic COmplete Survey (FRESCO) along with extremely deep existing observations from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) for a final filter set that includes 15 JWST/NIRCam filters and five HST/ACS filters. The high-redshift galaxy candidates were selected from their estimated photometric redshifts calculated using a template-fitting approach, followed by visual inspection from seven independent reviewers. We explore these candidates in detail, highlighting interesting resolved or extended sources, sources with very red long-wavelength slopes, and our highest-redshift candidates, which extend to z phot ∼ 18. Over 93% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates at z phot > 12. We also investigate potential contamination by stellar objects, and do not find strong evidence from spectral energy distribution fitting that these faint high-redshift galaxy candidates are low-mass stars. Using 42 sources in our sample with measured spectroscopic redshifts from NIRSpec and FRESCO, we find excellent agreement to our photometric redshift estimates, with no catastrophic outliers and an average difference of 〈Δz = z phot − z spec〉 = 0.26. These sources comprise one of the most robust samples for probing the early buildup of galaxies within the first few hundred million years of the Universe’s history.Peer reviewe

Abstract:

Reliable indirect diagnostics of LyC photon escape from galaxies are required to understand which sources were the dominant contributors to reionization. While multiple LyC escape fraction (f_esc) indicators have been proposed to trace favourable conditions for LyC leakage from the interstellar medium of low-redshift ‘analogue’ galaxies, it remains unclear whether these are applicable at high redshifts where LyC emission cannot be directly observed. Using a library of 14 120 mock spectra of star-forming galaxies with redshifts 4.64 ≤ z ≤ 10 from the SPHINX²⁰ cosmological radiation hydrodynamics simulation, we develop a framework for the physics that leads to high f_esc. We investigate LyC leakage from our galaxies based on the criteria that successful LyC escape diagnostics must (i) track a high-specific star formation rate, (ii) be sensitive to stellar population age in the range 3.5–10 Myr representing the times when supernova first explode to when LyC production significantly drops, and (iii) include a proxy for neutral gas content and gas density in the interstellar medium. O₃₂, Σ_SFR, M_UV, and H β equivalent width select for one or fewer of our criteria, rendering them either necessary but insufficient or generally poor diagnostics. In contrast, UV slope (β), and E(B − V) match two or more of our criteria, rendering them good f_esc diagnostics (albeit with significant scatter). Using our library, we build a quantitative model for predicting f_esc based on direct observables. When applied to bright z > 6 Ly α emitters observed with JWST, we find that the majority of them have 𝑓esc≲10 per cent⁠.

Abstract:

Local and low-redshift (z < 3) galaxies are known to broadly follow a bimodal distribution: actively star-forming galaxies with relatively stable star-formation rates and passive systems. These two populations are connected by galaxies in relatively slow transition. By contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems1–4. These galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence—so-called mini-quenching events5, 6. However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation7, 8. Early quenched galaxies have been identified out to redshift z < 5 (refs. 9–12) and these are all found to be massive (M⋆ > 1010 M⊙) and relatively old. Here we report a (mini-)quenched galaxy at z = 7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue (U–V = 0.16 ± 0.03 mag) but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4–6 × 108 M⊙) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.