The UV continuum slopes of early star-forming galaxies in JADES

Michael W Topping, Daniel P Stark, Ryan Endsley, Lily Whitler, Kevin Hainline, Benjamin D Johnson, Brant Robertson, Sandro Tacchella, Zuyi Chen, Stacey Alberts, William M Baker, Andrew J Bunker, Stefano Carniani, Stephane Charlot, Jacopo Chevallard, Emma Curtis-Lake, Christa DeCoursey, Eiichi Egami, Daniel J Eisenstein, Zhiyuan JiRoberto Maiolino, Christina C Williams, Christopher N A Willmer, Chris Willott, Joris Witstok

Research output: Contribution to journalArticlepeer-review

Abstract

The power-law slope of the rest-ultraviolet (UV) continuum (fλ ∝ 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 mF200W ≃ 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 mF356W − mF444W = 0.19 mag) than typical galaxies (median mF356W − mF444W = 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.
Original languageEnglish
Article numberstae800
Pages (from-to)4087-4103
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Volume529
Issue number4
Early online date22 Mar 2024
DOIs
Publication statusE-pub ahead of print - 22 Mar 2024

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