The UV Continuum Slopes of Early Star-Forming Galaxies in JADES

Abstract

The power-law slope of the rest-UV continuum ($f_{\lambda }\propto {\lambda }^{\beta }$) 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 JADES to investigate the UV slopes of 179 $z>9$ galaxies with apparent magnitudes of $m_{F200W}=26-31$, which display a median UV slope of $\beta =-2.4$. We compare to a statistical sample of $z=5-9$ galaxies, finding a shift toward bluer rest-UV colors at all ${\mathrm{M}}_{\mathrm{UV}}$. The most UV-luminous $z>9$ galaxies are significantly bluer than their lower-redshift counterparts, representing a dearth of moderately-red galaxies in the first $500$Myr. At yet earlier times, the $z>11$ galaxy population exhibits very blue UV slopes, implying very low attenuation from dust. We identify a robust sample of 44 galaxies with $\beta <-2.8$, which have SEDs requiring models of density-bounded HII regions and median ionizing photon escape fractions of $0.51$ to reproduce. Their rest-optical colors imply that this sample has weaker emission lines (median $m_{F356W}-m_{F444W}=0.19$ mag) than typical galaxies (median $m_{F356W}-m_{F444W}=0.39$ mag), consistent with the inferred escape fractions. This sample has relatively low stellar masses (median $\log (M/M_{\odot })=7.5$), and specific star-formation rates (median$=79/Gyr$) nearly twice that of our full sample (median$=44/Gyr$), suggesting they 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 colors, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures.

17 pages, 13 figures; submitted to MNRAS

Cited by in corpus (1)

• Spectroscopic confirmation of two luminous galaxies at $z\sim 14$