Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Michael W. Topping, Daniel P. Stark, Ryan Endsley, Lily Whitler, Kevin Hainline
| Summary:
The power-law slope of the rest-UV continuum
($f_{lambda}proptolambda^{beta}$) 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 $m_{rm F200W}=26-31$,
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
$rm~M_{UV}$. 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 $m_{rm F356W}-m_{rm
F444W}=0.19$ mag) than typical galaxies (median $m_{rm F356W}-m_{rm
F444W}=0.39$ 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$=79rm/Gyr$) nearly twice that of our
full sample (median$=44rm/Gyr$), 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.
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