Very Bright, Very Blue, and Very Red: JWST CAPERS Analysis of Highly Luminous Galaxies with Extreme Ultraviolet Slopes at z = 10

Abstract

We present JWST/NIRSpec PRISM observations of three luminous ( M UV  < −20) galaxies at z  ∼ 10 observed with the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS) Cycle 3 program. These galaxies exhibit extreme UV slopes compared to typical galaxies at z = 10. Of the three sources, two of them are a close pair (0 . ″ 22) of blue galaxies at z  = 9.800 ± 0.003 and z  = 9.808 ± 0.002 with UV slopes of β  = −2.87 ± 0.15 and β  = −2.46 ± 0.10, respectively, selected from PRIMER COSMOS NIRCam imaging. We perform spectrophotometric modeling of the galaxies, which suggests extremely young stellar ages and a lack of dust attenuation. For the bluest galaxy, its UV slope also suggests significant Lyman continuum escape. In contrast, the third source (selected from CEERS NIRCam imaging) at z  = 9.942 ± 0.002 exhibits a red UV slope with β  = −1.51 ± 0.08. We rule out the possibility of a strong nebular continuum due to the lack of a Balmer jump and find no evidence to support the presence of an active galactic nucleus continuum due to a lack of strong UV emission lines and no broad component to H γ or H β . Instead, it is most likely that the red UV slope is due to dust reddening ( A V  ≃ 0.9) implying a significant level of dust-obscured star formation only ≃480 Myr after the Big Bang. Under standard assumptions for dust attenuation, EGS-25297 would be the most intrinsically UV-luminous galaxy ( M UV , corr ≃ − 22 . 4 − 1.1 + 0.7 ) yet spectroscopically confirmed at z  ∼ 10. This work highlights that luminous galaxies at z  ≳ 10 have a diversity of dust properties and that spectroscopy of these galaxies is essential to fully understand star formation at z  ≳ 10.