Very Bright, Very Blue, and Very Red: JWST CAPERS Analysis of Highly Luminous Galaxies with Extreme Ultraviolet Slopes at z = 10
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.
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Donnan, C. T., Dickinson, M., Taylor, A. J., Haro, P. A., Finkelstein, S. L., Stanton, T. M., Jung, I., Papovich, C., Akins, H. B., Koekemoer, A. M., McLeod, D. J., Napolitano, L., Amorín, R., Begley, R., Burgarella, D., Carnall, A. C., Casey, C. M., Calabrò, A., Cullen, F., et al. (2025). Very Bright, Very Blue, and Very Red: JWST CAPERS Analysis of Highly Luminous Galaxies with Extreme Ultraviolet Slopes at z = 10. The Astrophysical Journal, 993(2), Article 224. https://doi.org/10.3847/1538-4357/ae0a1f
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