Very bright, very blue, and very red: JWST CAPERS analysis of highly luminous galaxies with extreme UV slopes at $\mathbf{z}=10$

Abstract

We present JWST/NIRSpec PRISM observations of three luminous ($M_{\mathrm{UV}}<-20$) galaxies at $z\sim 10$ observed with the 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 - arcsec) of blue galaxies at $z=9.800\pm 0.003$ and $z=9.808\pm 0.002$ with UV slopes of ${\hat{I}}^2=-2.87\pm 0.15$ and ${\hat{I}}^2=-2.46\pm 0.10$ 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 $z=9.942\pm 0.002$ exhibits a red UV slope with ${\hat{I}}^2=-1.51\pm 0.08$. 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 active galactic nucleus continuum due to a lack of strong UV emission lines and no broad component to H${\hat{I}}^3$ or H${\hat{I}}^2$. Instead, it is most likely that the red UV slope is due to dust-reddening ($A_{\mathrm{V}}\simeq 0.9$) implying a significant level of dust-obscured star-formation only $\simeq 480\mathrm{Myr}$ after the Big Bang. Under standard assumptions for dust attenuation, EGS-25297 would be the most intrinsically UV-luminous galaxy ($M_{\mathrm{UV},\mathrm{corr}}\simeq -22.4_{-1.1}^{+0.7}$) yet spectroscopically confirmed at $z\sim 10$. This work highlights that luminous galaxies at $z\gtrsim 10$ have a diversity of dust properties and that spectroscopy of these galaxies is essential to fully understand star-formation at $z\gtrsim 10$.

22 pages, 8 figures, accepted for publication in ApJ