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

TL;DR

This study uses JWST/NIRSpec observations to analyze three luminous galaxies at around redshift 10, revealing diverse UV slopes, stellar ages, and dust properties, and emphasizing the importance of spectroscopy for understanding early star formation.

Contribution

First spectroscopic analysis of luminous $z hicksim10$ galaxies showing diverse UV slopes and dust properties, highlighting the need for spectroscopy to understand early galaxy evolution.

Findings

Two galaxies have extremely blue UV slopes indicating young stars and possible Lyman continuum escape.

One galaxy shows a red UV slope due to dust reddening, implying significant dust-obscured star formation.

Spectroscopy reveals diversity in dust and star formation properties among luminous early galaxies.

Abstract

We present JWST/NIRSpec PRISM observations of three luminous ($M_{\rm UV}<-20$) galaxies at $z\sim10$ 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 $z=9.800\pm0.003$ and $z=9.808\pm0.002$ with UV slopes of $\beta=-2.87\pm0.15$ and $\beta=-2.46\pm0.10$ 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 $z=9.942\pm0.002$ exhibits a red UV slope with $\beta=-1.51\pm0.08$. 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$\gamma$ or H$\beta$. Instead, it is most likely that the red UV slope is due to dust-reddening ($A_{\rm V}\simeq0.9$) implying a significant level of dust-obscured star-formation only $\simeq480\, \rm Myr$ after the Big Bang. Under standard assumptions for dust attenuation, EGS-25297 would be the most intrinsically UV-luminous galaxy ($M_{\mathrm{UV,corr}}\simeq -22.4^{+0.7}_{-1.1}$) yet spectroscopically confirmed at $z \sim 10$. This work highlights that luminous galaxies at $z\gtrsim10$ have a diversity of dust properties and that spectroscopy of these galaxies is essential to fully understand star-formation at $z\gtrsim10$.