DREAMS: Taking the Temperature of a Massive Galaxy at z ~ 11

Astronomers present a detailed photometric and spectroscopic analysis of Janus-11, a galaxy at z = 10.7933 ± 0.0006 that is magnified by a factor of 1.6 by the galaxy cluster MACS J0416. It was observed with JWST/NIRCam as part of the PEARLS program and with JWST/NIRSpec MSA as part of the DREAMS program.

Janus-11 has M_UV = -19.78 (+0.12/-0.11) and a stellar mass of log(M*/M_sun) = 8.75 (+0.08/-0.10), placing it among the most massive known z>10 galaxies. Its A_V = 0.54 ± 0.09 mag and UV slope β = -1.71 ± 0.26 are compatible with a red monster classification, though it is among the least obscured members of this population; this could hint at a transition from the red monster to the blue monster phase.

The detection of [OIII]4363 enables the highest-redshift metallicity measurement based on the [OIII]4363 direct method to date: 12 + log(O/H) = 7.74 (+0.11/-0.10), with an electron temperature T_e(O++) = (2.1 ± 0.2) × 10^4 K. The object is spatially resolved with an effective radius of 136 ± 9 pc; its emission lines have narrow FWHM (≲230 km/s, or σ_v ≲100 km/s), and there are no significant detections of high-ionization emission lines or broad components, favoring a star-forming galaxy classification, although an AGN contribution cannot be excluded.

Janus-11 joins the small sample of spectroscopically confirmed and characterized sources at z>10, providing new information on massive systems with non-negligible dust attenuation and well-measured metallicity in this epoch.