Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Brown Dwarfs

Abstract

Abstract From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts z  >  10 and as high as z = 14. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at z  >  15 may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright z  ∼ 14 galaxies should be visible at redshifts as high as z  ∼ 20–30, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 pc. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr −1 and (24 ± 3) mas yr −1 , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.