Little Red Dots or Brown Dwarfs? NIRSpec Discovery of Three Distant Brown Dwarfs Masquerading as NIRCam-Selected Highly-Reddened AGNs

Résumé : Cold, substellar objects such as brown dwarfs have long been recognized as contaminants in color-selected samples of active galactic nuclei (AGNs). In particular, their near- to mid-infrared colors (1-5 $\mu$m) can closely resemble the V-shaped ($f_{\lambda}$) spectra of highly-reddened accreting supermassive black holes ("little red dots"), especially at $6 < z < 7$. Recently, a NIRCam-selected sample of little red dots over 45 arcmin$^2$ has been followed up with deep NIRSpec multi-object prism spectroscopy through the UNCOVER program. By investigating the acquired spectra, we identify three of the 14 followed-up objects as T/Y dwarfs with temperatures between 650 and 1300 K and distances between 0.8 and 4.8 kpc. At $4.8^{+0.6}_{-0.1}$ kpc, Abell2744-BD1 is the most distant brown dwarf discovered to date. We identify the remaining 11 objects as extragalactic sources at $z_{\rm spec} \gtrsim 5$. Given that three of these sources are strongly-lensed images of the same AGN (Abell2744-QSO1), we derive a brown dwarf contamination fraction of $25\%$ in this NIRCam-selection of little red dots. We find that in the near-infrared filters, brown dwarfs appear much bluer than the highly-reddened AGN, providing an avenue for distinguishing the two and compiling cleaner samples of photometrically selected highly-reddened AGN.