Little Red Dots or Brown Dwarfs? NIRSpec Discovery of Three Distant Brown Dwarfs Masquerading as NIRCam-selected Highly Reddened Active Galactic Nuclei

Abstract

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 μm) can closely resemble the V-shaped (f λ ) 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 arcmin2 has been followed up with deep NIRSpec multiobject prism spectroscopy through the UNCOVER program. By investigating the acquired spectra, we identify 3 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 − 0.1 + 0.6 kpc, A2744-BD1 is the most distant brown dwarf discovered to date. We identify the remaining 11 objects as extragalactic sources at z spec ≳ 5. Given that three of these sources are strongly lensed images of the same AGN (A2744-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.

UNCOVER: JWST Spectroscopy of Three Cold Brown Dwarfs at Kiloparsec-scale Distances

“…Section 5 summarizes our results. We note that these sources and the analysis of the data presented here have also been reported contemporaneously by Langeroodi & Hjorth (2023).…”

UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5

“…In this section we briefly describe the UNCOVER survey (Section 2.1 Bezanson et al 2022) and the Micro-Shutter Assembly (MSA)/PRISM spectroscopy and reductions in Section 2.2. Many exciting results have already come from the PRISM spectroscopy of the compact red sources, including a triply imaged z = 7 AGN (Furtak et al 2023b), a broad-line AGN at z = 8.5 (Fujimoto et al 2023a;Kokorev et al 2023), and three brown dwarfs which show similar SEDs as red AGN (Burgasser et al 2024;Langeroodi & Hjorth 2023). We have also reported two z > 12 galaxies (Wang et al 2023), and some of the faintest known targets in the epoch of reionization (Atek et al 2023a).…”

“…One obvious contaminant is brown dwarf stars. A simple color cut, excluding all sources bluer than F115W − F200W < −0.5 (blue box in Figure 4), would remove the majority of brown dwarfs from high-z galaxy selections in high-latitude deep fields (see also Langeroodi & Hjorth 2023). Based on the simulations from Burgasser et al (2024) in the UNCOVER area we expect ∼5 low-metallicity halo brown dwarfs, expected to have the bluest F150W − F200W colors (see Figure 4).…”

Extremely Red Galaxies at z = 5–9 with MIRI and NIRSpec: Dusty Galaxies or Obscured Active Galactic Nuclei?

“…At the same time, brown dwarfs also exhibit an upturn in their SEDs starting around 3.0 μm and peaking at ∼4.5 μm, which leads to red LW colors. Brown dwarf candidates have already been identified photometrically due to their peculiar blue-red SEDs using color-color thresholds similar to the ERO selection criteria (e.g., Hainline et al 2023;Holwerda et al 2023;Nonino et al 2023;Wang et al 2023), and recent NIRSpec observations have confirmed the stellar nature of handful of them (Langeroodi et al 2023;Burgasser et al 2024).…”