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

Abstract

We report JWST/NIRSpec spectra of three distant T-type brown dwarfs identified in the Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) survey of the Abell 2744 lensing field. One source was previously reported as a candidate T dwarf on the basis of NIRCam photometry, while two sources were initially identified as candidate active galactic nuclei. Low-resolution 1–5 μm spectra confirm the presence of molecular features consistent with T dwarf atmospheres, and comparison to spectral standards infers classifications of sdT1, T6, and T8–T9. The warmest source, UNCOVER-BD-1, shows evidence of subsolar metallicity, and atmosphere model fits indicate T eff = 1300 K and [M/H] ∼ −1.0, making this one of the few spectroscopically confirmed T subdwarfs known. The coldest source, UNCOVER-BD-3, is near the T/Y dwarf boundary with T eff = 550 K, and our analysis indicates the presence of PH3 in the 3–5 μm region, favored over CO2 and a possible indicator of subsolar metallicity. We estimate distances of 0.9–4.5 kpc from the Galactic midplane, making these the most distant brown dwarfs with spectroscopic confirmation. Population simulations indicate high probabilities of membership in the Galactic thick disk for two of these brown dwarfs, and potential halo membership for UNCOVER-BD-1. Our simulations indicate that there are approximately 5 T dwarfs and 1–2 L dwarfs in the Abell 2744 field down to F444W = 30 AB mag, roughly one-third of which are thick disk members. These results highlight the utility of deep JWST/NIRSpec spectroscopy for identifying and characterizing the oldest metal-poor brown dwarfs in the Milky Way.

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).…”

“…Of the 17 targets from L23 with UNCOVER PRISM spectra, 11 have unambiguous evidence for broad emission lines, including three which are multiple images of the same target (Furtak et al 2023b). Three targets do not show clear evidence for broad emission lines, and a further three are brown dwarfs (Burgasser et al 2024). Therefore 11/17 = 65% of the sample are confirmed as broad-line AGN.…”

“…We targeted with the NIRSpec/PRISM 17 sources that were photometrically identified in the UNCOVER photometry (L23). Of these, 14 are extragalactic sources and three turn out to be cool brown dwarfs (Burgasser et al 2024). Three of the extragalactic sources are confirmed as multiply lensed images of the same source, A2744-QSO1 (Furtak et al 2023a(Furtak et al , 2023b, leaving 12 unique nonstellar sources.…”

“…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). Some contaminants at redder colors would remain: ∼2 metal-rich brown dwarfs that reside in the Galactic thin disk are expected.…”

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).…”