Three Brown Dwarfs Masquerading as High-Redshift Galaxies in JWST Observations

Zhijun Tu, Shu Wang, Xiaodian Chen, Jifeng Liu

First listed 2025-01-28 | Last updated 2025-01-28

Abstract

We report the spectroscopic identification of three brown dwarf candidates -- o005_s41280, o006_s00089, and o006_s35616 -- discovered in the RUBIES using James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) PRISM/CLEAR spectroscopy. We fit these sources with multiple substellar atmosphere models and present the atmospheric parameters, including effective temperature ($T_\mathrm{eff}$), surface gravity, and other derived properties. The results suggest that o005_s41280 and o006_s35616, with $T_\mathrm{eff}$ in the ranges of 2100--2300 K and 1800--2000 K, are likely L dwarfs, while o006_s00089, with $T_\mathrm{eff} < 1000$ K, is consistent with a late T dwarf classification. The best-fit model spectra provide a reasonable match to the observed spectra. However, distinct residuals exist in the $Y$, $J$, and $H$ bands for the two L dwarf candidates, particularly for o006_s35616. Incorporating the extinction parameter into the fitting process can significantly reduce these residuals. The distance estimates indicate that these candidates are about 2 kpc away. The analysis of the color-color diagram using multiple JWST NIRcam photometry suggests that cooler T dwarfs, such as o006_s00089, overlap with little red dots (LRDs), while hotter L dwarfs, like o005_s41280 and o006_s35616, tend to contaminate the high-redshift galaxy cluster. These findings suggest a brown dwarf contamination rate of approximately 0.1% in extragalactic deep field surveys, with L dwarfs being more frequently detected than cooler T and Y dwarfs.

Short digest

From RUBIES NIRSpec PRISM/CLEAR (0.6–5.3 μm) spectra, the authors confirm three compact RUBIES sources (o005_s41280, o006_s00089, o006_s35616) are brown dwarfs rather than distant galaxies. Sonora Elf Owl fits yield Teff ≈2100–2300 K and 1800–2000 K for the two L-dwarf candidates and <1000 K for the late‑T object, with distances around 2 kpc; adding an extinction term notably improves Y/J/H residuals, especially for o006_s35616. NIRCam color–color planes place the cool T dwarf within the little red dot locus while the hotter L dwarfs sit among high‑z galaxy selections, implying ≈0.1% brown‑dwarf contamination in deep extragalactic surveys dominated by L dwarfs. The work pins down how specific substellar SED shapes infiltrate LRD/high‑z samples and shows that low‑resolution prism spectra efficiently weed them out.

Key figures to inspect

• Figure 1: Compare observed PRISM spectra to best‑fit models for each object to see the H2O/CH4‑shaped V‑profile (∼1–2.4 μm) and the percent residuals; use the RGB cutouts to confirm point‑source morphology that would masquerade as compact high‑z systems.
• Figure 2: For o006_s35616, inspect the with‑vs‑without extinction fits to see how adding AV suppresses the pronounced Y/J/H residuals and brings the model into agreement—evidence that modest line‑of‑sight reddening affects the apparent L‑dwarf SED.
• Figure 3: Read the placement of o005_s41280, o006_s00089, and o006_s35616 on NIRCam color–color and color–magnitude planes relative to RUBIES sources and LRDs to understand why the T dwarf overlaps the LRD locus while L dwarfs intrude on high‑z galaxy selections, quantifying the contamination pathway.