Two late-T dwarfs at kiloparsec distances revealed by JWST UNCOVER survey

TL;DR

The paper exploits JWST UNCOVER and MegaScience deep-field data to search for brown dwarfs at kiloparsec distances, using a two-step process of colour-based candidate selection and SED fitting to both stellar atmosphere models and high-redshift galaxy templates, followed by NIRSpec spectroscopic validation. It identifies two robust late-T dwarfs and a possible L subdwarf in the Abell 2744 field, deriving photometric distances and Galactic heights that place them across the thin/thick disc and halo. The results show a BD surface density of $0.094$ arcmin$^{-2}$ to a depth of $F277W = 29.24$ mag, with T5–T7 dwarfs underrepresented relative to the solar neighbourhood at kiloparsec distances, while T8–T8.9 dwarfs in the thick disc approach local densities. The work highlights both the power and the current limitations of broad-band infrared photometry for distant substellar detection and the essential role of spectroscopy to distinguish BDs from high-redshift galaxies in JWST deep fields, advancing our understanding of substellar demographics beyond the solar neighbourhood.

Abstract

We conducted a search for brown dwarf candidates in a James Webb Space Telescope deep field around A2744 to investigate the space density of these objects at kiloparsec distances. Our methodology employed an initial selection based on photometric colours, followed by spectral energy distribution fitting to both stellar atmospheric models and high-redshift galaxy templates. This approach yielded two robust T dwarf candidates and one possible L subdwarf candidate. The T dwarfs have estimated Galactic heights of 0.43 and 0.86 kpc, likely residing near the outer edges of the Galactic thin and thick discs, respectively. We measure a T dwarf surface number density of 0.094 per squared arcmin in the UNCOVER field, lower than previous predictions but consistent at the order-of-magnitude level. We also provide space number density estimates for T5-T8.9 dwarfs across different effective temperature and spectral type bins, finding that T5-T7 dwarfs out to 2 kpc have significantly lower densities than their solar neighbourhood counterparts, whilst T8 dwarfs within the thick disc exhibit densities comparable to local values. Our analysis demonstrates that broad-band near- to mid-infrared photometry provides high sensitivity to late-T dwarfs but is relatively less sensitive to L and early-T dwarfs. Spectroscopy is typically required to distinguish photometric candidates of L dwarfs, early-T subdwarfs, and high-redshift galaxies in JWST deep fields. This study demonstrates the potential for expanding our understanding of brown dwarf distributions and characteristics at unprecedented distances, offering new insights into substellar populations beyond the solar neighbourhood.

Figures (10)

• Figure 1: JWST NIRCam colour–colour diagrams showing the three new BD candidates (red circles) and three known BDs (black diamonds) in the UNCOVER field. Grey points represent all sources in the UNCOVER catalogue, with black contour lines indicating the 25th, 50th, 68th, 75th, 95th, and 99th percentiles of the source density distribution. Solid black lines delineate our selection criteria defined in Equations \ref{['eq1']}–\ref{['eq2']}. Coloured points show theoretical predictions from the Sonora atmospheric models Marl21 for $T_{\rm eff}$ ranging from 400 to 2200 K (in steps of 50 K for $T_{\rm eff} < 1000$ K and 100 K for $T_{\rm eff} > 1000$ K), with surface gravities of $4.0 \leq \log g \leq 5.5$ and metallicities of $\rm [M/H] = 0.0$ (upper panels) and $\rm [M/H] = -0.5$ (lower panels). The colour scale indicates temperature, transitioning from orange (hottest) to blue (coolest). Models with identical $\log g$ values are connected by lines: red ($\log g = 4.0$), orange ($\log g = 4.5$), green ($\log g = 5.0$), and blue ($\log g = 5.5$). Model magnitudes have been converted from the Vega to the AB photometric system using the following offsets: 0.97 (F115W), 1.14 (F150W), 1.53 (F277W), and 2.97 (F444W).
• Figure 2: Flowchart of our selection process. The number of remaining candidates at each step is indicated.
• Figure 3: Upper panel: JWST NIRSpec/PRISM spectrum and NIRCam photometry of the T6 dwarf standard SD1624. The transmission profiles of NIRCam filters used in the UNCOVER survey are shown below the spectrum. Lower panel: Best-fitting solar-metallicity ATMO model ($T_{\rm eff} = 1100$ K, $\log g = 5.5$) to the NIRCam photometry of SD1624 (red points).
• Figure 4: Best-fitting solar-metallicity ATMO models (blue spectra) to the observed SEDs (red points) of three known BDs (left-hand panels) and three new BD candidates (right-hand panels). JWST NIRCam postage stamp images (F090W, F115W, F150W, F200W, F277W, F356W, F444W; $1\arcsec \times 1\arcsec$ field of view) are shown below each corresponding SED, with north up and east to the left. Interstellar extinction ($A_V$) was fixed at 0.0 for all fits. For UNCOVER-BD-1, NIRSpec spectral fitting Bur24 yields a best-fitting LOWZ model meis21 with $T_{\rm eff} = 1300$ K, $\log g = 5.0$, and $\rm [M/H] = -1.0$. The best-fitting galaxy template to UNCOVER-DR3-59579 is also overplotted.
• Figure 5: Best-fitting LOWZ models (black lines and diamonds) to the SEDs (red crosses) of three known BDs (left panels) and three BD candidates (right panels). The model parameters $T_{\rm eff}$, $\log g$, [M/H], C/O, and $\log_{10}K_{zz}$ (fixed at 2.0), along with the $\chi^2_{\rm L}$ values of the fits, are indicated.