A search for late-type brown dwarfs in the Euclid Quick Data Release 1

TL;DR

Mid-to-late T dwarfs in Euclid Q1 were sought via Y_E-J_E and J_E-H_E photometric cuts guided by Sanghi 2024, followed by low-resolution NISP spectroscopy and template analysis. The two-stage validation (cosine similarity with Burgasser 2017 and Theissen 2022 templates, then visual inspection) identifies 15 high-confidence candidates, eight new; spectral types range T2–T7. Photometric types and distances are estimated with the Sanghi 2024 absolute magnitudes and a color-based k-NN classifier, yielding distances from ~30 to ~185 pc. The results illustrate Euclid’s capability to probe faint ultracool dwarfs and imply a potentially large expansion of the substellar census in future survey releases.

Abstract

We present the identification and characterization of 15 mid-to-late T dwarf candidates in the Euclid Quick Release 1 (Q1) dataset, based on a combined photometric and spectroscopic analysis. Candidates were initially selected via color-based cuts in the Euclid $Y_E - J_E$ and $J_E - H_E$ color-color space, targeting the region occupied by ultracool dwarfs in synthetic photometry from the Sanghi et al. (2024) sample. From an initial pool of 38,845 sources, we extracted low-resolution near-infrared spectra from the Euclid NISP instrument and applied a two-stage validation procedure that included spectral template fitting followed by visual inspection. Eight of the 15 validated candidates are newly identified objects with no prior literature association. We examined their morphological and photometric properties and compared them with established spectral standards. Photometric distances were derived using band-averaged distance modulus estimates. We discuss the limitations and promise of the Euclid survey for ultracool dwarf studies, and demonstrate the potential for discovering substellar populations beyond the reach of current wide-field surveys.

Figures (7)

• Figure 1: Color–color diagram showing the Euclid $Y_E - J_E$ vs. $J_E - H_E$ color space. The grayscale background represents the density of sources from the Euclid Q1 dataset, limited to high-quality detections and magnitudes brighter than 23 mag in all three bands. Overplotted are synthetic Euclid colors of M6–T9 ultracool dwarfs from the Sanghi+2024 sample, colored by spectral type. Mid-to-late T dwarfs clearly deviate from earlier-type UCDs, forming a distinct population toward redder $Y_E - J_E$ and bluer $J_E - H_E$ colors. The black dashed lines indicate the adopted color selection boundaries ($0.1 < Y_E - J_E < 0.9$ and $J_E - H_E < -0.1$) used to identify candidate mid-to-late T dwarfs in the Euclid Q1 dataset.
• Figure 2: Comparison of smoothed candidate spectra (blue curves) with the Burgasser templates (dark gray curves) for the 15 selected candidates. The orange curves represent flagged values (e.g., due to low quality or artifacts). The shaded gray area around each spectrum represents the flux uncertainty. Key molecular absorption features are indicated by horizontal black lines (FeH, CH$_4$, and H$_2$O). Specifically, CH$_4$ bands span 1.28 – 1.44µm in the $J$-band and 1.6 – 1.76µm in the $H$-band; H$_2$O bands span 1.325 - 1.55µm in the $J$-band and 1.72 - 2.14µm in the $H$-band. While several spectra exhibit clear methane absorption features characteristic of mid-to-late T dwarfs (e.g., at 1.3µm, 1.6µm), others show considerable noise or deviations from the templates, reflecting the limitations of low-resolution slitless spectroscopy and varying data quality in the Euclid Q1 release. The reduced chi-square fit statistic, displayed in the bottom-left corner, provides a quantitative measure of the fit quality.
• Figure 3: Euclid $20\arcsec \times 20\arcsec$ cutouts of the 15 brown dwarf candidates in four bands $VIS$, $Y_E$, $J_E$, $H_E$, and a color composite (left to right). Each candidate is centered within the red circle. The VIS cutouts illustrate that most candidates are barely detected at optical wavelengths, as expected for mid-to-late T dwarfs. In contrast, the majority are clearly visible in the $Y_E$, $J_E$ and $H_E$ bands. Although most candidates appear compact and isolated, some might be slightly blended by nearby background sources. All images are oriented with North up and East to the left.
• Figure 4: Comparison of smoothed candidate spectra (blue curves) with the Theissen templates (dark gray curves) for the 15 selected candidates. As shown in Figure \ref{['fig:comparison-with-burgasser-templates']}, the orange curves denote flagged values, while the gray shading indicates the flux uncertainty. Key molecular absorption features are marked by horizontal black lines. The inferred spectral types based on the Theissen templates are generally consistent with those derived from the Burgasser templates and agree within one subtype.
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