Euclid: Quick Data Release (Q1) -- A photometric search for ultracool dwarfs in the Euclid Deep Fields

We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria are based only on colour ($I_\mathrm{E}-Y_\mathrm{E}>2.5$). The combined requirement for optical detection and stringent signal-to-noise ratio threshold ensure a high purity of the sample, but at the expense of completeness, especially for T dwarfs. The detections range from magnitudes 19 and 24 in the near-infrared bands, and extend down to 26 in the optical band. We discuss Euclid’s capability to identify UCD candidates based on its photometric passbands. The average surface density of detected UCDs on the sky is approximately 100 objects per $\mathrm{deg}^2$, including 20 L and T dwarfs per $\mathrm{deg}^2$. This leads to an expectation of at least 1.4 million ultracool dwarfs in the final data release of the Euclid Wide Survey, including at least $300,000$ L dwarfs, and more than 2600 T dwarfs, using the strict selection criteria from this work.

💡 Research Summary

This paper presents a comprehensive photometric search for Ultracool Dwarfs (UCDs) using data from the first quick release (Q1) of the Euclid space mission. UCDs, encompassing the lowest-mass stars, brown dwarfs, and free-floating planetary-mass objects, are crucial for understanding the low-mass end of star formation. The study analyzes the three Euclid Deep Fields (EDFs), covering a total area of approximately 63 square degrees.

The authors utilized the merged photometric catalog (MER) from Q1, which contains data from Euclid’s visible (I_E) and near-infrared (Y_E, J_E, H_E) instruments. To isolate point sources—primarily stars and UCDs—from the vast number of galaxies in the catalog, a series of stringent morphological and quality filters were applied. These included cuts on ellipticity, light concentration parameters (MUMAX_MINUS_MAG), and a strict signal-to-noise ratio (S/N > 4) requirement in the I_E, Y_E, and H_E bands. This process yielded a clean sample of 688,957 point sources.

The core selection criterion for identifying UCD candidates was based on a single, powerful color index: I_E - Y_E > 2.5. This leverages the extreme red optical-to-near-infrared color that is a hallmark of UCDs due to their cool atmospheres. The combined requirement for optical detection and high S/N ensured a high-purity sample, minimizing contamination from background galaxies or instrumental artifacts. However, the authors acknowledge that this comes at the expense of completeness, particularly for the faintest T dwarfs, which emit very little optical light.

The search resulted in a catalog of 5,306 new UCD candidates, spanning spectral types from late M to late T, including approximately 1,200 L and T dwarfs. Follow-up spectroscopic observations confirmed the nature of 546 of these candidates, comprising 329 L dwarfs and 26 T dwarfs. Using these spectroscopically confirmed benchmarks, the paper provides empirical Euclid color sequences as a function of spectral type, establishing a valuable reference for future studies relying solely on Euclid photometry.

Key statistical results include an estimated sky surface density of detected UCDs of about 100 objects per square degree, with roughly 20 L and T dwarfs per square degree. By extrapolating these densities to the final planned area of the Euclid Wide Survey (EWS), the study predicts that Euclid will detect at least 1.4 million UCDs in its final data release. This immense sample is expected to include a minimum of 300,000 L dwarfs and more than 2,600 T dwarfs, even when applying the strict selection criteria from this work.

In conclusion, this study successfully demonstrates Euclid’s powerful capability to conduct large-scale photometric searches for UCDs. The high-purity catalog from the early Q1 data release validates the mission’s potential to revolutionize the census of these elusive low-mass objects. The anticipated final yield will provide an unprecedented dataset to constrain the initial mass function at its lowest masses and to investigate the formation pathways of brown dwarfs and planetary-mass objects.