Abstract :
The Euclid mission is transforming our view of the low-mass Universe by enabling the systematic exploration of ultracool dwarfs (UCDs), from late-M dwarfs to free-floating planetary-mass objects, across diverse Galactic environments. Within the Euclid SUBSTELLAR project, we are building the largest homogeneous census of brown dwarfs and free-floating planets to date, expecting to increase the known population by two orders of magnitude. Euclid’s combination of wide-area, deep, high-resolution near-infrared imaging and low-resolution near-infrared spectroscopy enables the identification of UCDs over unprecedented volumes of the Galaxy, reaching masses of ~4 MJup at the distance of Orion. Using Early Release Observations (ERO) data, we illustrate this potential through studies of the σ Orionis cluster and the neighboring Horsehead Nebula region. In σ Orionis, Euclid observations have improved the characterization of the initial mass function (IMF) in the substellar regime, while complementary infrared data are allowing us to extend the search for UCDs into the highly reddened environments of the Orion B molecular cloud. Preliminary results reveal numerous new embedded UCD candidates, opening a window onto the formation of very low-mass objects in environments shaped by ionization fronts and stellar feedback.
We are also taking the first steps toward constraining the very low-mass field IMF using Euclid Quick Data Release 1 (Q1) and Data Release 1 (DR1). UCD candidates are selected by combining Euclid photometry and spectroscopy while accounting for contamination and survey completeness, providing the statistically robust samples needed to probe the poorly constrained lowest-mass end of the IMF. Field measurements offer a crucial counterpart to studies in young clusters, enabling us to investigate how the formation environment and subsequent dynamical evolution shape the present-day substellar population. Together, these complementary approaches demonstrate Euclid’s potential to bridge stellar and planetary populations and to facilitate discussions on fundamental questions about the shape, low-mass cutoff, and universality of the IMF, laying the foundation for the mission’s broader legacy in substellar astrophysics.
