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Ultracool dwarfs encompass a wide variety of compact stellar-like objectswith spectra classified as late-M, L, T and Y. Most of them have beendiscovered using wide-field imaging surveys. The Virtual Observatory has provento be of great utility to efficiently exploit these astronomical resources. Weaim to validate a Virtual Observatory methodology designed to discover andcharacterize ultracool dwarfs in deep extragalactic surveys like ALHAMBRA andCOSMOS. Three complementary searches based on parallaxes, proper motions andcolours, respectively were carried out. A total of 897 candidate ultracooldwarfs were found, with only 16 previously reported in SIMBAD. Most of the newUCDs reported here are likely late-M and L dwarfs because of the limitationsimposed by the utilization of optical ($Gaia$ DR2 and r-band) data. Wecomplement ALHAMBRA and COSMOS photometry with other catalogues in the opticaland infrared using VOSA, a Virtual Observatory tool that estimates effectivetemperatures from the spectral energy distribution fitting to collections oftheoretical models. The agreement between the number of UCDs found in theCOSMOS field and theoretical estimations together with the low false negativerate (known UCDs not discovered in our search) validates the methodologyproposed in this work, which will be used in the forthcoming wide and deepsurveys provided by the Euclid space mission. Simulations of Euclid numbercounts for UCDs detectable in different photometric passbands are presented fora wide survey area of 15,000 square degrees, and the limitations ofapplicability of Euclid data to detect UCDs using the methods employed in thispaper are discussed.

Ultracool Dwarfs in deep extragalactic surveys using the Virtual Observatory: ALHAMBRA and COSMOS