New ultra-cool and brown dwarf candidates in Gaia DR2

Context. The second Gaia data release (Gaia DR2) contains high precision positions, parallaxes and proper motions for 1.3 billion sources. The resulting Hertzsprung-Russel diagram reveals fine structures all over the mass range. Aims. This paper aims to investigate the content of Gaia DR2 at the low-mass end and to characterize ultra-cool and brown dwarfs. Methods. We first retrieve the sample of spectroscopically confirmed ultra-cool and brown dwarfs in Gaia DR2. We use their locus in the precise Hertzsprung-Russel diagram to select new candidates, and to investigate their properties. Results. The number of spectroscopically confirmed objects recovered in Gaia DR2 corresponds to 62 %, and 71 %, of the expected number of objects with and estimated Gaia magnitude Gest $\leq$ 21.5, and 20.3, respectively, significantly filling the gap with Gaia DR1. Furthermore Gaia DR2 contains ~13 000 $\geq$M7 and 679 new L candidates. A tentative classification suggests that a few hundreds are young or subdwarf candidates. Their distance distribution shows that the solar neighborhood census is still incomplete. Conclusions. Gaia DR2 offers a great wealth of information on the low-mass objects. It provide a homogeneous and precise catalog of candidates worthwhile to be further characterized with spectroscopic observations.