HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them

Dark galaxies are small, DM-dominated halos whose gas remains in hydrostatic and thermal equilibrium and has never formed stars. They are of particular interest because they represent a strong prediction of the LCDM model. As of today, only a handful of candidates have been detected, the most intriguing of which being Cloud-9. Using several state-of-the-art hydrodynamical simulations, we aim to predict the abundance of dark galaxies expected within our Local Group (LG), characterise their properties and provide guidance for their potential detection. We analyse LG simulations with constrained initial conditions, run with different codes, implementing different baryonic physics, feedback prescriptions, and employing two distinct values of SF density threshold, n_th=0.13 and 10 cm^-3, to select samples of dark and bright galaxies harboured in haloes of similar mass. We demonstrate that dark galaxies exist in such simulations, though their number is larger in simulations that use a higher, more realistic n_th. These galaxies, whose gas remains diffuse and never forms stars, predominantly inhabit less-concentrated, higher-spin DM halos than their luminous counterparts. Dark galaxies are typically found in low-density regions at the outskirts of the LG, and their evolution across z indicate that both the DM and gas densities in their surroundings were consistently lower than those found around bright galaxies, making them less susceptible to interactions, mergers, or gas inflows. We estimate that up to 8 dark galaxies should be detectable in HI emission within 2.5 Mpc of the MW, with the FAST telescope, accounting for its sky coverage and minimum M_HI and N_HI. Current hydrodynamical simulations of galaxies, combined with upcoming HI surveys, will offer a direct and powerful test of LCDM through their ability to predict and measure properties of dark galaxies within and beyond the LG.