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

Dark galaxies are small, dark matter-dominated haloes whose gas remains in hydrostatic and thermal equilibrium and has never formed stars. These systems are of particular interest because they represent a strong prediction of the Lambda cold dark matter (łcdm) model. As of today, only a few dark galaxy candidates have been detected so far, 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 within our Local Group, characterise their physical properties, and provide guidance for their potential observational detection. We analyse Local Group simulations with constrained initial conditions, each run with different codes, implementing different baryonic physics, feedback prescriptions, and employing two distinct values of star-formation 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 all such simulations, though their number is larger in simulations that use a higher, more realistic n_̊m th. These galaxies, whose gas remains diffuse and never forms stars, predominantly inhabit less-concentrated, higher-spin dark matter haloes than their luminous counterparts. Dark galaxies are typically found in low-density regions at the outskirts of the Local Group. Their formation and evolution across redshift indicate that both the dark matter and gas densities in the surroundings of dark galaxies were consistently lower than those found around bright galaxies, making them less susceptible to interactions, mergers, or gas inflows. We estimate that up to eight dark galaxies should be detectable in ̋Isc emission within 2. 5, ̊m Mpc of the Milky Way, with the FAST radio telescope, accounting for its sky coverage and minimum ̋Isc mass and column density. Current hydrodynamical simulations of galaxies, combined with upcoming ̋Isc surveys, will offer a direct and powerful test of łcdm through their ability to predict and measure properties of dark galaxies within and beyond the Local Group.