Guacimara García-Bethencourt

Context. 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 (ΛCDM) model. As of today, only a few dark galaxy candidates have been detected so far, the most intriguing of which being Cloud-9. Aims. 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. Methods. 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. Results. We demonstrate that dark galaxies exist in all 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 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 H I emission within 2.5 Mpc of the Milky Way, with the FAST radio telescope, accounting for its sky coverage and minimum H I mass and column density. Conclusions. Current hydrodynamical simulations of galaxies, combined with upcoming H I surveys, will offer a direct and powerful test of ΛCDM through their ability to predict and measure properties of dark galaxies within and beyond the Local Group.

Within a simulated Milky Way-like galaxy, we identify and analyse analogues of the Gaia-Enceladus (GE), Kraken, and Sequoia mergers that each matches remarkably well observational results, including in velocity and chemical abundance space, and their distributions in the j_z-Energy plane. The Kraken analogue is the earliest merger and has the highest total mass ratio. Consistent with previous studies, it is chemically indistinguishable from old in situ stars at the time of its accretion. The GE and Sequoia analogue events accrete at similar times in our simulation, both along filaments but from opposite sides of the main galaxy. The mean stellar ages of the GE and Sequoia analogues are both similar and, from our simulation results, we see that they can be separate entities and still naturally reproduce the observed properties of their stellar remnants at the present day, including the significant retrograde velocities of the Sequoia analogue remnant stars and the difference in the tracks of the two galaxies through chemical abundance space. Our results provide supporting information about the properties of these three merger events, and show for the first time that they can all be reproduced with a fully cosmological simulation, providing a possible self-consistent evolutionary pathway for the Milky Way’s formation.