IAUS379: Dynamical Masses of Local Group Galaxies

In the hierarchical clustering scenario based on the $\Lambda$ cold dark matter ($\Lambda$CDM) model, sub-galactic dark matter halos (subhalos) are of crucial importance in building up the larger structures via merging processes. Moreover, dwarf galaxies around the Milky Way (MW) are ideal sites for studying the nature of dark matter since they are dark-matter-dominated systems.

We investigate the mass evolution histories and the evolutional tracks on the $r_\mathrm{max}$-$V_\mathrm{max}$ plane of the subhalos associated with MW-sized host halos based on the ultra-high resolution cosmological $N$-body simulation, Phi-4096 by Ishiyama et al. (2021), where $V_\mathrm{max}$ and $r_\mathrm{max}$ denote the maximum circular velocity and the radii where the circular velocity becomes $V_\mathrm{max}$, respectively. We report that the evolutional tracks of dwarf-galaxy-mass halos show an interesting feature compared to the ones of massive halos shown in the literature. The mass evolution of subhalos clearly shows two phases: the accretion phase and the stripping phase. First, more than 90% of our sample subhalos increase their masses gradually until the redshift $z=1-2$ (accretion phase), and then the masses decrease due to the tidal stripping driven by the host halo (stripping phase). Thus, we show quantitatively that tidal stripping plays an essential role in the dynamical evolution of subhalos.

In addition, the mass density profile of subhalos has a long-standing disagreement between the observations and the predictions by the pure CDM simulations, known as the cusp-core problem. Standing on a hypothesis that DM halos are formed primarily with central cusps and then some dynamical processes lead them to form cores, we provide a ‘core-to-cusp transformation model’ that reverts the properties of cored profile to ones of the cusp profile. In constructing a model, some appropriate conditions are imposed about how the transition should occur, and detailed physical processes are not considered here. The reverted observational properties, including $r_\mathrm{max}$ and $V_\mathrm{max}$, show excellent agreement with the theoretical prediction from the cosmological $N$-body simulation. This result supports the description that the CDM creates initial cusps in the centre of DM halos, and some dynamical process gives rise to the cusp-to-core transformation.