Emergence of a stellar cusp by a dark matter cusp in a low-mass compact ultrafaint dwarf galaxy

Abstract

Recent observations have been discovering new ultra-faint dwarf galaxies as small as ∼ 20 pc in half-light radius and ∼ 3 km s −1 in line-of-sight velocity dispersion. In these galaxies, dynamical friction on a star against dark matter can be significant and alter their stellar density distribution. The effect can strongly depend on a central density profile of dark matter, i.e. cusp or core. In this study, I perform computations using a classical and a modern analytic formulae and N -body simulations to study how dynamical friction changes a stellar density profile and how different it is between a cuspy and a cored dark matter haloes. This study shows that, if a dark matter halo has a cusp, dynamical friction can cause shrivelling instability which results in emergence of a stellar cusp in the central region < ∼ 2 pc. On the other hand, if it has a constant-density core, dynamical friction is significantly weaker and does not generate a stellar cusp even if the galaxy has the same line-of-sight velocity dispersion. In such a compact and low-mass galaxy, since the shrivelling instability by dynamical friction is inevitable if it has a dark matter cusp, absence of a stellar cusp implies that the galaxy has a dark-matter core. I expect that this could be used to diagnose a dark matter density profile in these compact ultra-faint dwarf galaxies.

Gravitational probes of dark matter physics

“…We strongly endorse the development of novel ways to estimate the density profile of small galaxies with photometry alone, as proposed by Ref. [248].…”

“…This is on account of their paucity of stars (but see Refs. [247,248] for ideas on photometric estimates of density profiles). For these systems, we are in the near-term limited to estimates of the mean density within the half-light radius of each system.…”

“…As we show below, new ideas are desperately needed for large swaths of M vir , possibly leading to better use of astronomical objects already identified as interesting. For example: the implications for two-body gravitational relaxation in ultrafaint dwarf galaxies [112,247,248] We now turn to more technical issues, namely the mapping between dark-matter microphysics and the macroscopic implementation of that physics in simulations. Importantly, the algorithms to model dark matter properties and their accuracy used to simulate dark matter and galaxies together must be well-matched to the observed system under consideration.…”

Review of Solutions to the Cusp-Core Problem of the ΛCDM Model

“…Recently, DFT studies have shifted their focus from GCs to DF on stars. Using analytical approaches, as in Hernandez [143], or numerical techniques, in Inoue [144], on ultra-faint satellite galaxies of our galaxy, was shown that DM fractions imply DFT for the stars themselves and are incompatible with cusped dark matter haloes, at these tiny scales, out of reach of SNF models, and even maybe of DFBC solutions.…”

Review of solutions to the Cusp-core problem of the $Λ$CDM Model

“…Recently, DFT studies have shifted their focus from GCs to DF on stars. Using analytical approaches, as in Hernandez [144], or numerical techniques, in Inoue [145], on ultra-faint satellite galaxies of our galaxy, was shown that DM fractions imply DFT for the stars themselves and are incompatible with cusped dark matter haloes, at these tiny scales, out of reach of SNF models, and even maybe of DFBC solutions.…”