Cuspy dark matter density profiles in massive dwarf galaxies

TL;DR

This study measures dark matter density profiles in six nearby dwarf galaxies using high-resolution ALMA data, finding cuspier profiles than some simulations predict, which impacts understanding of galaxy formation.

Contribution

First detailed measurement of dark matter density slopes in massive dwarf galaxies using CO rotation curves and infrared data.

Findings

Average slope $eta^* = 0.6$, indicating cuspier profiles.

Observed slopes are steeper than those predicted by FIRE and NIHAO simulations.

Dwarf galaxies studied are on the high stellar mass end and show more cuspy profiles.

Abstract

Rotation curves of galaxies probe their total mass distributions, including dark matter. Dwarf galaxies are excellent systems to investigate the dark matter density distribution, as they tend to have larger fractions of dark matter compared to higher mass systems. The core-cusp problem describes the discrepancy found in the slope of the dark matter density profile in the centres of galaxies ($\beta^*$) between observations of dwarf galaxies (shallower cores) and dark matter-only simulations (steeper cusps). We investigate $\beta^*$ in six nearby spiral dwarf galaxies for which high-resolution CO $J=1-0$ data were obtained with ALMA. We derive rotation curves and decompose the mass profile of the dark matter using our CO rotation curves as a tracer of the total potential and 4.5$\mu$m photometry to define the stellar mass distribution. We find $\langle\beta^*\rangle = 0.6$ with a standard deviation of $\pm0.1$ among the galaxies in this sample, in agreement with previous measurements in this mass range. The galaxies studied are on the high stellar mass end of dwarf galaxies and have cuspier profiles than lower mass dwarfs, in agreement with other observations. When the same definition of the slope is used, we observe steeper slopes than predicted by the FIRE and NIHAO simulations. This may signal that these relatively massive dwarfs underwent stronger gas inflows toward their centres than predicted by these simulations, that these simulations over-predict the frequency of accretion or feedback events, or that a combination of these or other effects are at work.