Determination of dark matter distribution in Ursa Major III and constraints on dark matter annihilation*

Abstract The recently discovered satellite dwarf galaxy Ursa Major III provides a promising opportunity to explore the signatures resulting from dark matter (DM) annihilation owing to its proximity and large J -factor. Given the absence of an excess of γ -ray signatures originating from Ursa Major III, observations of γ -rays, such as those from Fermi-LAT, can be utilized to set constraints on the DM annihilation cross-section. In this study, we determined the DM density profile and considered the relationship between DM density and velocity dispersion at different locations within Ursa Major III through Jeans analysis. We calculated the J -factor of Ursa Major III for s -wave annihilation along with the effective J -factors for p -wave and Sommerfeld enhanced annihilation scenarios. Employing these derived J -factors, we set stringent constraints on DM annihilation cross-sections in three scenarios. Given the substantial impact of member star identification on the J -factor of Ursa Major III, we further calculated J -factors under the exclusion of the largest velocity outlier. Our analysis reveals a notable reduction in the median value and an increase in the deviation of J -factors, thereby leading to considerably weaker constraints.