The distribution of mass in the Orion dwarf Galaxy

Abstract

Dwarf galaxies are good candidates to investigate the nature of dark matter (DM), because their kinematics are dominated by this component down to small galactocentric radii. We present here the results of detailed kinematic analysis and mass modelling of the Orion dwarf galaxy, for which we derive a high-quality and high-resolution rotation curve that contains negligible non-circular motions and we correct it for the asymmetric drift. Moreover, we leverage the proximity (D = 5.4 kpc) and convenient inclination (47°) to produce reliable mass models of this system. We find that the universal rotation curve mass model (Freeman disc + Burkert halo + gas disc) fits the observational data accurately. In contrast, the NFW halo + Freeman disc + gas disc mass model is unable to reproduce the observed rotation curve, a common outcome in dwarf galaxies. Finally, we attempt to fit the data with a modified Newtonian dynamics (MOND) prescription. With the present data and with the present assumptions on distance, stellar mass, constant inclination and reliability of the gaseous mass, the MOND ‘amplification’ of the baryonic component appears to be too small to mimic the required ‘dark component’. The Orion dwarf reveals a cored DM density distribution and a possible tension between observations and the canonical MOND formalism.