The scatter about the 'Universal' dwarf spheroidal mass profile: a kinematic study of the M31 satellites And V and And VI

Abstract

While the satellites of the Milky Way (MW) have been shown to be largely consistent in terms of their mass contained within one half-light radius (M_half) with a 'universal' mass profile, a number of M31 satellites are found to be inconsistent with these relations, and seem kinematically colder in their central regions than their MW cousins. In this work, we present new kinematic and updated structural properties for two M31 dwarf spheroidals (dSph), And V and And VI, using data from the Keck Low Resolution Imaging Spectrograph (LRIS) and the DEep Imaging Multi-Object Spectrograph (DEIMOS) instruments and the Subaru Suprime-Cam imager. We measure systemic velocities of v_r=−393.1 ± 4.2 and −344.8 ± 2.5 km s^(−1), and dispersions of σ_v= 11.5^(+5.3)_(−4.4) and 9.4^(+3.2)_(−2.4) km s^(−1) for And V and And VI, respectively, meaning these two objects are consistent with the trends in σv and r_half set by their MW counterparts. We also investigate the nature of this scatter about the MW dSph mass profiles for the 'classical' (i.e. M_V < −8) MW and M31 dSph. When comparing both the 'classical' MW and M31 dSph to the best-fitting mass profiles in the size–velocity dispersion plane, we find general scatter in both the positive (i.e. hotter) and negative (i.e. colder) directions from these profiles. However, barring one exception (CVnI) only the M31 dSph are found to scatter towards a colder regime, and, excepting the And I dSph, only MW objects scatter to hotter dispersions. The scatter for the combined population is greater than expected from measurement errors alone. We assess this divide in the context of the differing disc-to-halo mass (i.e. stars and baryons to total virial mass) ratios of the two hosts and argue that the underlying mass profiles for dSph differ from galaxy to galaxy, and are modified by the baryonic component of the host.

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