Andromeda XXI -- a dwarf galaxy in a low density dark matter halo

Understand

Andromeda XXI (And XXI) has been proposed as a dwarf spheroidal galaxy with a central dark matter density that is lower than expected in the Standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) cosmology.

• In this work, we present dynamical observations for 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low density outlier.
• We measure a systemic velocity of $v_r=-363.4\pm1.0\,{\rm kms}^{-1}$ and a velocity dispersion of $\sigma_v=6.1^{+1.0}_{-0.9}\,{\rm kms}^{-1}$, consistent with previous work and within $1\sigma$ of predictions made within the modified Newtonian dynamics framework.
• We also measure the metallicity of our member stars from their spectra, finding a mean value of ${\rm [Fe/H]}=-1.7\pm0.1$~dex.