Deep Imaging of Grus II and Horologium II: Structure and Extent of Two Ultra-Faint Milky Way Satellites

Abstract

We present deep, wide-field Magellan/Megacam imaging of the ultra-faint Milky Way (MW) satellites Grus II (Gru II) and Horologium II (Hor II), with the aim of deriving improved constraints on their distances, luminosities, and structural parameters, while also searching for possible signs of tidal disturbance. Our photometry reaches approximately 3 magnitudes deeper than the discovery data, enabling robust measurements of these quantities. Both systems exhibit color-magnitude diagrams consistent with old ($\sim$12.5 Gyr), very metal-poor stellar populations. We find Gru II to be at a distance of $52.3 \pm 1.9$ kpc, with a half-light radius of $6.8 \pm 0.5$ arcmin (103 $\pm$ 9 pc), ellipticity $ε= 0.25 \pm 0.07$, and absolute magnitude $M_V = -4.07 \pm 0.50$ mag. Hor II is further away at a distance of $72.4^{+5.9}_{-5.5}$ kpc and more compact, with $r_h = 2.1 \pm 0.2$ arcmin (44$^{+6}_{-5}$ pc), $ε= 0.32^{+0.20}_{-0.16}$, and $M_V = -2.10 \pm 0.44$ mag. Both galaxies lie within the typical size-luminosity locus of MW ultra-faint dwarfs. Gru II shows an asymmetric morphology including multi-directional clumpy features, some of which may be suggestive of tidal disturbance. We further identify and spectroscopically confirm a new distant member just outside $3r_h$ in Gru II, providing independent evidence for member stars at large projected radii. In contrast, Hor II appears regular, with no significant extended structure detected to the surface-brightness limits of our data.