We present a detailed study of the faint Milky Way satellite Draco II ( Dra II ) from deep CFHT/MegaCam broadband g and i photometry and narrow-band metallicity-sensitive CaHK observations , along with follow-up Keck II/DEIMOS multi-object spectroscopy .
Forward modeling of the deep photometry allows us to refine the structural and photometric properties of Dra II : the distribution of stars in colour-magnitude space implies Dra II is old ( 13.5 \pm 0.5 Gyr ) , very metal poor , very faint ( L _ { V } = 180 ^ { +124 } _ { -72 } { L _ { \odot } } ) , and at a distance d = 21.5 \pm 0.4 { kpc } .
The narrow-band , metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ( { [ Fe / H ] } = -2.7 \pm 0.1 dex ) .
Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations , the velocity dispersion of the system is still only marginally resolved ( \sigma _ { vr } < 5.9 { km s ^ { -1 } } at the 95 per cent confidence level ) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity ( \langle v _ { r } \rangle = -342.5 ^ { +1.1 } _ { -1.2 } { km s ^ { -1 } } ) .
We further show that the spectroscopically confirmed members of Dra II have a mean proper motion of ( \mu _ { \alpha } ^ { * } , \mu _ { \delta } ) = ( 1.26 \pm 0.27 , 0.94 \pm 0.28 ) { mas / yr } in the Gaia DR2 data , which translates to an orbit with a pericenter and an apocenter of 21.3 ^ { +0.7 } _ { -1.0 } and 153.8 ^ { +56.7 } _ { -34.7 } { kpc } , respectively .
Taken altogether , these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy .
The low-significance extra-tidal features we map around the satellite tentatively support this scenario .