With a central surface brightness of \mu _ { 0 } = 29.3 mag .
per sq .
arcsec , and half-light radius of r _ { half } = 3.1 ^ { +0.9 } _ { -1.1 } kpc , Andromeda XIX ( And XIX ) is an extremely diffuse satellite of Andromeda .
We present spectra for \sim 100 red giant branch stars in this galaxy , plus 16 stars in a nearby stellar stream .
With this exquisite dataset , we re-derive the properties of And XIX , measuring a systemic velocity of < v _ { r } > = -109.0 \pm 1.6 kms ^ { -1 } and a velocity dispersion of \sigma _ { vr } = 7.8 ^ { +1.7 } _ { -1.5 } kms ^ { -1 } ( higher than derived in our previous work ) .
We marginally detect a velocity gradient along the major axis of And XIX of \frac { { d } v } { { d } \chi } = -2.1 \pm 1.8 ~ { } { kms } ^ { -1 } kpc ^ { -1 } .
We find And XIX ’ s mass-to-light ratio is higher than galaxies of comparable stellar mass ( [ M / L ] _ { half } = 278 ^ { +146 } _ { -198 } M _ { \odot } / L _ { \odot } ) , but its dynamics place it in a halo with a similar total mass to these galaxies .
This could suggest that And XIX is a ‘ ‘ puffed up ’ ’ dwarf galaxy , whose properties have been altered by tidal processes , similar to its Milky Way counterpart , Antlia II .
For the nearby stream , we measure v _ { r } = -279.2 \pm 3.7 ~ { } { kms } ^ { -1 } , and \sigma _ { v } = 13.8 ^ { +3.5 } _ { -2.6 } ~ { } { kms } ^ { -1 } .
We measure its metallicity , and find it to be more metal rich than And XIX , implying that the two features are unrelated .
Finally , And XIX ’ s dynamical and structural properties imply it is a local analogue to ultra diffuse galaxies ( UDGs ) .
Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted .