A detailed study of Andromeda XIX, an extreme local analogue of ultra
  diffuse galaxies
  With a central surface brightness of $\mu_0=29.3$ mag. per sq. arcsec, and
half-light radius of $r_{\rm half}=3.1^{+0.9}_{-1.1}$~kpc, Andromeda XIX (And
XIX) is an extremely diffuse satellite of Andromeda. We present spectra for
$\sim100$ 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 $v_r=-109.0\pm1.6$ km/s and a velocity
dispersion of $\sigma_v = 7.8^{+1.7}_{-1.5}$ km/s (higher than derived in our
previous work). We marginally detect a velocity gradient along the major axis
of ${\rm d}v/{\rm d}r = -2.1\pm1.8$ km/s kpc$^{-1}$. We find its mass-to-light
ratio is higher than galaxies of comparable stellar mass ($[M/L]_{\rm half} =
278^{+146}_{-198}M_\odot/L_\odot$), 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 $v_r=-279.2\pm3.7$ km/s, and $\sigma_v=13.8^{+3.5}_{-2.6}$
km/s. 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
