A PAndAS view of M31 dwarf elliptical satellites: NGC147 and NGC185
  We exploit data from the Pan-Andromeda Archaeological Survey (PAndAS) to
study the extended structures of M31's dwarf elliptical companions, NGC147 and
NGC185. Our wide-field, homogeneous photometry allows to construct deep
colour-magnitude diagrams (CMDs) which reach down to $\sim3$ mag below the red
giant branch (RGB) tip. We trace the stellar components of the galaxies to
surface brightness of $\mu_g \sim 32$ mag arcsec$^{-2}$ and show they have much
larger extents ($\sim5$ kpc radii) than previously recognised. While NGC185
retains a regular shape in its peripheral regions, NGC147 exhibits pronounced
isophotal twisting due to the emergence of symmetric tidal tails. We fit single
Sersic models to composite surface brightness profiles constructed from diffuse
light and star counts and find that NGC147 has an effective radius almost 3
times that of NGC185. In both cases, the effective radii that we calculate are
larger by a factor of $\sim2$ compared to most literature values. We also
calculate revised total magnitudes of $M_g=-15.36\pm0.04$ for NGC185 and
$M_g=-16.36\pm0.04$ for NGC147. Using photometric metallicities computed for
RGB stars, we find NGC185 to exhibit a metallicity gradient of
[Fe/H]$\sim-0.15$ dex/kpc over the radial range 0.125 to 0.5 deg. On the other
hand, NGC147 exhibits almost no metallicity gradient, $\sim-0.02$ dex/kpc from
0.2 to 0.6 deg. The differences in the structure and stellar populations in the
outskirts of these systems suggest that tidal influences have played an
important role in governing the evolution of NGC147.
