Hubble Space Telescope imaging of the extremely metal-poor globular
  cluster EXT8 in Messier 31
  We recently found the globular cluster (GC) EXT8 in M31 to have an extremely
low metallicity of [Fe/H]=-2.91+/-0.04 using high-resolution spectroscopy. Here
we present a colour-magnitude diagram (CMD) for EXT8, obtained with the Wide
Field Camera 3 on board the Hubble Space Telescope. Compared with the CMDs of
metal-poor Galactic GCs, we find that the upper red giant branch (RGB) of EXT8
is about 0.03 mag bluer in F606W-F814W and slightly steeper, as expected from
the low spectroscopic metallicity. The observed colour spread on the upper RGB
is consistent with being caused entirely by the measurement uncertainties, and
we place an upper limit of sigma(F606W-F814W)=0.015 mag on any intrinsic colour
spread. The corresponding metallicity spread can be up to sigma([Fe/H])=0.2 dex
or >0.7 dex, depending on the isochrone library adopted. The horizontal branch
(HB) is located mostly on the blue side of the instability strip and has a tail
extending to at least M(F606W)=+3, as in the Galactic GC M15. We identify two
candidate RR Lyrae variables and several UV-luminous post-HB/post AGB star
candidates, including one very bright (M(F300X)=-3.2) source near the centre of
EXT8. The surface brightness of EXT8 out to a radius of 25 arcsec is well
fitted by a Wilson-type profile with an ellipticity of epsilon=0.20, a
semi-major axis core radius of 0.25", and a central surface brightness of 15.2
mag per square arcsec in the F606W band, with no evidence of extra-tidal
structure. Overall, EXT8 has properties consistent with it being a "normal",
but very metal-poor GC, and its combination of relatively high mass and very
low metallicity thus remains challenging to explain in the context of GC
formation theories operating within the hierarchical galaxy assembly paradigm.
