The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III , a dwarf spheroidal ( dSph ) companion to M31 .
The resulting color-magnitude ( c-m ) diagrams reveal for the first time the morphology of the horizontal branch ( HB ) in this dwarf galaxy .
We find that like Andromeda I and Andromeda II , and like most of the Galactic dSph companions , the HB morphology of And III is predominantly red , redder indeed than that of both And I and And II despite And III having a lower mean metallicity .
The And III HB morphology is also somewhat redder than that of the Galactic dSph Draco , which has a similar mean abundance to And III .
We interpret this red HB morphology as indicating that the bulk of the And III population is \sim 3 Gyr younger than the age of the majority of Galactic globular clusters .
Nevertheless , the And III c-m diagram does reveal the presence of a few blue HB stars , and a number of RR Lyrae variables are also evident in the data .
This indicates that And III does contain an “ old ” population of age comparable to that of the Galactic globular clusters .
There is no evidence , however , for any young stars in And III despite a claimed association between this dSph and an HI cloud .
As was the case for And II , but not And I , no radial gradient was detected in the And III horizontal branch morphology .
The mean V magnitude of the horizontal branch is 25.06 \pm 0.04 leading to ( m - M ) _ { 0 } = 24.38 \pm 0.06 for this dwarf .
And III is then \sim 75 kpc from the center of M31 , comparable to the Galactocentric distances of Sculptor and Draco .
Comparison with standard globular cluster red giant branches indicates a mean abundance for And III of < [ Fe/H ] > = –1.88 \pm 0.11 , the lowest mean abundance of any of M31 ’ s companions .
This value , however , is consistent with the absolute magnitude – mean abundance relation followed by dSph galaxies .
The same comparison yields an intrinsic abundance dispersion for And III of \sigma _ { int } ( [ Fe/H ] ) = 0.12 , a low value compared to And I and And II and to the Galactic dSphs of comparable luminosity to And III .
If confirmed by future spectroscopic studies , this low value would suggest that And III retained relatively little of the enrichment products generated during its evolutionary history .
The list of candidate variables reveals one definite and one probable Anomalous Cepheid variable stars in And III .
Such variables are common in Galactic dSphs , so their discovery in And III is not unexpected .