Structural properties of the M31 dwarf spheroidal galaxies

The projected structures and integrated properties of the Andromeda I, II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysed based upon resolved counts of red giant branch stars. The observations were taken as part of the Isaac Newton Telescope Wide Field Survey of M31 and its environs. For each object, we have derived isopleth maps, surface brightness profiles, intensity-weighted centres, position angles, ellipticities, tidal radii, core radii, concentration parameters, exponential scale lengths, Plummer scale lengths, half-light radii, absolute magnitudes and central surface brightnesses. Our analysis probes to larger radius and fainter surface brightnesses than most previous studies and as a result we find that the galaxies are generally larger and brighter than has previously been recognised. In particular, the luminosity of Andromeda V is found to be consistent with the higher metallicity value which has been derived for it. We find that exponential and Plummer profiles provide adequate fits to the surface brightness profiles, although the more general King models provide the best formal fits. Andromeda I shows strong evidence of tidal disruption and S-shaped tidal tails are clearly visible. On the other hand, Cetus does not show any evidence of tidal truncation, let alone disruption, which is perhaps unsurprising given its isolated location. Andromeda II shows compelling evidence of a large excess of stars at small radius and suggests that this galaxy consists of a secondary core component, in analogy with recent results for Sculptor and Sextans. Comparing the M31 dwarf spheroidal population with the Galactic population, we find that the scale radii of the M31 population are larger than those for the Galactic population by at least a factor of two, for all absolute magnitudes. This difference is either due to environmental factors or orbital properties, suggesting that the ensemble average tidal field experienced by the M31 dwarf spheroidals is weaker than experienced by the Galactic dwarf spheroidals. We find that the two populations are offset from one another in the central surface brightness – luminosity relation, which is probably related to this difference in their scale sizes. Finally, we find that the M31 dwarf spheroidals show the same correlation with distance-from-host as shown by the Galactic population, such that dwarf spheroidals with a higher central surface brightness are found further from their host. This again suggests that environment plays a significant role in dwarf galaxy evolution, and requires detailed modelling to explain the origin of this result.