Andromeda II dwarf spheroidal galaxy
Andromeda II (And II) is a dwarf spheroidal galaxy about 2.22 Mly away. It is part of the Local group of galaxies and is a satellite galaxy of the Andromeda Galaxy (M31) but it's also situated closely to the Triangulum Galaxy (M33), it is not quite clear if it is a satellite of the one or the other galaxy.
Spectra observations
Observations of spectra for stars inside Andromeda II indicates an average velocity Vr of -188 ± 3 km/s and velocity dispersion of 9.2 ± 2.6 km/s. This gives a mass to light ratio of M/Lv of +21
−14 standard stellar masses which implies that And II contains a significant amount of dark matter. Measures of the spectra of 42 red giants has revealed an average metallicity of <[Fe/H]> = -1.47 ± 0.19 and a dispersion of 0.35 ± 0.10 dex.
The color-magnitude diagram of And II shows that most of stars in And II have ages between 6 and 9 Gyr. However, the observation of RR Lyrae variables and blue horizontal-branch stars demonstrates the existence of a population segment with an age greater than 10 Gyr. And II differs from And I in that it does not show a radial gradient in horizontal-branch morphology. Additionally, the dispersion in abundance is significantly larger in And II as compared to And I. This implies that these two dwarf spheroidal companions to the Andromeda galaxy have very different evolutionary histories. This raises the question of whether there is a correlation between a radial horizontal-branch gradient and the metallicity dispersion between dwarf spheroidal galaxies.
