| Andromeda II | |
|---|---|
 Andromeda II, as taken by theHubble Space Telescope | |
| Observation data (J2000epoch) | |
| Constellation | Pisces |
| Right ascension | 01h 16m 29.8s[1] |
| Declination | +33° 25′ 09″[1] |
| Redshift | −188 ± 3 km/s[1] |
| Distance | 2.22 ± 0.07Mly (680 ± 20kpc)[2][3] |
| Apparent magnitude (V) | 13.5[1] |
| Characteristics | |
| Type | dSph[1] |
| Apparent size (V) | 3.6′ × 2.52′[1] |
| Notable features | satellite galaxy ofM31 |
| Other designations | |
| Andromeda II Dwarf Spheroidal,[1] PGC 4601,[1][4] And II[5] | |
Andromeda II (And II) is adwarf spheroidal galaxy about 2.22Mly away in theconstellation Pisces. While part of theLocal Group, it is not quite clear if it is a satellite of theAndromeda Galaxy or theTriangulum Galaxy.[citation needed]
It was discovered bySidney Van den Bergh[6] in a survey of photographic plates taken with thePalomar 48-inch (1.2 m) Schmidt telescope in 1970 and 1971, together withAndromeda I,Andromeda III, and the presumable non- or background galaxyAndromeda IV.[7]
Andromeda II has also been given the aliasPisces II by Martin et al. (2009), who also proposed aliases for several other satellite galaxies of the Andromeda Galaxy[8] However, that name was later used by a different group unaware of these names, for a separate galaxy.[9]
Using theKeck telescope, Côté et al. 1999 observedspectra for seven stars inside Andromeda II. From this data, they found 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
−10 solar units which implies that And II contains a significant amount ofdark matter. Also in 1999, Côté, Oke, & Cohen used the Keck to measure the spectra of 42red giants. From this, they deduced an averagemetallicity of <[Fe/H]> = −1.47 ± 0.19 and a dispersion of 0.35 ± 0.10 dex.[5]
In 1999, Da Costa et al. studied thecolor-magnitude diagram of And II and discovered that most of stars in And II have ages between 6 and 9 Gyr. However, the observation ofRR Lyrae variables andblue horizontal-branch stars demonstrates the existence of a population segment with an age greater than 10 Gyr. And II differs fromAnd I in that it does not show a radialgradient inhorizontal-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 correlationbetween a radial horizontal-branch gradient and the metallicity dispersion between dwarf spheroidal galaxies.[5]
And II appears to be in the possession of a stellar stream, a feature that is indicative of a merger event in the past.[10] The characteristics of And II can best be explained by the merger of two disky dwarf galaxies, some 5 billion years ago.[11]
| Location | Andromeda Galaxy → Andromeda subgroup →Local Group →Local Sheet →Local Volume →Virgo Supercluster →Laniakea Supercluster →Pisces–Cetus Supercluster Complex →Local Hole →Observable universe →Universe Each → may be read as "within" or "part of". | .jpg) |
|---|---|---|
| Satellite galaxies |
*It is uncertain whether these are companion galaxies of the Andromeda Galaxy | |
| Catalogued stars | ||
| Other | ||
| Location | Triangulum Galaxy →Triangulum subgroup →Local Group→Local Sheet→Local Volume→Virgo Supercluster→Laniakea Supercluster →Pisces–Cetus Supercluster Complex →Local Hole →Observable universe →Universe Each arrow (→) may be read as "within" or "part of". |  |
|---|---|---|
| H II regions | ||
| Suspectedsatellite galaxies | ||
| Catalogued stars | ||
| Other | ||
