Andromeda II (And II) is a
dwarf spheroidal galaxy
A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are comp ...
about 2.22
Mly
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
away in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
Pisces. While part of the
Local Group
The Local Group is the galaxy group that includes the Milky Way.
It has a total diameter of roughly , and a total mass of the order of .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...
, it is not quite clear if it is a satellite of the
Andromeda Galaxy
The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...
or the
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (''New General Catalogue)'' 598. With the D25 isophotal diameter of , the Triangulum Galax ...
.
It was discovered by
Sidney Van den Bergh
Sidney Van den Bergh, OC, FRS (born 20 May 1929 in Wassenaar) is a retired Dutch-Canadian astronomer.
He showed an interest in science from an early age, learning to read with books on astronomy. In addition to being interested in astronomy. ...
in a survey of photographic plates taken with the Palomar 48-inch (1.2 m) Schmidt telescope in 1970 and 1971, together with
Andromeda I
Andromeda I is a dwarf spheroidal galaxy (dSph) about 2.40 million light-years away in the constellation Andromeda. Andromeda I is part of the local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It is roughly 3.5 degr ...
,
Andromeda III
Andromeda III is a dwarf spheroidal galaxy about 2.44 million light-years away in the constellation Andromeda. It is part of the Local Group and is a satellite galaxy of the Andromeda Galaxy (M31). The galaxy was discovered by Sidney van den Be ...
, and the presumable non- or background galaxy
Andromeda IV
Andromeda IV (And IV) is an isolated irregular dwarf galaxy. The moderate surface brightness, a very blue color, low current star formation rate and low metallicity are consistent with it being a small (background) dwarf irregular galaxy, perhap ...
.
Nomenclature
Andromeda II has also been given the alias
Pisces II by Martin et al. (2009), who also proposed aliases for several other satellite galaxies of the Andromeda Galaxy
However, that name was later used by a different group unaware of these names, for a separate galaxy.
Spectra observations
Using the
Keck telescope
The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state
In the United States, a state is a constituent political entit ...
, Côté et al. 1999 observed
spectra for seven stars inside Andromeda II. From this data, they found an average velocity V
r of −188 ± 3 km/s and velocity dispersion of 9.2 ± 2.6 km/s. This gives a mass to light ratio of M/L
v of 21 solar units which implies that And II contains a significant amount of
dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
. Also in 1999, Côté, Oke, & Cohen used the Keck to measure the spectra of 42
red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
s. From this, they deduced an average
metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
of <
e/H = −1.47 ± 0.19 and a dispersion of 0.35 ± 0.10 dex.
In 1999, Da Costa et al. studied the
color-magnitude diagram of And II and discovered that most of stars in And II have ages between 6 and 9 Gyr. However, the observation of
RR Lyrae variable
RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and ...
s 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
"And I" is a song written by American contemporary R&B, R&B singer Ciara, and produced by Adonis Shropshire for Ciara's first album, ''Goodies (Ciara album), Goodies'' (2004). It was released as the album's fourth and final single (music), singl ...
in that it does not show a radial
gradient
In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gradi ...
in
horizontal-branch
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) ...
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.
History
And II appears to be in the possession of a stellar stream, a feature that is indicative of a merger event in the past. The characteristics of And II can best be explained by the merger of two disky dwarf galaxies, some 5 billion years ago.
See also
*
List of Andromeda's satellite galaxies
The Andromeda Galaxy (M31) has satellite galaxies just like the Milky Way. Orbiting M31 are at least 13 dwarf galaxies: the brightest and largest is M110, which can be seen with a basic telescope. The second-brightest and closest one to M31 is ...
References
External links
SEDS: Dwarf Spheroidal Galaxy Andromeda II
{{DEFAULTSORT:Andromeda 02
Dwarf spheroidal galaxies
Local Group
Andromeda Subgroup
Pisces (constellation)
04601
Triangulum Subgroup