Andromeda II
Encyclopedia
Andromeda II is a dwarf spheroidal galaxy
about 2.22 Mly
away in the constellation
Andromeda
. 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.
It was discovered by Sidney Van den Bergh
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 III
, and the presumable non- or background galaxy Andromeda IV
(Van den Bergh 1972).
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 solar units which implies that And II contains a significant amount of dark matter
. Also in 1999, Côté, Oke, & Cohen used the Keck to measure the spectra of 42 red giant
s. From this, they deduced an average metallicity
of <[Fe/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
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
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.
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
about 2.22 Mly
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Andromeda
Andromeda (constellation)
Andromeda is a constellation in the northern sky. It is named after Andromeda, the princess in the Greek legend of Perseus who was chained to a rock to be eaten by the sea monster Cetus...
. It is part of the Local group of galaxies and is a satellite galaxy
Satellite galaxy
A satellite galaxy orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a large number of objects which are not connected to each other, it has a center of mass, which represents a weighted average of the positions of each component object...
of the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
(M31) but it's also situated closely to the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
(M33), it is not quite clear if it is a satellite of the one or the other galaxy.
It was discovered by Sidney Van den Bergh
Sidney van den Bergh
Sidney Van den Bergh, OC, FRS is a retired Canadian astronomer.Born in the Netherlands, son of businessman and politician Sidney James van den Bergh and grandson of Unilever co-founder Samuel van den Bergh, he showed an interest in science from an early age, learning to read with books on 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
Andromeda I is a dwarf spheroidal galaxy 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 . It is roughly 3.5 degrees south and slightly east of M31...
, Andromeda III
Andromeda III
Andromeda III is a dwarf spheroidal galaxy about 2.44 million light-years away in the constellation Andromeda. And III is part of the Local group of galaxies and is a satellite galaxy of the Andromeda Galaxy . And III was discovered by Sidney van den Bergh on photographic plates taken in 1970 and...
, and the presumable non- or background galaxy Andromeda IV
Andromeda IV
Andromeda IV might be an irregular satellite of the Andromeda Galaxy; however it is more probably not a galaxy at all, but a loosely bound star cluster or some other background feature....
(Van den Bergh 1972).
Spectra observations
Using the Keck telescope Côté et al. 1999 observed spectraElectromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
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 solar units which implies that And II contains a significant amount of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
. Also in 1999, Côté, Oke, & Cohen used the Keck to measure the spectra of 42 red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
s. From this, they deduced an average metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
of <[Fe/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 variable
RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra....
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
Andromeda I
Andromeda I is a dwarf spheroidal galaxy 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 . It is roughly 3.5 degrees south and slightly east of M31...
in that it does not show a radial gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
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.