Our Milky Way galaxy is not a lone sentinel, set in space and time It is a member of a small cluster of galaxies called the Local Group. The Local Group includes about 27 near galaxies that share a common center of gravity, and the best known members are (besides the Milky Way), the Andromeda Nebula (M31), the Triangulum Nebula (M33), and the Magellanic Clouds.
The members of the Local Group exhibit a definite tendency toward sub-clustering. Perhaps the most striking example is provided by the Andromeda galaxy (M31) and its two companion galaxies M32 and NGC 205. Closer to home, the two Magellanic Clouds form a compact binary system, that, along with our galaxy, create a loose triplet. This triplet and possibly the three nearest dwarf spheroidal systems form one of two major sub-groups. The other sub-group consists of NGC 147, NGC 185, NGC 205, M32, M33 and M31.
The entire Local Group appears to be an out-riding member of a super clustering of galaxies centered on the more distant Virgo Cluster (see Supergalaxy). The local group contains a rather typical distribution of types of galaxies and extends over a volume 1 megaparsec in diameter. The group contains three spiral galaxies, each about 15 to 50 kiloparsecs in diameter: the Milky Way, Andromeda, and the Triangulum Nebula (M33). There are four irregular galaxies of some 3 to 10 kiloparsecs across, including the large and Small Magellanic Clouds. The other galaxies are ellipticals, including 4 regular ellipticals, 2 to 5 kiloparsecs across, two of which are the companions to the Andromeda galaxy. The remaining members are dwarf ellipticals, mostly less than 2 kiloparsecs across.
By far, the largest members of the Local Group are the Andromeda galaxy (M31) and our own Milky Way. Both are super-giant-spiral galaxies and the center of mass for the entire group is located along a line connecting the two, about 2/3 of the distance from our galaxy toward M31. It is believed that both M31 and our own galaxy are in a very slow orbit about the common barycenter. M31 (Andromeda) is both larger and brighter than our galaxy by about 50 per cent.
In addition to galaxies, the Local Group contains a number of intergalactic globular clusters, some of which may be out-riding members of our own galaxy. The cluster NGC 5694 appears to be moving through our galaxy in a hyperbolic orbit and can therefore be considered a true "intergalactic tramp." The known dwarf spheroidal galaxies and intergalactic clusters are mostly located rather close to the galaxy. The total mass of all dwarf spheroidal systems and intergalactic clusters is negligible compared to the mass of the galaxy or the mass of M31. 300 such clusters with 3x10x5 the mass of the Sun each would have only a total mass of 1x10x8 mass of the Sun, which is less than 0.1% of the total mass of bur galaxy.
New candidates for membership in the local group continue to be found. Some of these newly discovered galaxies have been difficult to find even though they are very close, because they occur in the plane of our galaxy, and are thus hidden from view by dust. Others are very dim and have gone unnoticed; three such dwarf ellipticals were discovered in 1972. Another was discovered in 1975 and it is so close to our galaxy that it disrupts the otherwise regular spiral arm structure.
In 1967 and 1968, two large galaxies were discovered in the direction of Perseus, along the galactic plane. The intervening dust had prevented their earlier discovery and these objects first appeared on infrared plates. These two objects, Mafeii I and II as they have been called, are not much farther away from us than is the Andromeda Galaxy. This puts them on the outskirts of the Local Group, but with a high velocity such that they could only be passing through our local cluster of galaxies, rather than being a permanent member.
Maffei I is a giant elliptical galaxy and Maffei II a spiral galaxy. More recent distance estimates put the Maffei galaxies some 5 megaparsecs away from us. They are not at this point considered to be members of the local group but belong to a nearby grouping of galaxies called the Ursa Major-Camelopardalis Cloud. Astronomer Gerard De Vaucouleurs states that the Maffei do not contribute to the location of the barycenter of our Local Group but dominate the small cluster of galaxies mentioned above, not far from us.
Perhaps the most important of all the external systems or galaxies is the Great Nebula (as it was at first, called) in Andromeda (NGC 224), the Andromeda galaxy. Aside from being the only super-giant spiral galaxy that is distinctly visible to the naked eye, M.31 (as the Andromeda galaxy is most often called) is bound to our own galaxy through mutual gravitational attraction. The Milky Way and M31 are in a bound orbit and share a center of mass that is located about two-thirds of the distance between the two galaxies, in a line toward the direction of M31. In other words, our own galaxy and the one in Andromeda dominate the local group of galaxies. M31 is considerably larger than our own, complete with a similar spiral structure and a well-defined semi-stellar nucleus.
Andromeda contains all of the stellar mater a we would expect to find in a normal super-giant spiral: star clouds, globular clusters, open clusters and clouds of nebulosity. Over forty Cepheid variables, 180 stellar associations, many novae have been discovered. The famous supernova of 1885, S. Andromeda, occurred very close to the nuclear region of M31. This particular supernova reached an absolute magnitude of about -14.0 (100 million Suns), which is brighter than many entire galaxies!
The Andromeda galaxy is a super-giant spiral with an integrated magnitude of V = 3.48 (apparent). Adopting an apparent distance modulus of (m-M) sub-v = 24.5 gives an absolute magnitude M sub-v = -21.0, which makes M31 the brightest member of the local group. M31 covers an area of 75'x245' on the sky. At an estimated distance 'of 690 kpcs, these dimensions correspond to 15x50 kiloparsecs.
This rather large ratio suggests that the galaxy is seen almost edge on. Estimates of the inclination range from 75.5 to 79 degrees. Inspection of the spiral arms suggests that the fundamental plane of the galaxy has been slightly warped, so that the spiral arms are not all strictly coplanar. Possibly these deformations are due to tidal interactions with it's companions M32 (NGC 221) and NGC 205. M31 and the Milky Way are the two most massive objects in the local group and contain 90% of the total mass of the group. The most significant fact of all is that Our Galaxy and Andromeda are approaching each other and this may indicate that the local group is contracting!
M32 (NGC 221) is a small elliptical galaxy that is in close proximity to M31. It has been suggested that some of the irregularities in the spiral pattern of M31 may be the consequence of a deformation produced on M31 by the presence of M32. The other close companion to M31 is the highly elongated elliptical galaxy NGC 205. NGC 205 appears as an open barred spiral, one of whose extensions is pointing toward the center of M31 and which might be interpreted as a direct tidal interaction with the gravitational field of M31.
Two companion galaxies to M31, although more distant, are the dust-free elliptical galaxy NGC 147 and NGC 185. NGC 185 contains large quantities of dust and gas along with bright B stars. The great Triangulum Nebula, M33 (NGC 598), is another member of the Andromeda group. M33 is a spiral of type Sc II-III which covers an area 68'x40' on the sky at an estimated distance of 730 kpcs.
M33 has been found to contain types of variable stars which are also known to occur in our galaxy, star clusters, large amounts of neutral hydrogen, and a helium abundance that does not differ much from that observed in the Milky Way. NGC 6822 is a dwarf irregular galaxy located rather close to the plane of the Milky Way. The main body of the nebula has dimensions of approximately 20'x10', which corresponds to 2.7xl.3 kpc at an assumed distance of about 470 kpc. A number of bright HII regions are located outside the main 'bar' of the nebula. NGC 6822 is an Ir IV-V type galaxy and is slightly fainter than the SMC. IC 1613 is a dwarf irregular galaxy of the type Ir V and is similar, though much smaller, than the Magellanic Clouds. IC 1613 appears to be-a very old galaxy that is undergoing much star formation at the present time.
The Dwarf Spheroidal Systems in the Local Group
An entirely new type of sidereal organization was discovered by the astronomer Shapley in 1938. The Dwarf Speroidal Systems, as they are called, consist only of resolved stars and no gas or dust has been observed in these galaxies. They can best be described as "super" globular clusters with a very low surface brightness. The dimensions of these galaxies are of the size of a small galaxy rather than of a large globular cluster. Morphologically the dwarf spheroidal galaxies occur at the end of a sequence which starts among the normal elIiptical galaxies and passes through the dwarf ellipticals (such as NGC 147 & 185).
The dwarf galaxies in Fornax and Sculptor were the first discovered followed by those in Draco, Leo ( I & II) and Ursa Minor. Globular clusters and dwarf systems do not have semi-stellar nuclei such as those, which are observed in M32 and NGC 205. The faint ellipticals NGC 147 and NGC 185 are intermediate between dwarf systems and the brighter elliptical galaxies. NGC 147 has only a faint nucleus and NGC 185 has no nucleus at all. A new and very near dwarf galaxy, discovered in 1975, appears to be disrupting the otherwise regular spiral structure of our galaxy! Dwarf spheroidal systems are difficult to detect due to their low surface brightness, but it is estimated ,that a very great number of these systems exist filling the space between the larger and brighter galaxies.
Copyright (c) 1997-99 Michael Erlewine