Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
External Galaxies

 

  Local Triplet

  "Snickers"

  The Local Group of Galaxies

  The Motion of the Local Group

  Clustering among Galaxies

  The Local Supergalaxy

  Galaxies: Listing

 

 

 

Local Triplet

The nearest neighbors to the Milky Way are the Magellanic Clouds in the southern hemisphere, where they appear as large nebulous naked-eye objects. The Large Magellanic Cloud (LMC) or Nubecula Major (the 'Greater Little Cloud') and the Small Magellanic Cloud (SMC) or Nubecula Minor, the 'Lesser Little Cloud') appear to the naked-eye as detached portions of the Milky Way and they are, in fact, satellites of our galaxy. The Magellanic Clouds are a close binary pair of Irregular dwarf galaxies that, along with our galaxy, form a loose triplet sharing a common barycenter and located about 5 kiloparsecs from the center of our galaxy in the direction of the LMC.

 

The Magellanic Clouds

The Magellanic Clouds are located at less than one tenth the distance to the Andromeda galaxy (M.31) and are near enough that it is possible to obtain the color-magnitude diagrams of star clusters. A 20 inch telescope working on the clouds is therefore equivalent to the 200 inch telescope working on the more distant Andromeda galaxy. In the Magellanic Clouds, we do get the kind of view or perspective that we need to estimate the nature of our own Milky Way and other external galaxies. Here is a summary of data on these members of the Local Group as of 1968: (Other data is given in list of members of the Local Group of galaxies)

 

LMC SMC
Geometric Center (1950) 81° = R.A. 12°= R.A.
-69°48 = Declination -73°06 = Declination
Apparent diameter 11.8° 4.2°
Inclination 65° 30°
Neutral H Mass ( Sun=1) 5.4x10 4.8x10
Total Mass (Sun=1) 6x10 1.5x10
Radial Velocity (km.sec) +270 +168
Note: Inclination is defined as the angle between the line of sight and the line perpendicular to the fundamental plane of a galaxy.

 

The Magellanic Clouds are dwarf Irregular galaxies, and the LMC has a Hubble type and luminosity classification Ir or SBc III-IV, and the SMC is classified Ir IV or Ir IV-V. The stellar content of the clouds is similar to our own galaxy with the following possible differences:

  • Some of the globular clusters in the Clouds differ significantly from their galactic counterparts. The very red colors of stars near the tips of the cluster color-magnitude relations are very striking.
  • Galactic Cepheids seem to differ from those in the Clouds in color and in the mean relation between period and pulsation amplitude.
  • There is less convincing evidence for differences between novae, giants with Mv = 0 and the giant branches of young clusters in the clouds and in the galaxy. (nine novae have been observed in the Clouds; four in the SMC, and five in the LMC)
  • The interstellar gas in the SMC, and perhaps in the LMC as well, is not as dusty as that in the Galaxy.
  • The ratio of gas mass to total mass is much higher in the Clouds than it is in the Galaxy. This shows that the Magellanic Clouds are less evolved than the Galaxy.
  • The observed differences between the clouds and the galaxy occur among both old and young stellar populations. Possibly small differences in the helium and/or heavy element abundance are responsible for the observed differences between stars in the Galaxy and the Magellanic Clouds.

 

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Snickers

A new member of our Local Group was discovered in 1975. It is a dwarf satellite of our galaxy, like the two Magellanic Clouds, but is located at less than a third of their distance! It has been hidden behind the rim or equatorial plane of the galaxy, the very nearest part of that rim, in Gemini and Auriga. It was detected only by its rapidly moving hydrogen clouds. It is estimated that 1 percent of the stars of magnitude 15 and below which appear in this part of the Milky Way must really belong to this new galaxy. The little galaxy is brushing so close to the Milky Way that is has been torn out into the shape of a long streamer by tidal forces; hence its enormous angular extent of over 45 degrees, from its core (55,000 light years away) near Almeisan in Gemini to its leading tip beyond Capella (Almeisan = Gamma Gemini, Capella = Alpha Auriga). The little dwarf galaxy has been unofficially christened "Snickers," due to its proximity to the Milky Way.

Most imortant: Here is a large and very near galaxy, a kind of a third Ma ellanic Cloud, that covers a significant portion of the heavens in the anti-center direction of our galaxy. On the Ecliptic, it extends from later Gemini through the first 10 degrees of Cancer, with a central core around the 6th degree of Cancer. "Snickers" is close enough to disrupt the outer portion of the spiral-arms of our galaxy!

 

New member of the Local Group

 

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The Local Group of Galaxies

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.

 

Positions of the 12 members of the Local Group

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.

 

Galaxy Diagram

 

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.

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The Motion of the Local Group

Recent investigations (1976) into the nature of the motion of our Galaxy and the entire local group of galaxies indicates that the Milky Way is moving almost edge-on through space and that the leading edge is in the anticancer direction. Our galaxy along with the local group appears to be moving with an approximate velocity of 454 km/sec toward a point in the constellation Perseus, roughly in the direction of NGC 1499, the California Nebula.

The direction is as listed:

R. A.Decl.LongitudeLatitudeIntersectL2B2
063 00 14 +35 26 11 067 35 13 +14 04 05 064 56 59 163 00 00 -11 00 00

 

The direction of this motion is approximately at a right angle to the direction of the center of the local supercluster, the Virgo Cluster. A question astronomers are attempting to decide is: is the local supercluster rotating, and are we orbiting the Virgo cluster? We cannot be in a bound Keplerian-type of orbit, for at our present distance from the center of the Virgo Cluster, such an orbit would take a time period of ten times the age of the Universe!

It is suggested that we may have moved away from a closer orbit in the same fashion that the ends of spiral-arms trail away from the nuclei of galaxies. At any rate, it appears that we are moving at about a right angle or edge-on toward SGL (supergalactic longitude) = 351° and SGB = -26°. We are moving, so it appears, away from the center of the Supergalaxy (the Virgo Cluster) and slightly below the Supergalactic plane.

As we have seen, galaxies are often members of pairs, triplets and groups of increasing multiplicity. In fact, grouping or clustering (as it is called) is the rule rather than the exception. The Large Small Magellanic Clouds form a close pair and along with our galaxy, a loose triplet. M.31 (Andromeda) is the major component of a triplet, including the two elliptical galaxies M.32 and NGC 205, and of a loose group with M.33 and the smaller ellipticals NGC 147 and 185.

Both the M.31 group and our galaxy's group are associated with a larger grouping: the local group. There are other groups similar to our own relatively nearby and the clustering phenomena does not stop with groups of galaxies. Galaxies appear to be arranged in a hierarchy of clusters. In fact, the tendency toward clustering increases with higher order structuring.

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Clustering among Galaxies

Thus galaxies tend to occur in small intense knots, the knots in clusters, and the clusters in larger clusters, and so on until vast clouds of clusters are formed.

The general kinds of grouping look like this:

  • A Cluster contains hundreds or even thousands of galaxies with a marked tendency to concentrate toward some center in the clusters.
  • A Group of galaxies contains several, perhaps up to 100 members but these groups do not show any marked concentration toward a center.
  • A Cloud of galaxies is just a large group containing hundreds or thousands of members gathered together in an irregular structure with no definite concentration toward a center.
  • A Cloud of Groups is a distribution of galaxies containing many groups in which the concentration of galaxies in the spaces between the groups is larger than the concentration of galaxies in the general field.
  • A Cloud of Galaxies is the largest agglomeration of matter so far known to us, Supergalaxy or Metagalaxy. There are double clusters, triple clusters and so on. Large bodies of this type may contain as many as 100,000 member galaxies.

The nearest groups of galaxies to our local group are the M.81 group of galaxies and the Sculptor group near the galactic south pole. A table of most of the major clusters and groups within 12-15 megaparsecs of our position is given elsewhere. Almost all of these clusters occur between us and the very large cluster of galaxies in Virgo at a distance of some 50 million light years from us (15.3 Mpc). A more or less dense cloud of galaxies extends from the Virgo cluster to about our own position and our local group may be considered to be a minor irregularity or secondary agglomeration near the very edge of the system of galaxies which is ellipsoidally distributed around the center of the Virgo cluster, the local supergalaxy.

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The Local Supergalaxy

The center of the local supergalaxy is either in or near the great Virgo cluster of galaxies located at a distance of 12 to 16 Mpc. Our position at the very edge of this supercluster is even more extreme than our somewhat out-riding position in the disk of our galaxy. Virtually all the matter concentrated in the supergalactic plane is to one side of our position and in the general direction of the zodiac signs Virgo and Libra.

A glance at the star maps (located elsewhere) will make the orientation of the supergalactic plane clear to the reader. The Supergalactic plane dominates the Autumn Equinox and the signs Virgo & Libra, just as the galactic plane dominates the solstice axis and the signs Sagittarius/Capricorn and Gemini/Cancer. It is of more than passing interest to notice that these two vast systems are at a right-angle (84°) to one another! and that the centers of the two systems (GC at 266°, SGC at 181° zodiac longitude) almost coincide with the zero points of Libra and Capricorn! A very meaningful analysis of the traditional Sun-Sign interpretation can be made with these facts alone. The midpoint between these two cosmic centers is at about the middle of Scorpio (Tropical).

Here are some facts concerning our Supergalaxy: It is a vast flattened supersystem or 'cloud of clusters' and the plane of maximum concentration defines the supergalactic equator. The overall diameter is about 40 megaparsecs and the thickness some10 megaparsecs and a volume of 16,000 Mpc (cubed) that contains about 10 (to the 15th) solar masses distributed among the tens of thousands of member galaxies.

A much larger cluster of galaxies is that in the direction of Coma Berenices. The Coma cluster (as it is called) is a dense knot of an elliptical shape about six times as distant as the Virgo cluster and containing perhaps 10 times as many galaxies! Some 3000 such very large clusters have been catalogued. Other supergalaxies have been discovered. The nearest other supergalaxy is the Southern Supergalaxy that can be seen almost edge-on extending through the constellations Cetus, Fornax, Eridanus and Horologium to Dorado. Its apparent nucleus is marked by a dense group of more than a dozen galaxies (NGC 1365, 1374, 1379, 1380, 1381, 1387, 1389, 1399, 1404, 1437 & 1427).

At a distance modulus of 27.0, it is only slightly farther away than the Virgo cluster and is of similar size,. Astronomers feel (at this time) that there is little likelihood that supergalaxies are themselves part of still larger structures. Recent research suggests that the tendency toward clustering falls off rapidly after a mass of supergalactic proportions is reached. A list of some of the major giant clusters and superclusters is given elsewhere. At this point, well over a million external galaxies have been counted by astronomers and such research is still at an early stage.

Click on the image to see a bigger view.

Super Galaxy, click to see bigger view

Distribution of nearby groups of galaxies over the celestial sphere in Supergalactic coordinates. Group of galaxies shown here are within 10 to 16 megaparsecs. Note marked concentration toward the Supergalactic plane (horizontal line).

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Galaxies: Listing

The existence of galaxies external to our own ("Island Universes" as they were first called) was not considered an established fact until the early 1900s. The galactic nebulae (bright diffuse nebulae) and the so-called external nebulae were thought of as one.

Today over a million external galaxies have been counted and we are yet in the early stages of deep space exploration. The astronomer Hubble introduced a system of galaxy classification in 1925 that, with some revision, is still in general use. It recognizes three main classes of galaxies: (1) Elliptical shaped galaxies, (2) Spiral shaped galaxies, and (3) Barred spiral galaxies.

There are also a large group of galaxies that are classified as "Irregular" in shape. Among the Spirals there are three stages Sa, Sb and Sc and these are distinguished according to the relative size of the nuclear or central bulge (decreasing from Sa to Sc) and the relative strength of the arms (increasing from Sa to Sc). Elliptical galaxi6l have a smooth structure from a bright center out to indefinite edges and they differ only in ellipticity, from round (E0) to a 3:1 axis ratio (E7). Spiral galaxies show their typical spiral arms or whorls emerging either directly from a bright round nucleus (ordinary spirals) or at the ends of a diametrical bar (barred spirals). Irregular galaxies are either of the Magellanic Cloud type or chaotic, and difficult to classify in the Hubble method. This method was later revised to include the SO Or lenticular type of galaxy, which shares the smooth structure of the ellipticals, but has a definite, nucleus, occasional interstellar matter, and luminosity similar to the spirals. There are also two varieties of the barred spiral, the classical S-shaped spirals (s) and the ringed type (r) in which the arms start at the rim of an inner ring. Transition types exist between all the main types. There also appear to be large numbers of 'dwarf galaxies' that are small, have low surface brightness, and are difficult to detect. Dwarfs exist only as ellipticals or Magellanic irregulars. The beautiful spirals appear to only occur among the giant galaxies.

 

Galaxy Types

 

Click on the image to see a bigger view.

Super Galaxy, click to see bigger view Super Galaxy, click to see bigger view Super Galaxy, click to see bigger view Super Galaxy, click to see bigger view Super Galaxy, click to see bigger view

 

 

 

Copyright © 1997 Michael Erlewine

 

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