AGNs are a minority in both spiral and elliptical galaxies, but they are the source of extreme energy output. Within them almost exclusively are the quasars (see page 20-6) that are the visible manifestations of matter falling into Black Holes. There is growing evidence that supermassive B.H.'s are at the center of most (perhaps all) larger galaxies that have a bright central bulge. AGNs reached their peak around 4 billion years after the Big Bang, having taken some time to build up to the condition in which huge energy outputs result from their numbers of quasars, and are less frequent in younger galaxies. Although still not proved from observations, many astronomers believe that one or more AGN episodes took place in both elliptical and spiral galaxies at some stage(s) of their histories. The relationships between AGNs and Starbursts (described later), and their mutual association with Black Holes, will be established on page 20-4.
As alluded to in previous paragraphs, between star groupings in the arms and central region of spiral galaxies there remains much Hydrogen gas and dust in large clots from which more stars will form later. The gas is ionized (HII) and radiates at several discrete wavelengths. The Wide Field Imager (WFI) of the 2.2 meter MPG/ESO telescope at the southern hemisphere La Silla Observatory has imaged the spiral galaxy NGC300 with a filter that selectively passes ionized Hydrogen radiation, so that the stars are screened out leaving only the Hydrogen clots. As seen below, these clots are irregular in shape but widespread:
Sky surveys (especially with the Hubble Space Telescope) indicate that spiral galaxies contain a large number of individual stars, clusters, and even small satellite galaxies, and considerable Hydrogen gas and dust, dispersed in galactic space around the central disk in what is called the halo region. A halo appears to be a roughly spherical envelope that surrounds galaxies in general. Haloes develop around protogalaxies and aid in the subsequent development of each type. The density of gas and dust within the halo space is overall less than that within the central disk. Globular star clusters (see below) are the most distinct entity in this distribution. This next figure is a simple diagram of the four principal components of spiral galaxies; the green marks the halo region:
The galaxy NGC 5746 possesses a distinct gas halo, seen in this HST image as a uniform blue region. Its population of halo stars appears to be low.
The genesis of the spiral type of galaxy is fairly well understood. It starts with gravitational action within a denser part of the intergalactic medium. Dark matter exerts a control over the resulting collapse of hydrogen gas in the protogalactic molecular cloud. This collapse tends to be asymmetric (uneven). Turbulence within aids in setting the contracting cloud into rotation. When dense enough, the gas begins to organize into individual stars. The cloud further contracts preferentially in one direction and a disc shape results from centrifugal flow within the star and gas assemblage. The angular speed of the stars rotating about a center varies with distance outward; this imparts a curvature to the pattern of stars within the disc. The stars tend to locate in streamers that become the arms of the spiral galaxy. This diagram is a simplified version of this model (there are variants involving collisions, shock waves, density waves, etc. that modify the basic idea):
Also enclosed by a dark matter halo is the second major galactic type, the Elliptical Galaxy,, marked by mostly old stars (populations up to 1011 individuals). Ellipticals comprise about 15% of regular types. Such a galaxy is now believed to originate through collisions, tidal disruption and other interactions, between small galaxies or even large spirals. leading to merging and destruction of the spiral arms (some ellipticals may have formed in the early Universe simply by a collapse mechanism still poorly understood). Elliptical galaxies rotate more slowly than spiral ones, so that the tendency to evolve into a flattened disc is thwarted. Elliptical (the majority are almost spherical) galaxies, generally more massive than spiral galaxies, usually occur in groups or clusters. Both Giant and Dwarf varieties are known. The typical elliptical galaxy contains a larger percentage of red stars than found in spiral galaxies (those have more blue or hotter stars than red); however, being more compact ellipticals are usually brighter than spirals. Recent observations of elliptical galaxies have found that there are still many younger blue stars. Elliptical galaxies, although more massive than spirals, contain much lower amounts of dust and are gas-poor which suggests that overall they contain a larger fraction of older stars than in the more abundant spirals. Here is a typical example; in this and many other images of ellipticals, the individual stars are not resolved - they are so densely packed that the galaxy image appears to be uniformly bright:
The Giant Elliptical Galaxy is probably the brightest of any category of galaxies. It can contain as many as a trillion stars. Among the best known is Messier 87 (M87) shown below as seen in visible light.
Giant Ellipticals are strong sources of radiation beyond the visible range (discussed on page 20-4). Although we are "jumping the gun" a bit, it is instructive to show M87 as an X-ray source (detected by Rosat) and as a Radio source:
As may be the case for most elliptical galaxies, which holds that many (most) of these form by collisions (see below), the Giant Galaxy type almost certainly results from multiple elliptical galaxy collisions, as depicted in this simulation:
Both spiral and elliptical galaxies can group in clusters from a few tens to hundreds of thousands of individual galaxies. Such clustering is a direct consequence of the uneven distribution of Dark Matter into halos that developed soon after the Big Bang. The image below is a giant cluster of many hundreds of elliptical galaxies (most burned to the red stage) lying at 9 billion light years from Earth. The cluster density makes for an apparently huge single entity but is actually the glow from many close-spaced ellipticals. The image is a composite of ESA's XMM-Newton X-ray space telescope and ground imagery taken through the European Space Observatory (Vis-IR) telescope in Chili.
This HST view (within the Coma Cluster) shows a Giant Elliptical Galaxy on the left and a rather diffuse Spiral Galaxy on the right; being at similar distances the relative sizes are valid:
Elliptical galaxies tend to occur in clusters of this one type, but with a few gas-poor spiral galaxies within a cluster. Spiral galaxies are more scattered in space.
Rare among these principal galaxy types is the so-called Ringed Galaxy. This example is known as Hoag's Object, found in the constellation Serpens and situated about 600,000,000 light years from Earth. In size, its diameter is 120,000 l.y., slightly larger than the Milky Way. Its central nucleus consists of densely packed yellow (old) stars which together resemble an elliptical galaxy. The ring consists mainly of younger blue stars. In the gap in between there is a dearth of stars of either type.
The origin of Ringed Galaxies is still uncertain but a stage of redistribution after the collision of two galaxies is a plausible explanation.
A second example has brought out a bit of humor among the astronomer clique. Look at this Hubble image of Ringed Galaxy ARP 147 (on the right):
Those who viewed this image noted that the spiral galaxy on the left was shaped almost as a "1". This led one wag to rate this scene as "a perfect 10".
There is another galaxy type - the Lenticular Galaxy - that some consider deserving of its own category. Generally, most such galaxies are equivalent to the SO group at the base of the two spiral branches of the Hubble galaxy classification. In side view, a lenticular galaxy is just that - a double convex shape, much like an optical lens. When seen face on (as from the top), the SO type has no distinct or discernible individual spiral arms but in the part beyond the center (which may be a massive core but a few have very non-descript cores) individual stars are evident but distributed randomly and at various densities. When Lenticular galaxies are examined in this way, they show rudimentary spiral arms, suggesting they are an incipient spiral transitional to that class or are a degenerate spiral no longer typical of that class. Most of their stars are old (yellow) both in the core and the surroundings, which makes this type similar to elliptical galaxies - except for its pronounced disk shape. However, gas and dust seem in short supply, suggesting that little subsequent evolution is likely. Several examples of Lenticular Galaxies are shown in this next sequence; see their captions for details.
About 10% of galaxies are neither spiral nor elliptical but have what can be described as "irregular" shapes. As a type example, here is NGC 1569, about 7 million light years away:
Many of these irregular galaxies are actually two (or more) colliding galaxies (discussed in more detail on page 20-4). Some are described as "peculiar galaxies", a term coined by Halton Arp who compiled a catalog of these in 1966. A classic peculiar galaxy is the pair NGC 6621/6622:
Globular star clusters are intermediate between simple star clusters and galaxies - each is an aggregate of 100,000 to a million stars. These are much like miniature elliptical galaxies but have far fewer individual stars. Like the latter many seem to have a predominance of old stars. The largest concentration of these stars is in the interior of the cluster. Typical Globular Cluster densities are several hundred stars per cubic light-year (compared with typical densities of 0.01 to 0.1 stars per (ly)3, which is the norm for most galactic space). Because of the higher densities within clusters, the frequency of star collisions (normally a rare event) is considerably greater.
Although some clusters are found around elliptical galaxies, the globular clusters mostly occur in much larger numbers within the halos of spiral galaxies, i.e., in orbits at all angles to the galactic plane within an imaginary sphere that may be 200,000 light years or more in diameter. The Milky Way contains less than 200 such clusters; Andromeda almost 500. Most globular clusters contain old (> 7 billion years) stars. Globular clusters have proved to be a primary means of determining the ages of the oldest stars in the Universe. The halo region around a galaxy also contains millions of isolated (non-clustered) individual stars, or small groupings. Below is globular cluster NGC6093:
The largest globular cluster around the Milky Way is NGC5139, estimated to contain up to 10 million stars.
Some globular clusters are more open and contain less stars, as holds for cluster M3:
The Wide Field Camera on the Hubble Space Telescope has captured a view of just how dense are the stars in globular clusters. Here is an image of a small part of the Omega Centauri globular cluster, just outside our galaxy about 13000 light years away. At least 30000 stars appear in this segment of the cluster; most of these are similar in size and luminosity to the Sun, and some of the larger ones (yellow) are Red Giants. The cluster is 12 billion years or older in age. A large number of blue-white stars formed early on have since lost their luminosity as they converted to white dwarfs and neutron stars.
M13, the Great Hercules Globular Cluster, is imaged first in full below; the second diagram shows it again with an inset of the central region in the upper left, and two insets on the right in which individual stars are separated to give an indication of actual spacing:
Astronomers have concluded that globular clusters formed mostly during the early stages of galaxy formation (and since most galaxies appear old, clusters are ancient cosmic features - they are old) and then became much rarer as the Universe expanded. It is now known that clusters have been forming continuously over time from denser pockets of Hydrogen in the halo regions. Young(er) clusters have been found around galaxies a few billion light years or less from the M.W. These contain stars with concentrations of heavier elements that could only have reached those levels after many episodes of stellar explosions; thus many of their stars must be young. Star clusters have been observed near galaxies that collide, indicating that one process of formation is related to interactions between merging galaxy pairs. Thus globular clusters probably formed at maximum rates in the early Universe but intermediate age and even young clusters indicate that this component of galactic systems can develop at any time.
Occasionally, telescopes locate masses of small and large stars that are still in the process of organizing into individual globular clusters. Such is the case for this grouping that is part of the Large Magellanic Cloud (a galaxy group discussed on the next page). 30-Doradus is seen here as a color composite with red contributed by x-radiation, blue from UV radiation, and green from ionized Hydrogen gas:
Recently, another class of galaxies has been discovered. Called "fuzzy" clusters because of their appearance, those few found so far occur in the plane of a galaxy rather than well out in its halo (as do most globular clusters), are larger 50-100 l.y. across (globulars are usually 15-20 l.y.), and consist of dominantly old red stars. Here is a view made by the HST and supported by Keck Telescope observations that shows a fuzzy cluster on the upper left; a farther out globular cluster appears to its right.
Still another category of globular clusters, long predicted, as at last been imaged and verified. This defining image is shown below; note the four small boxes:
These clusters are not associated with galaxies as is the usual case. They are isolated in intergalactic space, a fact that has led them to be called "orphan clusters". They contain up to a million stars. Although only a few have been detected so far, they likely are fairly common throughout the Universe. The favored explanation is that they were torn from parent galaxies by other galaxies and dragged into open space; alternatively, they may just be incipient clusters trying to build to full-scale galaxies.
In the Milky Way, there are star clusters (mainly in the halo) that consist mainly of old stars. There may be only a few thousand, up to 100000, individual stars. The grouping is controlled by mutual gravitational attraction. This type is called an Open Cluster. The Pleides, shown on page 2a, is one example. Here is another (M11, the Wild Duck Cluster, about 4 light years away):
The above galaxy types have in common the fact that they show some type of geometric organization. But objects in the Universe that are galaxy-size but seem to be irregular in shape have been given the colloquial name of "blobs". Recent studies of these blobs indicate that when at least some are resolved in infrared light, a number of spherical objects appear. This image pair gives a good example: