Local Group:
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The biggest and brightest Local Group members are the Milky Way Galaxy and
the brightest Messier objects: M31 and M33. Next in line would be M32 and
the two Magellanic Clouds (the LMC and SMC). The Clouds are big and close,
so we have good detailed studies of them. The rest are smaller objects,
either irregular galaxies or dwarf ellipticals. Two other more distant and less luminous irregular galaxies that have gotten well deserved beatings with large telescopes are NGC 6822 (``Barnard's Galaxy'') and IC 1613. They are providing new insights for both the distance scale and the evolution of galaxies. Both have Cepheid variables, still the best way of determining distances within the nearest 10 million light-years, and both have current star-formation activity. For reasons as yet unknown, star formation in NGC 6822 is far more active than the otherwise similar IC 1613.
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In the basement of luminosity, we find the ``Seven Dwarfs'', the small,
very faint dwarf elliptical galaxies that surround the Milky Way Galaxy.
They are neither spirals nor irregulars. However, they are so different
from any other elliptical galaxies that we will follow convention and call
them Dwarf Spheroidals or DSph's. There might be a lot of these dwarfs
around other galaxies, but they have been hard enough to find nearby. A
recent exciting result has been the discovery that these objects are
completely dominated by dark matter, as we will discuss later. What is the criterion for inclusion in the Local Group? Proximity is the cleanest and often used to the exclusion of any other. If we grant membership to all galaxies within 4 million light-years, we have a club with 30 members, three of which barely got in. We can also use velocities to find out if the last three are on their way in or out. That is, we can accept all the fellow travelers. If we do this, the three squeakers become full members and we are pressed to include a few more distant objects such as Leo A and Pegasus, both small irregular galaxies. There are still other applicants on the waiting list. There are two faint irregular galaxies in Sextans, imaginatively designated Sextans A and B, both of which are well-resolved into stars. At the moment, they just barely fail to qualify. The bright irregular galaxy NGC 3109 also hovers on the periphery with a velocity that is only a little too large. The membership committee will have to carefully consider another small irregular object nominated by R. Kraan-Korteweg and G. Tammann, UGC-A86. A recent letter of recommendation has been published by A. Saha and J. Hoessel. Their supporting documents include a color-magnitude diagram that fixes a distance close enough for inclusion. The applications of past candidates, such as the heavily-obscured spirals NGC 6946 and IC 342, and the Maffei 1 and 2 infrared galaxies, have been declined. They fail on all counts, their velocities are all wrong and they are too distant.
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Hubble Classification:
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Hubble introduced the classification scheme illustrated in the following figure, which
separates most galaxies into elliptical, normal spiral, and barred spiral categories, and then
sub-classifies these categories with respect to properties such as the amount of flattening
for elliptical galaxies and the nature of the arms for spiral galaxies. The galaxies that do
not fit into these categories are classified separately as irregular galaxies.
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The "definitive" version of the Hubble classification was set out in Sandage's article in
Galaxies and the Universe. Compared to Hubble's original conception, this version adds the S0
(lenticular) class between ellipticals and spirals. Hubble hypothesized such an intermediate
class, but it was only recognized later. Galaxies are often called early (E and S0) or late
(Sb,Sc, Irr) in type, a remnant of early notions that galaxies physically evolve along the
Hubble sequence. Classification of an elliptical galaxy picture is straightforward, because there is so little structure present. Types E0-E7 are recognized, where the number gives the projected axial ratio. Specifically, an E0 galaxy appears circular (like M87), and in general for axial ratio b/a the number is 10(1-b/a). True ellipticals do not appear more flattened than E7, probably because there is a stability limit for nonrotating systems near this shape. Note that even the ellipticity is not completely well-defined, as many ellipticals have changes in ellipticity with radius or isophotal twists. Most appear to be triaxial systems. Spirals are divided into ordinary (S) and barred (SB) families, with a stage from S(B)a ("early" types, large bulge, tightly wound and fairly smooth arms) to S(B)c ("late" spiral, small bulge, loosely would arms, arms very textured with star-forming regions). As noted in the Hubble atlas, some spirals have arms patterns defined more by the dust lanes than by starlight per se.
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| Galaxy Properties by Hubble Class | ||||||
|---|---|---|---|---|---|---|
| E0-E7 | S0 | Sa | Sb | Sc | Irr | |
| Nuclear Bulge | "All Bulge" No disk |
Bulge & Disk | Large | -> | Small | None |
| Spiral Arms | None | None | Tight/Smooth | -> | Open/Clumpy | Occasional traces |
| Gas | Almost none | Almost none | ~1% | 2-5% | 5-10% | 10-50% |
| Young Stars HII Regions | None | None | Traces | -> | Lots | Dominates Appearance |
| Stars | All Old (~ 1010yr) |
Old | Some young | -> | -> | Mostly(?) young (but some v. old) |
| Spectral Type | G-K | G-K | G-K | F-K | A-F | A-F |
| Color | Red | Red | -> | -> | -> | Blue |
| Mass (Msun) | 108-1013 | (More) 1012-109 (Less) | 108-1011 | |||
| Luminosity (Lsun) | 106-1011 | (More) 1011-108 (Less) | 108-1011 | |||
Elliptical Galaxies:
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Elliptical galaxies, which essentially consist of only a nuclear bulge
component are subdivided among seven ellipticity classes from E0
(circular) to E7 (cigar shaped). Numerically the ellipticity is given by
10(a-b)/a, where a is the length of the major axis and b is the length of
the minor axis. Of course, the Hubble Classification does not tell us the
true shape of the galaxy (e.g. an E0 could be a "cigar" seen down its
barrell). Statistical arguments suggest that the distribution of galaxies
among the ellipticities is roughly uniform. Galaxies of this class have smoothly varying brightnesses, steadily decreasing outward from the center. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same color: they are somewhat redder than the Sun. Ellipticals are also devoid of gas or dust and contain just old stars. All ellipticals look alike, NGC 4881 is a good example (NGC stands for New General Catalog). Notice how smooth and red NGC 4881 looks compared to the blue spirals to the right. A few ellipticals are close enough to us that we can resolve the individual stars within them, such as M32, a companion to the Andromedia Galaxy.
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Hubble Space Telescope's exquisite resolution has allowed astronomers to
resolve, for the first time, hot blue stars deep inside an elliptical
galaxy. The swarm of nearly 8,000 blue stars resembles a blizzard of
snowflakes near the core (lower right) of the neighboring galaxy M32,
located 2.5 million light-years away in the constellation Andromeda. Hubble confirms that the ultraviolet light comes from a population of extremely hot helium-burning stars at a late stage in their lives. Unlike the Sun, which burns hydrogen into helium, these old stars exhausted their central hydrogen long ago, and now burn helium into heavier elements. The observations, taken in October 1998, were made with the camera mode of the Space Telescope Imaging Spectrograph (STIS) in ultraviolet light. The STIS field of view is only a small portion of the entire galaxy, which is 20 times wider on the sky. For reference, the full moon is 70 times wider than the STIS field-of-view. The bright center of the galaxy was placed on the right side of the picture, allowing fainter stars to be seen on the left side of the picture. Thirty years ago, the first ultraviolet observations of elliptical galaxies showed that they were surprisingly bright when viewed in ultraviolet light. Before those pioneering UV observations, old groups of stars were assumed to be relatively cool and thus extremely faint in the ultraviolet. Over the years since the initial discovery of this unexpected ultraviolet light, indirect evidence has accumulated that it originates in a population of old, but hot, helium-burning stars. Now Hubble provides the first direct visual evidence. Nearby elliptical galaxies are thought to be relatively simple galaxies comprised of old stars. Because they are among the brightest objects in the Universe, this simplicity makes them useful for tracing the evolution of stars and galaxies. |
S0 Galaxies:
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The "definitive" version of the Hubble classification was set out in Sandage's article in
Galaxies and the Universe. Compared to Hubble's original conception, this version adds the S0
(lenticular) class between ellipticals and spirals. Hubble hypothesized such an intermediate
class, but it was only recognized later. Galaxies are often called early (E and S0) or late
(Sb,Sc, Irr) in type, a remnant of early notions that galaxies physically evolve along the
Hubble sequence. Unfortunately, this nomenclature is opposite to that of the dominant stellar
population in these types, and to the early-late nomenclature in the Yerkes classification.
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Spiral Galaxies:
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Iregular Galaxies:
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Clusters of Galaxies:
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