Young, old, big, small, luminous and faint, the galaxies in the universe span an enormous range of properties. A million galaxies have been counted that are brighter than the 15th magnitude, peppering the heavens like a second tier to the starry night sky. These enormous island-colonies of stars harbor among their ranks members that have acheived considerable notoriety among earth- bound astronomers. The Milky Way is a spiral-type galaxy 100,000 light years in diameter containing perhaps 500 billion times the mass of the sun in the form of stars and interstellar gas. Yet, tipping the cosmic scales at over 2 trillion solar masses is the faint, 14th magnitude giant spiral galaxy UGC 12591 in the constellation Pegasus. Spirals of this size are believed to be rare. Usually the 'heavy-weights' of the universe are the elliptical galaxies, among them in particular the so-called 'cD's' found in the centers of rich clusters of galaxies. Over a period of billions of years these cD galaxies consume nearby galaxies in the cluster and incorporate their stellar contents into their own steadily growing mass. A recently discovered example of galactic canabalism in progress may be the galaxy Arp 220.
Arp 220, also called IC 4553, has a complex shape, neither elliptical nor spiral, with two nuclei separated by 20,000 light years leading astronomers to believe that two galaxies recently collided and merged. Located in the constellation Serpens Caput at a distance of 230 million light years, the collision has triggered an enormous burst of star- forming activity within the galactic nuclei, making Arp 220 one of the most luminous known galaxies. Each second, it radiates more energy than 2 trillion stars like the sun, mostly in the infrared, and within a region less than 7000 light years across. Adding to the uniqueness of this galaxy is the fact that it is one of only three known galaxies that produces maser emission: a coherent form of radio emission commonly found only under laboratory conditions. The active maser region of Arp 220 is 450 parsecs in diameter and brighter than 70 million ordinary star forming regions in our own Milky Way. It is not completely understood what the energy mechanism is that operates in such a small region of space, or how the maser mechanism is produced in such a large object. Although the merger of the two galaxys has already taken place, there are other objects in the universe which may be examples of such canabalism about to take place in the next few hundred million years. Consider the case of the galaxy cluster Shakhbazian 1.
This dense cluster of 24 galaxies about 2.3 billion light years away occupies a volume of space 650,000 light years across, only a few times the diameter of the Milky Way. The member galaxies, crowded together almost to the point of touching, each contain about 30 billion solar masses of stars and gas. As a comparison, the Milky Way is also a member of a cluster of 30 galaxies, but spanning a volume 16 million light years across. The outstanding question concerning this cluster of galaxies is "How did the 24 galaxies in Shakhbazian 1 become so densely packed into such a small volume?" We might also ask if this cluster on the verge of collapse into one large galaxy.
NGC 2685 is an interesting variation on the ring galaxy system. This bright galaxy in the constellation Ursa Major looks very much like an ordinary edge-on spiral, except for one remarkable feature: In addition to the customary disk of stars and gas, there are several thin filamentary strands that form a helical band perpendicular to the disk and centered on the galaxy's nucleus. The strands consist of knots of luminous star- forming regions and hydrogen gas with a diameter comparable to the disk. Twenty-two of these 'polar ring' galaxies have now been discovered. The normal formation of a galaxy, as we think we understand it, produces only one disk component per spiral galaxy, not the two associated with NGC 2685. This suggests that some other event interfered with the normal formation of these galaxies. A strong possibility is that NGC 2685 had a companion, perhaps like one of the Milky Way's Magellanic Clouds. This companion was captured into a polar orbit and its stars eventually merged with those of the galaxy, leaving behind the companion's interstellar medium. New generations of stars formed from this material to produce the luminous ring that we now see. It is possible that if the Magellanic Clouds had been closer to the Milky Way, they too would have created a polar ring around our galaxy. Perhaps a foreshadowing of this process can be found in the so-called Magellanic Stream, a string of hydrogen clouds trailing the Magellanic clouds extending thousands of light years along their orbit around the Milky Way.
We are all familiar in principle, if not in detail, with how lenses may be used to bend and re-focus light. Whether one considers a simple pair of eye glasses or the complex, compound lenses in a camera, the principle is the same. A little appreciated result of Einstein's Theory of General Relativity is that gravity bends light as surely as an optical lens. The deflection near the sun's limb is almost imperceptible but, nonetheless, measurable. An exciting discovery made recently in astronomy is that whole galaxies and clusters of galaxies can serve as gravitational lenses if the geometry is just right. What types of images do such galaxies produce? PG 1115+08 is one of the first known examples of a gravitational lens source. The image of a distant quasar has been split into three images by the gravitational bending of the light rays caused by a galaxy located along our line-of-sight to the quasar. If the background quasar is located almost exactly along our line-of-sight, a more unusual situation may result.
The object 2237+0305 is a faint spiral galaxy. When the slit of a spectroscope passes the light from only the disk region of the galaxy, a pattern of spectral lines can be identified whose doppler shift when interpreted by astronomers indicates a distance of 400 million light years to this system. Yet, when the light from the galaxy's nucleus is examined, a much larger red shift is deduced appropriate to a galaxy located 7 billion light years away. A recent resolution of this contradiction suggests that the image of a quasar located behind the galaxy has been focused in front of the galaxy as seen from our vantage point. The result is a comparatively nearby spiral galaxy with two incompatable distances.
Consider the object known only as the Leo Dark Cloud. This cloud of hydrogen gas forms a partial ring around two galaxies -- NGC 3384 and M 105 -- in the constellation Leo. The ring is over 600,000 light years in diameter and is slowly rotating with a period of about 4 billion years. The cloud has only been detected through its radio emissions and is completely invisible optically. One theory suggests that it may have been produced from the collision of two galaxies which stripped the interstellar hydrogen gas from both of them. Because the gas had some angular momentum to it left over from its rotation within each galaxy, the gas that was left behind continues to rotate in the form of a giant ring. The amount of matter in the ring is estimated as 2 billion times the mass of the sun, more than enough to form a separate galaxy of its own.
Even in our own neighborhood there may be yet other examples of galaxies only now beginning to form from scattered intergalactic hydrogen clouds. I Zwicky 18 is believed to be one of the youngest known nearby galaxies. Its total mass is about 800 million suns as compared to the Milky Way's total mass of about 200 billion. Located 32 million light years distant, this 16th magnitude galaxy has a bright blue color indicating the recent formation of thousands of young, luminous, massive stars of stellar classes O and B. The atomic composition of the light- emitting gas shows very few elements heavier than hydrogen, and oxygen seems to be about 37 times less abundant than in an old galaxy like the Milky Way. What this means is that the gas in this galaxy has not been recycled by many supernovae which would otherwise enrich the gas with elements heavier than helium.
Although the precise triggering mechanism that causes a cloud to collapse and produce stars is not fully understood, it may be possible that in fact a variety of agents can cause a cloud to loose its precarious balance and collapse. Collisions between individual clouds in a galaxy may be effective in compressing matter, as are the shock fronts produced by supernovae. Perhaps the most remarkable triggering mechanism ever proposed is the one thought to have stimulated the star formation process in Minkowski's Object. Adjacent to Minkowski's Object is the elliptical galaxy NGC 541 in the constellation Cetus, which also happens to be a radio galaxy containing a jet of ionized plasma emanating from its nucleus. Evidence now seems to indicate that the star formation in Minkowski's object was actually triggered by the jet from the nucleus of NGC 541 because the body of the jet can be traced all the way to the region where the new stars are forming in Minkowski's Object. If this turns out to be a correct interpretation, this will be the first case known where a jet from one galaxy can impact a neighboring galaxy and trigger new stars to form. The details of just how this is done are still unknown.
In a catalog of the most remarkable objects in the universe one would be terribly remiss if one did not include those which host some of the most titanic outpourings of matter and energy imaginable. In these 'active galaxies', great volumes of matter are being expelled from mysterious regions barely a light year in diameter located deep in the nuclear regions of the galaxies. The entire host galaxy reals from the explosion which triggers waves of star forming activity extending thousands of light years from the active, nuclear core region. Among the best studied examples of 'starburst' or 'mini-quasar' activity can be found in the great star systems called Cygnus-A and Centaurus-A.
Cygnus-A is a peculiar-looking, 15th magnitude galaxy
located in the constellation Cygnus which would probably never
have come under scrutiny were it not for the fact that it is the
host for the strongest radio source in the entire sky, excluding
the sun of course. Located 600 million light years away, this
galaxy is among the giants of the universe with a mass estimated
at 100 trillion times the sun's mass. It consists, apparently, of
two nuclei separated by 5500 light years, embedded in a galaxy
extending some 450,000 light years across. Just as for Arp 220,
the two nuclei of Cygnus-A are probably all that remain of two
separate galaxies that passed too close to each other and merged
together. The total radio power from Cygnus-A exceeds the total
electromagnetic output of over 2 trillion stars like our sun.
This in itself is not surprising, since the galaxy contains
enough mass to account for 100 trillion suns. What makes this
output phenomenal is that the radio emission does not come from
stars in the optical galaxy, instead, it originates in two
optically invisible regions located 160,000 light years on either
side of the optical galaxy! These clouds consist of a hot plasma
of electrons moving in magnetic fields at nearly the speed of
light. The clouds may have been produced and replenished over
time by jets of relativistic plasma ejected from the galactic
nucleus. Such double radio sources are not uncommon in the
universe and jets connecting the optical nucleus with the distant
radio lobes are often seen. What causes this peculiar ejection?
Current thinking suggests that in the cores of these galaxies,
gigantic billion-solar-mass black holes lurk. Surrounding them
are swirling disks of gas and stars from which some material
eventually falls into the black holes, releasing enormous
quantities of energy, radiation, and jets of plasma and magnetic
fields. The jets travel travel along a path of least resistence
perpendicular to the disk plane and the plasma eventually ends up
in dumbell-shaped lobes to either side of the galaxy.
We have taken quite a grand tour through the universe in search
for rare phenomena. There are many more that could have been
described. Every astronomer has his or her own list of peculiar
objects in the known universe. It's fun to sit around and chat
with collegues about weird bodies in space and to try to
understand how they fit in with our standard explanations of
stars, galaxies and other things. But today's peculiar objects
rather quickly become tomorrow's standard archetypes. They serve
as the necessary friction beneath the wheels of our steadily
advancing knowledge of the world around us. They also make
astronomy an exciting and dynamic subject, some of whose contents
remain fixed while others are forever changing.
We have taken quite a grand tour through the universe in search for rare phenomena. There are many more that could have been described. Every astronomer has his or her own list of peculiar objects in the known universe. It's fun to sit around and chat with collegues about weird bodies in space and to try to understand how they fit in with our standard explanations of stars, galaxies and other things. But today's peculiar objects rather quickly become tomorrow's standard archetypes. They serve as the necessary friction beneath the wheels of our steadily advancing knowledge of the world around us. They also make astronomy an exciting and dynamic subject, some of whose contents remain fixed while others are forever changing.