However galaxies might have originated, either their formative stages or subsequent evolutionary events led to the myriad galaxies now seen in the nighttime sky. We observe loose and tight spiral galaxies having mixtures of old and new stars, large and small ellipticals containing only old stars, amid dwarf irregular and explosively active galaxies, let alone the baffling quasars whose central engines may not house any stars at all.
With such a zoo of galaxy-like objects littering the Universe, we naturally wonder if any overall pattern or evolutionary scheme interrelates the many varied types of galaxies. The answer is none discerned presently. As best we know, no identifiable physical mechanisms underlie all the galaxies and no clear developmental bonds relate one type of galaxy to another. Whoever does discover strong evolutionary links among the galaxies, akin to those connecting stars in the next STELLAR EPOCH, let alone the elaborate evolutionary relations among life forms in the later BIOLOGICAL EPOCH, will get their names in textbooks forever.
Normal Galaxy Evolution Astronomers decades ago proposed an evolutionary progression among normal galaxies, starting with the near-spherical ellipticals that gradually became squashed ellipticals, eventually changing into closed spirals, followed by open spirals, and finally culminating in irregular galaxies. Figure 2.19 schematically illustrates this evolutionary scheme, whose central idea is that galaxies originate with a more or less spherical shape and, as they grow older, their rotation tends to flatten them, first producing some ellipticity and then some spiral arms, prior to their breaking up as aged irregular galaxies. However, therein lies a problem: This type of evolutionary notion requires all elliptical galaxies to be young and all irregular galaxies old—which isn’t the case at all. Observationally, elliptical galaxies are not young. They are populated with only old stars, nearly depleted of interstellar gas and dust, and display no evidence of active star formation.
On the other hand, given that the elliptical galaxies are so clearly old, then perhaps the evolutionary sequence runs in the opposite sense. Maybe irregulars are young and, having formed first, gradually evolve into ellipticals. It’s easy to imagine loose spiral galaxies wrapping up into tighter spirals and eventually becoming elliptical galaxies. But troubles abound here, too. Apart from the obvious puzzle of how beautiful spirals might have emerged from the contorted irregulars, it’s hard to reconcile this idea with the abundance of old stars observed in the irregular and loose-spiral galaxies. Simply put: If irregular and loose-spiral galaxies are the starting point in a scheme of galactic evolution, then all of them should be young. But they’re not. Virtually all irregulars and spirals contain a mix of old and new stars. The existence of old stars is inconsistent with the nature of a youthful galaxy. The fact that astronomers know of no “dead galaxies” doesn’t help our understanding either.
Alas, normal galaxies do not likely evolve directly from one type to another. Spirals don’t seem to be ellipticals with arms, nor do ellipticals appear to be spirals without arms. No unambiguous parent-child relationships connect these huge cosmic systems—other than to suggest that all galaxies are cousins that trace their birth to the same grandparent, namely, turbulent dark matter inhomogeneities in the aftermath of the big bang. Indeed, all galaxies’ dispositions probably result partly from the intrinsic physical conditions extant in the gas clouds from which they originated >10 billion years ago and partly from environmental interactions with other galaxies ever since.
Frankly, this contrast between intrinsic and surrounding influences isn’t much different from the way that biological species evolve, combining aspects of their internal genes with those of their external environment. In the above paragraph, we could replace the word galaxy with the word organism and still be reasonably correct. Apparently, the nature-versus-nurture struggle extends beyond the living world. All through this Web site, we shall be confronted with the issue of whether systems change intrinsically or in response to external events. The answer for astronomical galaxies, as for biological life, is probably both. And much like human life, wherein genes are estimated to influence well less than half of human behavior, environmental effects probably dominate changes among the galaxies too.
Intrinsic Changes Star formation apparently proceeds at different rates in spiral and elliptical galaxies; after all, spirals currently contain large amounts of interstellar gas and dust, whereas ellipticals contain little. This fact is clear for two reasons: Dusty regions are associated with spiral and not elliptical galaxies, and radio radiation from atomic hydrogen in spiral galaxies is strong whereas that from ellipticals is often weak or absent (implying that loose hydrogen gas is missing in the ellipticals). Astronomers suspect that early in the "life" of an elliptical galaxy, the star-formation rate was very high. The most massive stars soon exploded (since they use their fuel more rapidly, as discussed in the next STELLAR EPOCH), and the ensuing conflagration from many such explosions drove the remaining interstellar gas from the galaxy, thus eliminating the material needed for further star formation. We can envision such an outflow of gas as forming a "galactic wind," in analogy with the solar and stellar winds directly observed from the Sun and stars. Stellar explosions of this sort (supernovae) occur frequently enough even today to keep ellipticals swept clean of interstellar matter.
By contrast, in spiral galaxies, stars might not have initially exploded frequently enough to cause a catastrophic purging of interstellar space, so a sufficient amount of interstellar matter remains today to support active star formation. Thus the differing reservoirs of interstellar matter in spirals and ellipticals conceivably result from the different initial formation rates of massive stars, which later explode. Why the rates of star formation might have differed is an unsolved problem of galactic evolution that can be addressed only by observing galaxies as they were long ago. Since "looking out is looking back," such observations are possible in principle by studying galaxies at great distances. In practice, however, these observations are difficult because of the faintness of those remote galaxies.
Nonetheless, x-ray astronomy seems to offer a way to test these ideas. The existence of galactic winds is quite consistent with current x-ray observations of rich clusters, especially those clusters in which most of the galaxies are ellipticals. In many such cases, astronomers have recently found hot, x-ray-emitting intracluster gas whose total mass and chemical composition agree with the expected accumulations of galactic winds from the various member galaxies of the cluster. Removal of loose gas from the galaxies is further aided by any intracluster gas; such intracluster gas can purge matter from the galaxies as they move through it. Figure 2.20 is an x-ray image that dramatically illustrates how galaxies can be swept clean of any stray interstellar matter not bound in stars.
Environmental Changes Astronomers do have ample evidence that galaxies change in response to external, environmental factors, long after the first preglactic fragments originated. As already noted, given the size, scale, and groupings of galaxies, collisions and interactions among them are commonplace events. This is especially true for the dark-matter halos surrounding many spiral galaxies, including our own, and probably those around all galaxies. Computer simulations performed during the past decade show that these dark halos are strongly involved in, and influenced by, such galaxy interactions.
As galaxies orbit or encounter one another, halo material from one galaxy can become stripped by tidal forces exerted by the other. The freed matter often ends up in a common envelope surrounding both galaxies; occasionally it’s lost entirely to (that is, flung out of) the system. In this way, even smaller galaxies can severely distort larger ones, depending upon the angle and proximity of interaction and the energy transferred between them. In some cases, over the course of a hundred million years—a span of cosmic time that powerful computers can model in minutes—the simulations illustrate how close encounters between galaxies can cause spiral arms to appear where none existed before. The pinwheeling arms are literally drawn out of one or both galaxies, as they pass by in each others’ wakes like giant ships at sea.
Such environmental factors may be the sole source of galaxies’ spiral arms, implying that “arms” are evolutionary appendages, not products of birth. If so, then even our home Milky Way plausibly got its arms by interacting with another galaxy at some time in the past. Our Galaxy’s stellar census surely does contain evidence that it has feasted on its neighbors, now seen as remnant, elongated clumps of elderly stars captured into the Milky Way’s halo and disk billions of years ago. Perhaps the culprits were systems as small as the Magellanic Clouds now orbiting in the halo of the Milky Way, or the Sagittarius dwarf galaxy now being torn apart and subsumed by our Galaxy on its far side opposite the Sun. Previous, long-ago encounters with a larger, comparable galactic system, such as the nearby spiral galaxy, Andromeda, is another possibility. Andromeda does currently have a component of motion toward us, meaning that our two giant galactic systems are destined for a close encounter that could cause both to become tidally disrupted and eventually more elliptical. Even more dire (or spectacular, depending on one’s viewpoint), these two grand spirals might merge together during their next encounter—the result often glibly called Milkyomeda—though that won’t happen for another several billion years.
Examples abound. Consider two spherical galaxies, one a little smaller than the other, though each having a mass comparable to our Milky Way Galaxy. Now let the two galaxies experience a close encounter. As depicted in the various frames of Figure 2.21(a), the smaller galaxy can substantially distort the larger one. This figure is a computer-generated reenactment of the environmental changes produced exclusively by gravity. Note how the interaction causes the larger galaxy to sprout spiral arms where there were none before. The entire event transpired over several hundred million years—the kind of accelerated evolution that desktop computers can model in an afternoon.
Figure 2.21(b) is a photograph of a double galaxy having an uncanny resemblance to the final frame of Figure 2.21(c). Shown there are two galaxies having sizes, shapes, and velocities matching very closely those objects in the computer simulation. The magnificent spiral galaxy is M51, popularly known as the Whirlpool Galaxy. Its smaller companion is probably an irregular galaxy which, having drifted past M51 millions of years ago, managed to disturb it greatly.
This computer-generated encounter might conceivably be a valid model for the interaction of M51 and its companion. No one claims that the computer model accurately depicts a close encounter that did occur; nor does anyone suggest that M51 became a spiral galaxy specifically because of such a gravitational rearrangement. Still, the computer rendition does demonstrate a plausible way that these two galaxies might have interacted millions of years ago, and how spiral arms might generally be created or enhanced by such interactions.
The M81-M82 system is another example of a galactic interaction that probably rearranged much matter in at least one of these galaxies. Shown in Figure 2.22, the two galaxies seem safely separated by several hundred thousand light-years of nothingness. But as also noted in the same figure, the overlaid radio contours of neutral hydrogen show clear evidence of invisible gas linking the two galaxies. The galaxies’ relative motions, and of the intervening hydrogen gas, suggest that M81 glided past M82 about 200 million years ago, severely affecting the smaller M82 object. Unquestionably strong gravitational tides doubtless rearranged much matter, triggering bursts of new stars, exploding other stars, and generally causing some activity in the central regions of M82.
These and other systems tend to support the newly emerging idea that some of the more spectacular changes occurring in galaxies perhaps result from their interactions with other galaxies. Still, such close encounters are random events and don’t represent any genuine evolutionary sequence linking all spirals to all ellipticals and irregulars.
Mergers and Acquisitions Mergers and acquisitions may well be common among galaxies in clusters, triggering changes mostly in shape well after the galaxies' initial formation. To appreciate such evolutionary events, we need to contemplate extremely long durations of time. And that’s where computer simulations again come in handy. The simulations clearly show that interacting galaxies occasionally tend to gravitate toward one another, eventually merging. What’s more, those simulations imply that giant elliptical galaxies probably grew via generations of mergers with spiral galaxies, potentially explaining why the big ellipticals reside near the core of galaxy clusters and the somewhat smaller spirals toward their perimeter. Colloquially termed “galactic cannibalism” or “galaxy gobbling,” these are scenarios for which galaxies experience very close encounters, often in fact direct collisions. Still, the interactions are sluggish, their explosiveness muted. The last big impacts seemingly occurred 8-10 billion years ago, after which most galaxies, dispersed somewhat by cosmic expansion, have enjoyed a relatively peaceful existence.
Despite these fanciful terms, astronomers have acquired remarkable observational support for such cannibalism, as actual imagery shows smaller galaxies at or near the central regions of large galaxies, apparently in the process of being “digested” as the larger galaxy gobbles them up and consumes them. Such cannibalism may also explain why supermassive galaxies—those having roughly 10 times more mass than typical galaxies like our Milky Way—are often found near the centers of rich galaxy clusters. The relatively nearby Virgo cluster of galaxies, ~60 million light-years distant, offers a prime example. There, as shown earlier in Figure 2.20, a titanic, trillion-solar-mass galaxy known as Messier 87 resides in the middle of this cosmic archipelago, ostensibly ruling the cluster’s dynamics. Having dined on its companions, this supermassive galaxy now lies in wait, patiently awaiting more “food” to fall into the gravitational grip of its 3-billion-solar-mass black hole. The other, smaller galaxies swarming around in the outskirts of this and other galaxy clusters like it are almost surely destined to be someday integrated into the swelling central “beasts” at the heart of their evolving systems. Figure 2.23 is an astonishing image that has apparently captured this process at work!
Theoretical support for this "bottom-up" scenario is provided by computer simulations of the early Universe, which clearly imply that merging took place—much of it under the direction of, once again, gravity. Further evidence comes from recent observations indicating that galaxies at large distances from us (>6 billion light-years away, meaning that the light we see was emitted >6 billion years ago) appear distinctly smaller and more irregular than those found nearby. Figure 2.24(b) and (c) show some of these images. The vague bluish patches are apparently separate small galaxies, each containing only a few percent of the mass of the Milky Way Galaxy. Their irregular shape is thought to be the result of building-block mergers; the bluish coloration comes from young stars that formed during the merger process.
If galaxies did form by repeated mergers, how might we account for the differences between spirals and ellipticals? The answer is still blurred, although 2.25 might have some currency. One apparently important factor is just when and where stars first appeared—in the original blobs, during mergers, or later—and how much gas was used up or ejected from the young galaxy in the process. If many stars formed early on and little gas was left over, an elliptical galaxy would be a likely outcome, with many old stars on random orbits and no gas to form a central disk. Alternatively, if a lot of gas remained, it would tend to sink to a central plane and form a rotating disk—in other words, a spiral galaxy would form. However, it’s not known what determines the time, the place, or the rate of star formation. For this reason, it’s still an open question whether all spirals and ellipticals form in basically the same kind of environment or if each type forms under different circumstances.
We do have some important clues to guide us, though. For example, spiral galaxies are relatively rare in regions of high galaxy density, such as the central regions of rich galaxy clusters; sprials tend to hang out in the outskirts of clusters. Is this because they simply tended not to form near the center, or is it because their disks are so fragile that they are easily destroyed by collisions and mergers, which are more common in dense galactic environments? Computer simulations suggest that collisions between spiral galaxies can indeed destroy the spirals' disks, ejecting much of the gas into intergalactic space, thus creating hot intracluster gas now seen glowing in x rays, as implied earlier by Figure 2.20. Spirals do seem to be more common at large distances (that is, well in the past), implying that their numbers are decreasing with time, presumably the result of repeated collisions.
Nothing in this area of research is clear cut. The above ideas represent frontier thinking, which is itself evolving with each generation of astronomers. Puzzles abound at every turn: Some isolated elliptical galaxies reside in the “field” well outside clusters, which would seem hard to explain as the result of mergers. (Perhaps they’ve already gobbled up everything around them.) Spiral galaxies often populate the outskirts of galaxy clusters where encounters would seem to be rare and thus not conducive to the growth of spiral arms. (Perhaps they’re in wide orbits about the cluster core, obeying Kepler’s laws and spending most of their time far from the center.) And the irregular galaxies don’t seem to fit into any evolutionary scheme—unless, ironically, they’re the larger galaxies’ building blocks staring us right in the face. (Perhaps those irregulars that still exist are the survivors, having so far managed to avoid extinction.)
Simply stated, owing to their distance and therefore their dimness, galaxies are hard to observe and the observations even harder to interpret. Many galactic secrets still lurk within them, awaiting new probes and new insights by future generations of astronomers eager to solve one of the great unresolved riddles in all of science—the origin and evolution of normal galaxies, abundantly and ubiquitously scattered through the Universe.
Active Galaxy Evolution Evolutionary links between normal galaxies and active galaxies are more robust, though they, too, are hotly debated. A time sequence like that sketched in Figure 2.26(a), starting with quasars and proceeding to active galaxies and finally to normal galaxies, implying a continuous range of cosmic energy, has been bolstered in recent years. Adjacent objects along this sequence are almost indistinguishable from one another, meaning that all galaxies, regardless of type, might have similar “engines” at various stages of activity—such as supermassive black holes, which virtually all galaxies do seem to have at their cores. For example, weak quasars have some commonality with the most explosive of the active galaxies, whilst the feeblest active galaxies often resemble the most energetic members of the normal galaxies. Such a chain of cosmic verve suggests that galaxy-like objects originated as quasars ~12 billion years ago, after which their emissive powers gradually declined, becoming in turn active galaxies and eventually normal galaxies. This continuity among all galaxies has been strengthened recently as astronomers have become reasonably convinced that the black-hole energy-generation mechanism can account for the luminosities of quasars, active galaxies, and the central regions of most normal galaxies.
This unifying idea maintains that the quasars are actually ancestors of all (or most of) the galaxies. Consistent with the observed fact that quasars were more common in the past than they are today, galaxies do seem to have been more active long ago than they are now. Far too remote for us to resolve any individual stars within them, the quasars are detectable at great distances only because of their tremendously energetic central engines. Precisely because of their great distances, we perceive them as they once were in their blazing youth. As their core activity declined with time, quasars assumed forms and functions closer to those of more familiar and nearby galaxies. They essentially “wound down” while running out of fuel to feed their central black holes, eventually becoming the relatively quiescent normal galaxies now observed closer to us in space and time.
Should this view be proved correct, then maybe even our Milky Way Galaxy was once a brilliant quasar. Most ironic, if true. For decades, astronomers have struggled to decipher the herculean quasars, especially their prodigious energy emission, only to find, perhaps, that we live inside an old, burned-out one—a time-tamed version of a quasar that once lit up the far away and the long ago.
For this quasar evolutionary idea to hold, we ought to be able to see the vague outlines, however far away, of the more normal galaxies surrounding the quasars. Until quite recently, astronomers were hard-pressed to discern any galactic structure whatever in quasar images. However, the Hubble Space Telescope has done yeoman service since the mid-1990s by indeed finding “host” galaxies around some of the distant quasars. The evidence appears as dimly glowing “fuzz”—see Figure 2.27—now seen to be faintly enveloping a few dozen of the brighter quasars studied to date. Quasars really do seem to be residents within the centers of normal galaxies, rich in ordinary matter beyond their bright cores; the fuzz is apparently the accumulated soft emission of innumerable unresolved stars or stars-to-be. Some of the deepest, long-exposure quasar images even show suggestive evidence for spiral arms, such as in the figure here.
Although attractive, this quasar --> active galaxy --> normal galaxy evolutionary sequence has its drawbacks. Not all astronomers have yet embraced the idea, arguing that evolutionary links may not exist at all. As sketched in Figure 2.26(b), they suggest that the powerful quasars are merely extreme manifestations of the explosive phenomena seen in virtually all galaxies. After all, even the center of our own Milky Way is known to be expelling matter and radiation. The same can be said for active galaxies and quasars, though on vastly larger scales. Perhaps all these objects are part of the same family without there being any evolutionary sequence linking its members, just as evolutionary changes cannot be said to bridge different races within the human species. Each galaxy type or human race is distinctly different. One race of humans doesn’t evolve into another, and similarly one type of galaxy might not necessarily evolve into any other. Instead, all the galaxies might be quite ordinary galaxies that formed long ago, though some were endowed with especially explosive central regions. Those able to exercise their explosiveness more than others for some still unknown reason are called quasars, while those hardly able to fire up their cores much at all are called normal galaxies.
Why the quasars emit radiation so prodigiously, even violently, is also unknown, other than the notion that more fuel was available at earlier times. And for how long the quasars endure in their bright phase, adequately supplied with fuel, is also unknown; certainly they cannot do so indefinitely, lest their central black holes consume their whole being. The answers presumably lay buried within the relatively uncharted centers of galaxies, including the startling idea, now subject to much debate, that quasars originally formed and regularly flare as supermassive black holes themselves merged, especially during the GALACTIC EPOCH within a few billion years after the big bang.
Future research focused on the cores of galaxies will probably provide the best insights to decipher the secrets of the bright and shining quasars, whose troubling properties of huge energy yet small size once threatened to topple the laws of physics. Even if quasar details are sorely lacking, their main issues now seem reasonably solved and the laws of physics intact. Rather than jeopardizing our knowledge of the cosmos, these violent objects have become an integral part of the thread of understanding that binds our own Galaxy to the earliest epochs of the Universe in which we live.