Grappling With Galaxies: Basic Questions Persist

After years of passivity, the astronomy community is protesting telescope closings, cramped quarters, and scanty maintenance Our understanding of galaxies is at a primitive level, and we are still perplexed by basic questions. For instance: 1. Why do things such as galaxies exist at all, with their observed characteristic dimensions—typically 10^11 stars within radii of about 10^4 parsecs? 2. What is the dark matter that constitutes so large a part of galaxies?We know that more than 9O% of their mass is unaccounted for and that the visible content of galaxies is embedded in a diffuse “halo” of dark matter that may be 10 times as massive. 3. Why do the central nuclei of some galaxies flare up (especially at redshifts z=2) outshining the entire host galaxies, and what remnants of this activity are left behind? Possible answers are now coming into focus as astrophysicists and cosmologists increasingly concentrate on these questions, which are some of the most important ones in physics today. Galaxies must have evolved from small-amplitude fluctuations in the early universe—regions of above-average density that lagged behind the universal expansion and even formed-gravitationally-bound galaxies. We can infer how this happened by studying the structure and spatial distribution of present-day galaxies, and seek ing correlations among their properties. In the last few years more direct evidence has started to become available: Galaxies have been observed so far away that their light has taken much more than half the age of the universe on its journey toward us. We can now in practice, and not merely in principle, obtain “snapshots” of galaxies at early evolutionary stages—perhaps right back to their emergence from amorphous beginnings. What the dark matter consists of is still a mystery. It could be black holes—dead remnants of an early generation of massive stars—or it could be more than 10^12 very small stars (brown dwarfs), each less than a tenth of the Sun’s mass, whose centers are not hot enough to ignite nuclear fusion reactions and which are therefore very faint. A halo population of black holes, each in the range 10^2-10^6 masses, could manifest themselves indirectly via interaction with interstellar gas. The nearest brown dwarf would be less than a light-year away, perhaps detectable as a red object moving across the sky faster than a typical star (that is, with a higher so-called “proper motion”). If either brown dwarfs or black holes were numerous enough to account for the dark matter, there would be a real prospect of detecting the characteristic gravitational lensing that would occur when such an object lay almost exactly along the line of sight to a bright background source. Because the effec tive cross section is larger for a long pathlength, there is actually more chance of detecting lensing by compact objects in the halos of galaxies half way out to the Hubble radius than in our own Milky Way. A more exotic possibility is that the dark matter consists of elementary particles left over from the big bang—for instance, neutrinos, or hypothetical weakly interacting neutral particles such as the “photinos” that some theories predict. Indeed, some cosmologists argue quite compellingly that ordinary atoms are merely a trace constituent of the entire universe. Every cubic meter of our galaxy—including our immediate terrestrial environment—would then contain about 100,000 such particles, moving at a few hundred kilometers per second and interacting hardly at all with ordinary atomic nuclei, the characteristic cross-section being 10-36 square centimeters. From these numbers, one estimates that there would be a few collisions per day within every kilogram of material; in each such impact, the nucleus would recoil with a typical speed of 100 kilometers per second. These rare events could be detected by laboratory experiments. Various such experiments (mainly involving sensitive cryogenic techniques) are now being planned both in Europe and the United States. Even optimists might rate the chance of a positive result at less than 50%. However, even upper limits would set interesting constraints on some of the dark-matter candidates that particle physicists have already proposed. And the scientific payoff from a positive result—actual detection of the dominant constituent of the universe, and the discovery of a hitherto unknown class of particles—is so colossal that the modest investment in such experiments seems more than justified. Indeed, these searches seem among the most important high-risk experiments being contemplated in physical sciences today. There is no reason why everything in the universe should shine. We should therefore not really be surprised that the dominant component of our galaxy (and other galaxies) seems to be “dark.” The existence of the dark matter is inferred from the gravitational pull exerts on stars and gas. There are indeed, almost too many possible candidates; the main task is to narrow down the range of options. At the moment, I am personally “agnostic” and would lay roughly 259 odds each on brown dwarfs, black holes, or weakly interacting particles (leaving the last 25% for possibilties that haven’t yet been thought of). But it is encouraging that feasible observations or ex periments could detect (or at least constrain) each of the favored candidates. Particle theorists should eventually be able to firm up their predictions as to whether any particle species would survive in sufficient numbers to account for the dark matter. Astrophysicists and cosmologists are trying to narrow down the range of cosmogonic options by modeling the physical processes involved in galaxy formation and clustering, adopting several different hypotheses in turn about what the dark matter is, and seeing which, if any, assumption yields agreement with what we see. What a young galaxy would look like depends very much on what the dark matter in present-day galaxies actually is. For example, if black hole remnants of a bright an short-lived stellar population that formed early in galactic history constitute the dark matter, the young galaxies would be exceedingly luminous. If, on the other hand, weakly interacting photinos (or other such particles) are the gravitationally dominant constituent of our universe, then these particles would aggregate into galactic-scale self-gravitating swarms, into which the ordinary gas gradually sedimented before condensing into stars. According to the so-called “cold dark matter” cosmogony, galaxies and clusters are dynamically dominated by weakly interacting particles left over from the early universe, and evolved from scale-independent initial fluctuations. The dynamical aspects of this model have been explored in great detail in recent years, with encouraging results the next, much harder, step is to incorporate realistic gas dynamics dissipative ef fects, and star formation. The first bound systems probably on subgalactic scales) may have formed at a very large redshift. However, one can give general arguments that galaxy formation wasn’t completed until the universe was a few billion years old. The free-fall timescale for the outer parts of halos is > l0 ^ 9 years; and, if discs acquired their angular momentum via tidal torques, they must have been spun up whan the matter was at a radius 10 times larger than the present disc (that is, > l00 kiloparsecs for the material in a disc at 10 kiloparsecs radius). The universe would therefore be expected to be very different at z=2. Galaxies at these distances are now becoming detectable, so there will soon be direct evidence on galactic evolution. The quasar phenomenon was much more prevalent at redshifts of 2-3, and may be associated with particular phases in the host galaxies’ evolution. The dynamically inferred dark matter amounts to only 10% or 20% of the cosmological critical density. If actual density is as high as the critical value, the galaxy distribution must be less smooth than that of matter in general. The voids must be less empty than they look, and the galaxies must be “biased,” being formed more efficiently in overdense regions. There are ideas on how this might come about in a universe dominated by non-baryonic matter, but at the moment the popularity of a “flat” universe is based more on theoretical prejudice than on direct evidence. Martin J. Rees is the Plumian Professor of Astronomy at Cambridge University. He is also the director of the Institute of Astronomy in Cambridge. The preceding is the author’s summary of an invited paper he presented at the 20th General Assembly of the International Astronomical Union held last month in Baltimore.

Grappling With Galaxies: Basic Questions Persist

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