the stars originally in the halos of galaxies in clusters must currently permeate intergalactic space. Tidal forces between colliding galaxies during the first billion years of the cluster's existence stripped the outer halos of stars. Stripping was effective beyond a radius of about 100,000 light years in a typical galaxy.
From observations of the Doppler shifts of their spectra, we infer that the cluster galaxies move at rather high random velocities. Because we can measire the dimensions of a cluster, we can compute how much mass must be present within the rapidly moving galaxies to contain the expansion. (if this mass were not present, the galaxies would simply fly apart, there would be no cluster.) the result is surprising; the required amount of mass per galaxy is several times as large as that inferred by other types of measurements, usually of nearby galaxies, whose dynamics we can study in suffiecient detail to infer their masses. For example, by measuring the rate at shich a nearby galaxy is roatating, we can infer its mass. We can also measure the velocities of nearby galaxies ina number of isolated close pairs to determine the average mass of the pair.
We can make these statements rather more precise by introducing the mass-luminosity ratio. We measure luminosity directly and, for every unit of luminosity (usually expressed in units of solar luminosity), we can assign a certain number of units of mass (expressed in solar masses). Thus, the sun has a mass -luminosity ratio of 1; the visible regions of the Milky Way galaxy, which consist for the most part of stars less massive and considerably less luminous than the sun, have a mass-luminosity ratio between 200 and 400. Measurements of individial elliptical galaxies yield a mass-luminosity ratio of about 8, although this result is applicable only the central region luminous regions.
By studying radio emmision from meutral hydrogen, scientists have been able to measure the rate at which a spiral galaxy rotates. We can follow to the extreme parts of the galaxy should be more weakly bound. They should therefore experience a weaker centrifugal force and be rotating less rapidly But this is contrary to what is found. It appears from the measurements atht spirals have larger mass-luminosity ratios than we would predict from studying their luminous inner regions More mass must be present than we havepreviously realized. their net mass-luminosity ratios must be about 30 or even larger, Precisely what for this non luminous matter take in the out regions , or halos is not known.
rotation curves probe the outer regions of spiral galaxies, where there is little luminous matter. two different techniques have been used to study ellipticals, which are gas poor and therefore not amenable to rotation curve studies at large distances from the center of the galaxies. X-ray emmision has been discovered around ellipticals. the x-rays are produced by hot gas at about 10 million kelvins, gravitionally confined in the halos of the elliptical galaxies. To confine the gas reuires a considerable amount of mass: it is inferred that the ratio of total mass, including dark halo, to optical luminosity, which comes entirely from the inner regions, may be as large as 50.
Antoerh discovery also indicates a considerable amount of dark matter in the halo of the elliptical. Elliptical galaxies reveal the presence of faint shells on deep photographic plate. These shells extend out two or three times as far as the bulk of the starlight. As many as 20 shells have been discovered around one bright galaxy. the shells appear to be fossil "splashes" remaining from a merger of a smaller satellite galaxy into the core of the elliptical. the spacing of the shells are a measure of the gravitational field, and computer simulations of the merger result in a simple array of concentric shells. Modeling of the shells requires the presence of a massive dark halo.
Classical methods of mass determination, based on optical studies of the luminous inner regions, leave open the possibility of galaxies having considreable amounts of mass in their extended halos. GAlaxies could be very extended indeed, concievably filling most of space with exceedingly tenous halos, In clusters, the halos were stripped during collisions between the galaxies. However the excess mass should stil be present in the intergalactic medium. But the precise form of the dark mass poses a great astrophysical puzzle. the mass cannot be very luminous, or astonomers would be able to observe it directly. It cannot be gaseous, because gas, whether hot or cold, ionized or neutral, is difficult to hide. Many searches have been performed for intergalactic gas. Some gas has been discovered in rich clusters, but not enough to account for the mass discrepancy. Perhaps the most dramatic studies of dark matter in galaxy cluster have merged from the gravitational lensing by the cluster of background galaxies. the gravity field of the cluster bends the background light, acting as a lens, and produces images that are distorted into arcs, This effect was predicted by Albert Einstein but was first detected in the 1970's.
two hypostheses have merged to account for the mass that is inferred to be present in clusters and in galactic halos. One hypothesis argues that the dark mass is baryonic. It might consisst of stars of very low mass, which are so faint that they have escaped detection. Alternatively, many collapsed remnatns perhaps white dwarfs or even black holes of an early generation of massive stars constitute the hidden mass. A second hypothesis argues that the dark matter is nonbaryonic. it consists of one of the exotic particle species that wearlier hypothesized could exist in sufficientl quantity to yield a substantial fraction of dark matter in halos and in clusters amounts to only 10 percent of the critical density required to reverse the expansion of the universe, if we measure it as the ratio of hypothesized mass to be observed luminosity averaged over a suitably large region of psace.
Black holes would hav formed as a result of catastrophic stellar explosions, and the ensuing radiation shouldm in principle, be detecable. the current consensus is that if black holes account for the dark mass in clusters of galaxies they must have fmored sufficiently early in the universe for the cosmological redshift to have hidden the associated optical emission from our observations. at a redshift of, say 10, the protogalactic radiation produced when the massive stellar precursors of the black holes evolved and collapsed would now be visible only in ther infrared region of the spectrum. I the infrared, observations are extrmely difficult because of atmospheric emission (such as the terrestial airglow) and attenuation resulting from a sbosrtion by ozone, water vapor, and other molecules.
White dwarfs or neutron stars are a more conservative choice than black hole for that matter. They are the only dark matter candidates that we can unambiguosly state must exist, although whether enough actually exist is another matter. If they are to be numerous enough to account for dark matter, white dwarfs must hav been produced by a large number of stars of moderate mass, formed early in the evolution of the galaxy. We cannot exclude such a hypothesis, but we can seek ways to test it. for example, the white dwarfs would have cooled down, but they might still be dimly visible as reddish dwarfs. the ejecta produced when compact remnants, black holes, neutron stars, or white dwarfs formed would be chemically enriched and would show up in the composition of old stars. Studies suggest that remnants of very massive stars, either black holes or neutron stars, are implausible candidates for the dark matter unless the black holes are much more massive than ordinary stars, but white dwarfs are a possibility.
Stars of low mass also are a possible source of a small fraction of the dark mass. Stars of very low mass populating the halo of our galaxy would occasionally pass close enough to the sun to be recognizeable. Thet would appear as very faint nearby stars with appreciable proper motions and the high velocities characteristic of their halo origin. Becasue fewsuch objects are seen, the orbits of such stars must restrict them predominantly to the outer halo. Presumably, their orbits are most circular, the dynamical characteristics of these objects would make them distinct from the ordinary stars in our galaxy, which have appreciable velocities in the direction of the center, move in highly elongated orbits. Alternatively, these out halo stars could be "jupiters" essentially invisible giant planets that were not massive enough [less than 0.08 solar mass] to become stars.
if we possed an adequate theory of star formation, we sgould be able to choose between hypotheses of massive versus low mass star formation. Even if low mass stars predominate, there must also have been a considerable number of massive stars in the halo of a newly formed protogalaxy the Processed gases ejected during supernova explosions of the massive stars would accounts for the origin of the neriched intergalactic matter that is observed in rich clusters of galaxies. However, our knowledge of star formation is likely to remain so imprecise that direct observations will be required to determine the form of the dark mass if it is baryonic.
