Galaxy formation and evolution

How the diverse array of galaxies that are now observed originated and evolved into their present form is a topic of intense speculation.Evidence from structural properties. Some clues can be discerned in certain structural properties of galaxies. The most plausible explanation for the smooth and round light distribution of an elliptical galaxy is that the stars formed out of a collapsing gas cloud.

The rapidly changing gravitational pull experienced by different stars as the collapse proceeds has been shown by computer simulations to rearrange the stars into the observed shape.
The highly flattened disks of spiral galaxies must have formed during a similar collapse, but it is believed that most star formation did not occur until the rotating gas cloud had already flattened into a pancakelike shape. Had the stars formed at an earlier stage of the collapse, their rapid motions would have led to the formation of an elliptical galaxy. However, if the cloud stays gaseous until it flattens, much of the kinetic energy of the collapse is radiated away by gas atoms. Subsequent star formation is found to maintain a highly flattened, disklike shape, characteristic of spiral galaxies.

The flattening occurs in part because of the centrifugal forces in the rotating cloud. A confirmation of this picture has come from the discovery that elliptical galaxies rotate much less rapidly than spiral galaxies. This raises the question of the origin of the rotation itself. A natural explanation seems to lie in the action of the gravitational torques exerted by neighboring protogalaxies.
Evidence from composition. Another aspect of galaxies that has evolutionary significance is their composition, and, in particular, the distribution of heavy elements. The amount of heavy-element enrichment can be inferred from the color of the starlight, blue stars being metal-poor. Galaxies are found to be significantly bluer in their outermost regions and redder toward their central nuclei. The explanation seems to be that galaxies formed out of collapsing gas clouds that formed stars in a piecemeal fashion. As stars formed, they evolved, underwent nuclear reactions, produced heavy elements, and eventually shed enriched material (some stars even exploding as supernovae). Successive generations of stars formed out of the debris of earlier stars, and in this way the stellar content of galaxies systematically became enriched. The greatest enrichment would naturally occur toward the center of a galaxy, where the gaseous stellar debris tended to collect.

Interactions. Observations reveal many systems of interacting galaxies and close pairs ofgalaxies, which are possible candidates for later interactions. As galaxies move about within clusters, they will occasionally pass very near one another or even collide directly. Possible outcomes include loss of material from the galaxies' outer regions, transfer of material from one galaxy to another, merger of the two galaxies, modification of the galaxies' forms by tidal perturbations, and loss of gas and dust due to collisional heating. Whether a merger occurs mainly depends on the relative velocity difference of the two galaxies. If they pass each other too fast, the gravitational drag between them will not be efficient enough to change their trajectory and the passage does not result in merging.

Observations show an increase of interactions in the last few 109 years. The random velocity of a galaxy inside an association of galaxies increases with the mass connected to that association. Once the association forms and the galaxies within it start moving under its gravitational influence, the relative velocities in encounters of galaxies will become too high for mergers to occur and the number of mergers decreases. Looking backin time, the increase ofinter-actions is stronger in clusters of galaxies than in environments with few galaxies. This is a consequence of the higher number density of galaxies, which increases the probability of having an encounter.
Galaxies undergoing mergers experience dramatic morphological changes. Due to tidal forces the merging galaxies start deforming and develop very prominent features, most notably the so-called tidal arms. Mergers can trigger periods of intense star formation. Gas which was available in the disks of the progenitor spiral galaxies will be driven to the centers of the remnant galaxies and start forming stars in a starburst. It has been proposed that ultraluminous infrared galaxies like the Antennae galaxies are just galaxies undergoing mergers in which an extensive starburst occurs. Theoretical considerations suggest that the outcome of the interaction between two galaxies of similar size will be an elliptical galaxy. This is one of the most favored formation mechanisms for elliptical galaxies.

Origin. An outstanding and unresolved issue concerns the origin of the primordial gas clouds out of which the galaxies evolved. The cosmic background radiation yields a glimpse of the universe prior to the epoch of galaxy formation. The universe is now completely transparent to this radiation. However, about 500,000 years after the big bang, the radiation was sufficiently hot that matter was ionized, and the matter was also sufficiently dense to render the universe completely opaque to the radiation. To observe the background radiation now (some 1010 years later) is to see back to this early epoch, known as the decoupling epoch: at earlier times, matter and radiation were intimately linked, and subsequently the radiation propagated freely until the present time.

Theory of formation from fluctuations. The cosmic background radiation is very uniform, but fluctuations were discovered in 1992 by the Cosmic Background Explorer (COBE) satellite. These are at a level of 1 part in 105, and are on angular scales of several degrees. The precursor fluctuations of the primordial inhomogeneities that gave rise to the observed structures in the universe would be on scales of degrees, for the largest superclusters and voids, to arc-minutes, for galaxy clusters. A definitive measurement is not yet available for these angular scales, but without such primordial fluctuations galaxies could not have formed. The mutual action of gravity exerted between these infinitesimal fluctuations results in their gradual enhancement. Eventually, great gas clouds develop that will collapse to form galaxies. The required amplitude for these primordial seed fluctuations must be of the order of 1 part in 105, precisely what is measured on larger scales, to within uncertainties of at most a factor of 2.

Numerical simulations of galaxy clustering have enabled the spectrum of fluctuation length scales and amplitudes to be inferred. Galaxies are not randomly distributed, as would be "white" noise; rather, they are correlated. Given a galaxy at an arbitrary position, at a distance away, there is an excess probability, above random, of finding another galaxy. These correlations are large on scales less than 5 mega-parsecs (1.0 x 1020 mi or 1.5 x 1020 km), and are measured out to 20 Mpc (4 x 1020mior6 x 1020km). The parent fluctuations that gave rise to the galaxies must be similarly correlated, although the amplitude of the effect was much less.
A theory of the very early universe, first proposed in 1980, provides an explanation of the distribution of amplitudes of the fluctuations with scale, but accounts only qualitatively for their strength. According to this theory, the initial stages of the big bang were characterized by a period of rapid inflation during the first 10-35 sof the expansion. One consequence of an inflationary epoch is that quantum-statistical fluctuations are amplified up to scales of galaxies and of clusters of galaxies.

The predicted distribution of fluctuations is initially the same, on all scales, from that of galaxy clusters to the observable universe. On the largest scales, where there has been little time to develop deviation from the initial conditions, the distribution ofcosmic microwave background fluctuations measured by COBE in 1992 and in many subsequent experiments, especially the Wilkinson Microwave AnisotropyProbe (WMAP),is approximately consistent with that predicted by the simplest inflationary cosmological model. Thus, the most simple of cosmologies may contain the nascent seeds of future galaxies.
Isothermal and adiabatic fluctuations. The possible fluctuations in the early universe can be categorized into distinct varieties. Of particular importance for galaxy formation are density fluctuations that are found to generally be a combination of two basic types: adia-batic and isothermal.

Primordial adiabatic fluctuations are analogous to a compression of both matter and radiation. They are generic to almost all models of the early universe. In the absence of weakly interacting dark matter, the diffusive tendency of the radiation tends to smooth out the smaller adiabatic fluctuations. This process remains effective until the decoupling epoch, and galaxy formation occurs only relatively recently. However, the dominant presence of dark matter that does not interact with the radiation other than by gravity allows fluctuations to survive on all scales in the weakly interacting dark matter. The theory of fluctuation origin does not specify the strength of the fluctuations. However, the observations of the cosmic microwave background demonstrate that some 300,000 years after the big bang when the radiation was last scattered by the matter, the amplitude of the density fluctuations amounted to only a few parts in 104 on galaxy cluster scales. This means that massive galaxies and galaxy clusters formed relatively recently, although small galaxies could have formed when the universe was just a tenth of its present size. Galaxies form when the gaseous matter cools and condenses in the gravity field of the dark matter, forming gas clouds that subsequently fragment into stars.

In some variations of the standard model for structure formation, primordial isothermal fluctuations were also present in the very early universe. These consist of variations in the matter density, without any corresponding enhancement in the radiation density. Consequently, in the radiation-dominated early phase of the big bang, isothermal fluctuations neither grow nor decay, as the uniform radiation field prevents any motion. Once the universe becomes transparent, the matter fluctuations respond freely to gravity and grow if they are above a certain critical size. The smallest isothermal fluctuations that can become enhanced and form gas clouds contain about 106 solar masses. Galaxy formation occurs very early in this case.
Role of dark matter. The presence of weakly interacting dark matter is almost universally accepted by astronomers in order to account for the rotation curves of galaxies. The baryonic component of matter in the universe is known from calculations of the abundances of the light elements to amount to about 3% of the critical density for closing the universe. There is at least 10 times as much dark matter, which constitutes at least 90% of the mass of the universe. Consequently, dark matter dominates the growth of the primordial density fluctuations. The gravitational influence of this dark matter greatly aids this growth.

Baryon fluctuation growth is suppressed by interactions with the radiation prior to the epoch of decoupling of matter and radiation, whereas weakly interacting particles are able to cluster freely as long as the dominant form of density is ordinary matter rather than radiation. Since the density in the very early universe was dominated by radiation, fluctuation growth in the presence of dark matter is enhanced by about a factor of 10, equivalent to the expansion factor between the epochs of ordinary matter dominance when fluctuation growth first commences and the last scattering of the radiation. The associated fluctuations in the cosmic microwave background required in order to form structures by a given epoch are reduced by a corresponding factor. The detection of cosmic microwave background temperature fluctuations at a level of about one part in 105, initially by the COBE satellite and subsequently by more than 20 experiments, means that the precursor fluctuations of the largest structures, such as galaxy clusters, have been identified, in a statistical sense, in the sky. Dark matter plays an essential role in reconciling the level of the observed fluctuations with the limited growth period available since the universe was first matter-dominated, approximately 10,000 years after the big bang. This matter consists of massive weakly interacting particles whose existence is predicted by the theory of supersymmetry .

The implications of particle dark matter for structure formation are considerable. If the particles are massive, they are slowly moving at the onset of fluctuation growth, when the universe is first matter-dominated. Such particles are called cold dark matter. As structure develops, cold dark matter clusters, first on the smallest scales, then on progressively larger scales. This leads to a bottom-up scenario of hierarchical clustering. Unique predictions are made for both the microwave background fluctuations and the density fluctuations that are measured in large-scale structure studies.
Reconciliation of large-scale structure and galaxy formation with cold dark matter has proven remarkably successful on the largest scales. Most data point to a universe in which the density of cold dark matter is about 30% of the critical value. The cosmic microwave background temperature fluctuations demonstrate that the universe is at critical temperature to account for the locations of the observed angular peaks in the fluctuation distribution observed on the microwave sky. In a universe that has a near-Euclidean geometry, the positions of these peaks are displaced because of the bending of the light rays that have traversed the universe since the epoch when the microwave background photons were last scattered by the matter. A Euclidean geometry requires that the universe must be at critical density, if most of the energy density is in the form of the vacuum energy that is associated with the cosmological constant term introduced by Albert Einstein. One prediction of such a cosmological model is that the expansion of the universe is currently accelerating as a consequence of the nature of the additional energy. Data from use of distant supernovae to measure the deceleration of the universe suggest that the universe is indeed accelerating. Large-scale structure still provides asevere constraint on the nature of the dark matter.

In a universe with a critical density of dark matter, excessively strong clustering of galaxies occurs. Dark energy, which is uniform and smooth, does not participate in gravitational clustering. Three independent observational results contribute to make a strong case for a standard model of the modern universe. These are the cosmic microwave background temperature fluctuation peaks, the acceleration of the universe as inferred from the distances to remote supernovae, and the large-scale structure of the galaxy distribution. The standard model of the universe consists of 30% dark matter, 65% dark energy, and 5% baryons. Variations in the standard model introduce a component of hot dark matter. This matter consists ofneutrinos that are assumed to have a small mass, sufficient to account for about one-quarter of the critical density. However, the resulting mixture of hot and cold dark matter gives poor agreement with the astrophysical data on fluctuations at all scales. According to a much less accepted alternative viewpoint, the dark matter is entirely baryonic. It consists of very low mass stars or of burnt-out stars such as white dwarfs. In this case, the matter density of the universe is only about one-tenth of the critical density, with the rest of the critical density being made up of dark energy. One then finds that adiabatic fluctuations in a baryon-dominated universe, supplemented by a subdominant admixture of hot dark matter, can result in temperature fluctuations that agree with the observational constraints.

A further possible advantage of this interpretation is that by reducing the Hubble constant to about two-thirds of the currently preferred value, it is also possible to dispense with dark energy if one ignores the evidence for acceleration of the universe inferred from distant supernovae. The universe would then contain a critical density of baryons, along with some hot dark matter. Another scenario appeals to warm dark matter, for which a massive neutrino is the expected candidate. However, this option requires an admixture of isothermal fluctuations in order to allow early structure formation. The isothermal fluctuations are consistent with the cosmic microwave background data, provided they are subdominant. In this case, a complex baryon genesis scenario is required, in which matter is created with spatial inhomogeneities in the number of baryons relative to the number of photons. By far the simplest model is one in which the observed structure is seeded by primordial adia-batic density fluctuations generated during inflation.
Reference : McGraw - Hill Encyclopedia of Science and Technology


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