THE ORIGIN OF THE HUBBLE SEQUENCE

A brief review is given of the aspects of galaxy formation and evolution that are relevant to understanding the origin of the Hubble sequence. The variation along the Hubble sequence of the gas content and related properties of galaxies is the result of a variation in the rate of galactic evolution that is probably controlled mainly by the underlying mass distribution, which is thus the most fundamental characteristic to be explained by theories of galaxy formation. Much recent theoretical work and observational evidence, including the fossil record for our Galaxy, support the view that most large galaxies are formed by the merging or accretion of smaller units. The structure of the resulting galaxy depends on the nature of the subunits: if they are mainly stellar, the result will be a bulge-dominated galaxy, while if they are mostly gaseous, the result may be a galaxy with a dominant disk. Star formation processes and mergers are both expected to proceed most rapidly in the densest parts of the universe, resulting in the formation of mostly bulge-dominated galaxies in these regions. This may account for the observed relation between morphology and environment, although continuing environmental effects including gas removal and the destruction of disks by interactions may also play an important role. 1. The Systematic Properties of Galaxies Like all natural phenomena, galaxies exhibit a mixture of regularity and irregularity, or of order and chaos, in their properties. Hubble looked for the order among their properties, and noted several correlations that he adopted as the basis of his proposed classification scheme. As has been emphasized by Buta, Madore, and Gavazzi in this volume, galaxies may have a large number of measurable characteristics, but only a few parameters suffice to account for most of the variability in their properties. Principal component analysis shows that there are just two dominant parameters, and that they can be identified as ‘scale’ and ‘form’, or equivalently as luminosity and Hubble type (Whitmore 1984). This finding supports the utility of Hubble types as a predictor of other properties of galaxies, and also suggests that the Hubble classification scheme reflects important underlying regularities in the processes of galaxy formation and evolution. Although the Hubble types and luminosities of galaxies are almost completely uncorrelated in the sample studied by Whitmore (1984), some correlation is found when a larger range of types is considered, the later-type galaxies generally being fainter (de Vaucouleurs 1977). Some authors have even suggested that the basic properties of galaxies are largely determined by a single parameter, which may be the total mass (Tully, Mould, and Aaronson 1982), the virial velocity (Lake and Carlberg 1988b), or just the bulge mass (Meisels and Ostriker 1984). The classification sequence proposed by Hubble (1936) for spiral galaxies is based on three characteristics, all of which generally increase along the sequence: the diskto-bulge ratio, the openness of the spiral pattern, and the resolution of the arms into bright knots. The resolution of the arms reflects the level of star formation activity, and this depends on the gas content, which also increases systematically along the sequence. As has been discussed by Eder in this volume, the S0 class introduced by Hubble as a transition between spirals and ellipticals can, with present data, be understood as forming a smooth extension of the spiral sequence to systems with weak or absent spiral structure and with little or no gas or present star formation. Weak disk components even appear to be present in some elliptical galaxies, which may thus constitute a further continuous extension of the sequence to systems that are almost pure bulge (Bender 1990). The Hubble classification scheme does not, however, apply to dwarf galaxies, which differ fundamentally in several respects from the giant galaxies studied by Hubble; some of their properties have been reviewed by Ferguson in this volume. The dwarf galaxies do not exhibit distinct disk and bulge components, but they nevertheless vary greatly in their content of gas and young stars, and range from gas-poor dwarf ellipticals to gas-rich dwarf irregulars. The dwarf ellipticals and dwarf irregulars may in fact be closely related, and may form a sequence of objects that differ mainly in gas content (Kormendy and Djorgovski 1989; Da Costa 1992). It has been recognized since Hubble’s time that the morphological properties of galaxies are correlated with their environment: spiral galaxies mainly inhabit lowdensity field regions, while ellipticals are concentrated in dense clusters. Several studies have shown that the mix of morphological types shifts steadily toward earlier types with increasing density of the environment (Oemler 1974; Dressler 1980; Postman and Geller 1984; Haynes 1987). A related finding is that the fraction of blue star-forming galaxies in clusters decreases with increasing cluster size (Oemler 1992). The ‘morphology-density relation’ clearly contains important clues about the formation and evolution of galaxies, but its interpretation has continued to be a subject of much discussion (e.g., Dressler 1984; Salvador-Solé, Sanromà, and Jordana 1989; Whitmore and Gilmore 1991; Oemler 1992; see also the contributions of Salvador-Solé and Whitmore to this volume). The simplest and most obvious interpretation would be that the formation of disks is favored in low-density environments, while the formation of spheroids is favored in denser regions. However, environmental effects can also influence galactic morphology; for example, the galaxies in clusters may be gas-poor and typically of earlier Hubble types than field galaxies because of gas removal processes or because of interactions with other galaxies that have triggered enhanced star formation (Kenney 1990; Oemler 1992). The sweeping away of gas envelopes in dense environments might have a similar effect if gas infall is important for galactic evolution (Larson, Tinsley, and Caldwell 1980). It is even possible, as has been suggested by Whitmore in this volume, that the destruction of disks by violent interactions in forming clusters could account for the morphologydensity relation by destroying most of the late-type galaxies in dense environments; in this case, the morphology-density relation might actually be telling us more about galaxy destruction than about galaxy formation. The challenge is thus to account for the regularities embodied in the Hubble sequence and the morphology-density relation on the basis of an understanding of the processes of galaxy formation and evolution, and perhaps destruction. It will be argued in Section 2 that the variation of some of the properties of galaxies along the Hubble sequence results from a variation in the rate of galactic evolution that mainly depends on the underlying mass distribution. The most basic property of galaxies to be accounted for by theories of galaxy formation is then the internal mass distribution, as reflected for example in the disk-to-bulge ratio. Subsequent sections will discuss some current ideas about galaxy formation that are relevant to understanding the origin of the Hubble sequence, as well as some of the supporting evidence. 2. Galactic Evolution and the Hubble Sequence Of the properties of galaxies that vary along the Hubble sequence, the gas content and related properties such as the star formation rate and the spiral arm structure may be regarded as secondary characteristics that depend only on the rate at which galaxies deplete their initial gas supply. The early-type galaxies have evidently evolved faster than the late-type galaxies in this respect, having consumed more of their gas because of higher past rates of star formation. Direct evidence that the timescale for gas depletion increases along the Hubble sequence is provided by the results of Donas et al. (1987) for the star formation rates and gas depletion times in a large sample of spiral and irregular galaxies, which show that at least along the later part of the sequence, the gas depletion time increases by roughly a factor of 4 from the Sbc galaxies to the irregular galaxies. This dependence of the timescale for galactic evolution on Hubble type is an important feature of the Hubble sequence that must be accounted for by any understanding of star formation processes and star formation rates in galaxies (Larson 1983). Here we summarize briefly why the timescale for gas depletion might be expected to depend on basic structural properties such as the disk-to-bulge ratio and the underlying mass distribution. On the largest scales, star formation begins with the formation of giant molecular clouds and cloud complexes, and the star formation rate therefore depends on the rate of formation of these massive clouds and complexes. The formation of such large gas concentrations is almost certainly due primarily to the self-gravity of the interstellar medium in galaxies, and in this case the rates of cloud formation and star formation are controlled mainly by the timescale for gravitational accumulation of the gas (Larson 1987, 1988, 1992; Elmegreen 1990a,b). The gas layer in a galaxy is gravitationally unstable if the Toomre stability parameter Q = cκ/πGμ is smaller than unity, where c is the velocity dispersion of the gas, κ is the epicyclic frequency, and μ is the surface density; such gravitational instabilities may occur, for example, in irregular galaxies, and may be responsible for driving star formation in them. In spiral galaxies like our own, a true gravitational instability probably cannot occur because Q is actually somewhat larger than unity, but large density enhancements can still be generated by the closely related phenomenon of ‘swing amplification’, which is a kind of truncated instability (Toomre 1981, 1990). Evidence that swing amplification is responsi