The Intrinsic Flattening of e, so, and Spiral Galaxies as Related to Galaxy Formation and Evolution

The distribution of apparent axial ratios for 168 E, 267 50 + SBO, and 254 ordinary spiral galaxies confirms previous conclusions that ellipticals have only moderate intrinsic flattenings, q, which range from q = 1 to q 0.3. Ordinary spirals and SO's are intrinsically flatter, possessing thin disks of (q) 0.25with only small dispersion. Because of this difference, true spirals and SO systems cannot have evolved either from, or into, E systems. Evolutionary questions concern (a) why SO Im galaxies have flattened to a disk, while E galaxies have not; (b) why the spheroidal components of all galaxies contain only old stars, probably formed at a single epoch; and (c) why SO and early Sn galaxies have lost their young spiral-arm population, while Sb H Im galaxies have not. Analysis of the four properties, (1) the disk blue4ight surface brightness at r = 0, (2) the absolute length scale of the disk, (3) the size ratio of the spheroidal component to the total radius of the galaxy and (4) the mean total mass density (p), shows that galaxy type does not depend uniquely on systematic variations of these parameters along the Hubble sequence. The only strongly systematic variable is (P)H,. Observations suggest that the spheroidal subsystem is relaxed. If relaxation occurs by Lynden-Bell's process of a rapidly changing (rR free-fall time) gravitational potential in the collapsing protogalaxy, then stars in the spheroidal component must be the same age to within the collapse time (several times 108 years). Because the mean angular momentum per unit mass, I', of the spheroidal component is low, we suggest that the relative size of this component is determined by the relative amount of low-h matter in the protogalaxy, and that it is this low-h matter which fragments into stars during the rapid collapse phase; the high-h matter subsequently settles in gaseous form to the disk. The fundamental distinction between B and spiral systems probably lies in their primeval mass- angular momentum distribution. The morphological type of a spiral or 50 system appears to be essentially defined at the time of formation of the old disk stars.