Secular Evolution of Spiral Galaxies. III. The Hubble Sequence as a Temporal Evolution Sequence

Abstract

In the first two papers in this series, we have presented a dynamical mechanism by which an unstable spiral mode in a galactic disk can remain quasi-stationary on the order of a Hubble time at the expense of a continuous dissipative basic state evolution, manifesting as the secular redistribution of disk matter, as well as the secular heating of disk stars. In the current paper, we consider in detail the astrophysical consequences of this spiral-induced basic state evolution process and show that it could explain, among a spectrum of observational facts, the variation of the disk surface-density profiles across the Hubble types, as well as the well-known age-velocity dispersion relation of the solar-neighborhood stars. We seek to establish that the Hubble sequence for the classification of galaxies, when viewed in the reverse direction, is essentially a temporal evolution sequence. The spiral-induced secular mass redistribution process could lead to the evolution of the Hubble type of a spiral galaxy from late (Sd and Sc) to early (Sb and Sa). Furthermore, since similar radial mass-accretion processes could also be mediated by central stellar bars, or other central nonaxisymmetric structures, this leads to the continued galaxy evolution across Hubble type S0 and into disky elliptical galaxies. The secular evolution mechanism presented in this series of papers, together with other previously proposed evolution mechanisms, provides a framework for understanding the results of Hubble Space Telescope Deep Field and Medium Deep Survey, which showed that there is significant evolution of the number statistics of the Hubble types of galaxies with redshift, consistent with an evolution along the Hubble sequence from late to early. It also provides possible solutions to several long-standing problems in the theory and observation of galaxy formation and evolution, among them the cause of the steepening of the galaxy morphology-environment relation; the similarity between the Tully-Fisher and Faber-Jackson relations for spiral and elliptical galaxies, respectively; the existence of a fundamental-plane relation for elliptical galaxies; and the mass-angular momentum anticorrelation observed along the Hubble sequence.