Xiaolei Zhang
George Mason University, USA
Title: Secular evolution of galaxies and the formation of Hubble sequence
Biography
Biography: Xiaolei Zhang
Abstract
Nearly a century ago, Edwin Hubble was among the first (including also Sir James Jeans) to notice a systematic trend of variation of galaxy properties along a morphological sequence, from highly flattened disky shapes to gradually more bulgy shapes, and onward to elliptical galaxies of varying ellipticity. It took the accumulation of observational evidence and the advance of theoretical understanding of the subsequent decades for us to learn that this morphological sequence, classified by Hubble in the eponymous Atlas edited by Sandage, was likely an evolutionary sequence, i.e., the so-called early-Hubble-types (bulgy or elliptical ones) are the results of the gradual central concentration of matter from the so-called late-Hubble-types (disky ones). Galaxies, thus evolve throughout the cosmic time along the (reverse) Hubble sequence from the late to the early Hubble types. In the past 25 years, the we have gradually established a dynamical framework, built on the foundation of the density wave theory of galaxies, which demonstrated how this so-called secular morphological evolution of galaxies along the Hubble sequence can be accomplished through the nonlinear and collective interaction of the galaxy-disk matter with the density-wave modes (these modes are the intrinsic global instabilities on the parent galaxy disks, and they give galaxies the striking appearance of grand-design spiral arms and bars). The analytical predictions have received extensive confirmation from observations and from N-body simulations. The result of this work has important implications on the cosmological evolution of galaxies.