Bars, Spiral Structure, and Secular Evolution in Disk Galaxies

Simulations and observations of galactic bars suggest they do not commonly evolve into bulges, although it is possible that the earliest bars formed bulges long ago, when galaxies were smaller, denser, and had more gas. The most highly evolved of today’s bars may become lenses over a Hubble time. Most galaxies in the early Universe are extremely clumpy, with ∼ 10 −10 M⊙ blue clumps that resemble in color and magnitude the isolated field objects nearby. The presence of blue and irregular bars at high redshift suggests that some bars formed primarily in the gas phase accompanied by giant starbursts, rather than in pure stellar disks like most models. Secular and non-secular processes that cause galaxies to evolve are summarized.