Annu. Rev. Astron. Astrophys. 2005. 43: xxx-xxx Copyright © 2005 by Annual Reviews. All rights reserved

4.3. Towards a Scenario of Recent Bulge and Disk Formation in Intermediate-Mass Spirals

Because a significant part of recent star formation takes place in LIRGs, any overall picture of galaxy evolution requires a detailed panchromatic study. Optical/spectral properties of LIRGs are similar to those of other galaxies and only infrared measurements are able to describe how the star formation is distributed between the different galaxy types. Thus a complete study has to link the star formation revealed in the infrared to the morphological changes seen in the optical. This has been done by Hammer et al. (2005) using HST, ISO, VLA and VLT observations of the CFRS. A detailed comparison of the morphologies of distant (0.4 < z < 1.2) galaxies with the local galaxies shows the complete vanishing of the LCGs in the local Universe (by a factor ~ 10) and the decrease of mergers and irregulars (by a factor ~ 4). Almost all distant galaxies have much bluer central colors than local bulges, probably as a result of active star formation in the 1kpc central region of most distant spirals. This supports a relatively recent formation of bulges in many present-day spirals. This simultaneous changes in galaxy morphologies and central colors of distant galaxies together with the observed lower metallicities (Liang et al. 2004) and overall higher star-formation rates at high z are the ingredients for an updated scenario of bulge and disk formation in spirals. Hammer et al. (2005) propose three different phases of galaxy evolution: the mergers/interacting, the compact galaxy and finally the growth of disk phase. During the last 8 Gyrs, most luminous galaxies are expected to experience a major merger that suppresses the disk as matter is falling to the mass barycenter. This phase is associated with short (1 Gyr) and strong peaks of star formations. Most of galaxies in this phase are LIRGs. Then, the compact phase corresponds to a decrease over 0.6-2 Gyr of the enhanced star formation due to merging. A significant fraction of stars form in bulges and additional occurrence of gas infall may subsequently wrap around the bulge to form a new disk-like component. Finally, the star formation spreads over all the forming disk as fed by large amounts of gas infall. In this scenario, about half of the bulge stellar content was made earlier in their progenitors, before the last major phase of accretion. More than a third of the present-day stellar mass is formed at z <1. This scenario is in very good agreement with the hydrodynamical numerical simulations of Scannapieco & Tissera (2003) in which mergers, through secular evolution and fusions, transform galaxies along the Hubble sequence by driving a morphological loop that might also depend on the properties of the central potential wells, which are also affected by mergers. This very attractive scenario links in a simple way the distant and local galaxies; it will be confronted to the new panchromatic studies of Spitzer galaxies. Note that another possibility of buildup of dense central component in disk galaxies is internal secular evolution, as reviewed by Kormendy & Kennicutt (2004).