Early Evolution of Disk Galaxies: Formation of Bulges in Clumpy Young Galactic Disks

Abstract

A new idea is proposed for the origin of bulges in spiral galaxies. Numerical simulations of protogalactic collapse suggest strongly that galactic bulges have been assembled from massive clumps formed in galactic disks in their early evolutionary phase. These clumps result from the gravitational instability of the gas-rich disks of young galaxies. Owing to dynamical frictions, those massive clumps, individual masses of which can be as large as D109 are able to spiral toward the galactic center within a few M _ , Gyr. Inward transport of disk matter by this process leads to the formation of a galactic bulge. A simple analytical model has been constructed in which the clumpy evolution of a disk galaxy is controlled by two parameters : the timescale with which the primordial gas in the halo accretes onto the disk plane (i.e., the collapse timescale) and the initial mass fraction of the gas relative to the galaxy total mass. Under plausible assumptions for the variation of these parameters among spiral galaxies, the clumpy evolution model can explain an observed trend in which the bulge-to-disk ratio increases as the total mass or the internal density of the galaxy increases. This success suggests that the clumpy evolution of the galactic disk constitutes an important ingredient of disk galaxy evolution. Star formation in primeval disk galaxies takes place mostly in the clumps. The resulting knotty appearance of these systems may explain the peculiar morphology observed in a number of high-redshift galaxies.

Star formation sustained by gas accretion

“…2), it is expected to reach the disks in clumps often forming stars already (e.g., Dekel et al 2009;Ceverino et al 2010;Genel et al 2012b). Alternatively, the external gas streams may fuel the disks with metal-poor gas, so that gas mass builds up developing starbursts through internal gravitational instabilities (e.g., Noguchi 1999;Elmegreen et al 2008;Bournaud and Elmegreen 2009). In any case, the cold-flow accretion is bound to induce metal-poor starbursts.…”

The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time

“…A simplified physical picture of such objects was first described by Noguchi (1998Noguchi ( , 1999 and is nicely summarised by Genzel et al (2011). (For a more detailed theoretical treatment, see Dekel et al 2009a).…”

Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

“…This mass inflow is partly due to clump migration (Noguchi 1999;Elmegreen et al 2005;Bournaud et al 2007), and is partly a smooth inflow of disc gas outside the clumps (Gammie 2001;Bournaud et al 2011;Forbes et al 2012), generated by torques within the perturbed disc, that drive angular momentum outward. One should emphasize that the inflow in the disc is a robust feature of the instability, not limited to clump migration.…”