Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies

Abstract

Gas-rich disks in the early universe are highly turbulent and have giant starforming clumps. Models suggest the clumps form by gravitational instabilities, and if they resist disruption by star formation, then they interact, lose angular momentum, and migrate to the center to form a bulge. Here we study the properties of the bulges formed by this mechanism. They are all thick, slowly rotating, and have a high Sersic index, like classical bulges. Their rapid formation should also give them relatively high α−element abundances. We consider fourteen low-resolution models and four high-resolution models, three of which have supernova feedback. All models have an active halo, stellar disk, and gaseous disk, three of the models have a pre-existing bulge and three others have a cuspy dark matter halo. All show the same basic result except the one with the highest feedback, in which the clumps are quickly destroyed and the disk thickens too much. The coalescence of massive disk clumps in the center of a galaxy is like a major merger in terms of orbital mixing. It differs by leaving a bulge with no specific dark matter component, unlike the merger of individual galaxies. Normal supernova feedback has little effect because the high turbulent speed in the -2gas produces tightly bound clumps. A variety of indirect observations support the model, including clumpy disks with young bulges at high redshift and bulges with relatively little dark matter.

The CALIFA survey across the Hubble sequence

“…They conclude that the mass growth took place in a fairly uniform way, with the galaxies increasing their mass at all radii, thus, their R eff barely grows. These results seem to be supported by numerical simulation by Elmegreen et al (2008), which find that bulges can be formed by migration of unstable disks. Other observational evidence come from the detection of clumpy star-forming disks in galaxies at z ∼ 2 (Genzel et al 2008;Förster Schreiber et al 2011), which may indicate an early build up of bulges by secular evolution.…”

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.…”

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

“…The gravitational fragmentation of gas-rich and turbulent galactic discs into giant clumps has been addressed by simulations in the idealized context of an isolated galaxy (Noguchi 1999;Immeli et al 2004;Elmegreen et al 2005;Bournaud et al 2007;Elmegreen et al 2008;Bournaud & Elmegreen 2009), and in a cosmological context, using analytic theory (Dekel et al 2009, DSC) and cosmological simulations (Agertz et al 2009;Ceverino et al 2010;Genel et al 2012;Ceverino et al 2012;Mandelker et al 2014;Moody et al 2014). According to the standard Toomre instability analysis (Toomre 1964), a rotating disc becomes unstable to local gravitational collapse if its surface density is sufficiently high for its self-gravity to overcome the forces induced by rotation and velocity dispersion that resist the collapse.…”

“…Isolated-disc simulations have yielded results of mixed nature. Sticky-particles simulations (Elmegreen et al 2008) reported the formation of VDI-driven bulges with a classical, de-Vaucouleurs mass profile. In contrast, Inoue & Saitoh (2012) reported SPH simulations that produced pseudo-bulges with nearly exponential profiles.…”