Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids
Abstract
We present a simple theoretical framework for massive galaxies at high redshift, where most of the assembly and star formation occurred. The evolution is governed by the interplay between fueling by smooth and clumpy streams and stabilization by a spheroid, leading to a bimodality in galaxy type by z~3. Disks of giant clumps and high star formation rate (SFR) form when the streams are smoother than average. The streams maintain a dense disk that undergoes gravitational fragmentation into several giant clumps, each a few percent of the disk mass. The disk may reach a disk-to-total mass ratio d~0.5 due to the dark-matter halo before it settles into a steady state with a slowly growing bulge, d=0.5-0.25. The clump interactions self-regulate an unstable disk with a dispersion-to-rotation ratio sigma_r/V=0.3-0.15. Encounters and dynamical friction induce inward clump migration in 10 dynamical times, ~0.5Gyr, while the disk expands in response. The inflowing smooth streams replenish the draining disk and prolong the clumpy phase to several Gyrs. The clumps form stars at the accretion rate, ~100 Msun/yr, each clump converting into stars in 6-10 dynamical times. The migrating clumps coalesce dissipatively into a compact bulge. Passive spheroid-dominated galaxies form when the incoming streams contain more than average dense clumps. These clumps stir up turbulence in the disk and grow a dominant bulge; together they stabilize the disk and suppress in-situ clump and star formation. The spheroid is surrounded by a smooth gas disk, as in simulated wet-merger remnants. This scenario explains the bimodality observed at z~2, of giant-clump star-forming extended disks alongside with compact spheroids of suppressed SFR. We present first maps from cosmological simulations that reveal clumpy disks consistent with our analysis.Keywords
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