High-redshift clumpy discs and bulges in cosmological simulations

Abstract

We analyze the first cosmological simulations that recover the fragmentation of high-redshift galactic discs driven by cold streams. The fragmentation is recovered owing to an AMR resolution better than 70 pc with cooling below 10^4 K. We study three typical star-forming galaxies in haloes of approx. 5 10^11 Msun at z=2.3, when they were not undergoing a major merger. The steady gas supply by cold streams leads to gravitationally unstable, turbulent discs, which fragment into giant clumps and transient features on a dynamical timescale. The disc clumps are not associated with dark-matter haloes. The clumpy discs are self-regulated by gravity in a marginaly unstable state. Clump migration and angular-momentum transfer on an orbital timescale help the growth of a central bulge with a mass comparable to the disc. The continuous gas input keeps the system of clumpy disc and bulge in a near "steady state", for several Gyr. The average star-formation rate, much of which occurs in the clumps, follows the gas accretion rate of approx. 45 Msun/yr. The simulated galaxies resemble in many ways the observed star-forming galaxies at high redshift. Their properties are consistent with the simple theoretical framework presented in Dekel, Sari & Ceverino (2009). In particular, a two-component analysis reveals that the simulated discs are indeed marginally unstable, and the time evolution confirms the robustness of the clumpy configuration in a cosmological steady state. By z=1 the simulated systems are stabilized by a dominant stellar spheroid, demonstrating the process of "morphological quenching" of star formation (Martig et al. 2009) . We demonstrate that the disc fragmentation is not a numerical artifact once the Jeans length is kept larger than 7 resolution elements, i.e. beyond the standard Truelove criterion.Comment: 20 pages, 12 figures, accepted in MNRAS

Star formation sustained by gas accretion

“…If the gas is accreted through the cold-flow mode (Sect. 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).…”

“…The distances are normalized to the virial radius R vir , and the density ρ to the mean density of the universe ρ . Taken from van de Voort and smaller, and so they may identifiable through inhomogeneities produced in the disks (e.g., Ceverino et al 2010). …”

Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge

“…Being continuously fed by cold streams, the gas-rich disc tends to develop a violent disc instability (VDI), where the disc is turbulent and highly perturbed, with large transient features and in-situ giant clumps (Noguchi 1999;Immeli et al 2004;Genzel et al 2006;Bournaud et al 2007;Genzel et al 2008;Agertz et al 2009;Ceverino et al 2010Ceverino et al , 2012Mandelker et al 2014). Torques within the disc drive gas inflow and inward clump migration, which compactify the disc to drive central star formation in the form of a "blue nugget" (Barro et al 2013(Barro et al , 2014Dekel & Burkert 2014;Zolotov et al 2014), grow a massive rotating stellar bulge (Genzel et al 2006(Genzel et al , 2008Bournaud et al 2011;Ceverino et al 2010, and feed the central black hole (Bournaud et al 2011).…”

Galaxy formation hydrodynamics: From cosmic flows to star-forming clouds

“…Rapid internal evolution with bulge formation by giant clump instabilities in highredshift disks has now been reproduced with various codes and in cosmological simulations (Agertz et al 2009a;Ceverino et al 2010). A major unknown is the survival of the giant clumps to star formation feedback, in particular radiative pressure from young stars (Murray et al 2010).…”

“…Things are a bit easier at redshift z > 1 with giant clumps of star formation. They have been directly captured in cosmological simulations (Agertz et al 2009a;Ceverino et al 2010). They have not been "resolved", though: these giant clumps survive feedback because they keep a relatively low density and hence a relatively low star formation efficiency.…”