Rapid Formation of Exponential Disks and Bulges at High Redshift from the Dynamical Evolution of Clump‐Cluster and Chain Galaxies

Abstract

Many galaxies at high redshift have peculiar morphologies dominated by 10 8 -10 9 M kpc-sized clumps. Using numerical simulations, we show that these ''clump clusters'' can result from fragmentation in gravitationally unstable primordial disks. They appear as ''chain galaxies'' when observed edge-on. In less than 1 Gyr, clump formation, migration, disruption, and interaction with the disk cause these systems to evolve from initially uniform disks into regular spiral galaxies with an exponential or double-exponential disk profile and a central bulge. The inner exponential is the initial disk size, and the outer exponential is from material flung out by spiral arms and clump torques. A nuclear black hole may form at the same time as the bulge from smaller black holes that grow inside the dense cores of each clump. The properties and lifetimes of the clumps in our models are consistent with observations of the clumps in high-redshift galaxies, and the stellar motions in our models are consistent with the observed velocity dispersions and lack of organized rotation in chain galaxies. We suggest that violently unstable disks are the first step in spiral galaxy formation. The associated starburst activity gives a short timescale for the initial stellar disk to form.

Radio-loudness of active galaxies and the black hole evolution

“…These may form from a cold gas streaming/dropping onto the galaxy from cosmological filaments. Such an inflow is predicted to take place in galaxies with the dark matter halo masses smaller than 10 12 M ⊙ (Dekel & Birnboim, 2006), and is supposed to protect the disk galaxies against destruction by the multiple minor mergers (Bournaud, Jog, & Combes, 2007). The modified spin paradigm described above is consistent with the recent finding that the radio-loudness correlates with stellar brightness profiles in the nuclear portions of active galaxies (Capetti & Balmaverde, 2006, 2007.…”

Galaxy Growth at Early Times from 3D Studies

“…Elevated velocity dispersions were also measured from high-resolution IRAM/Plateau de Bure interferometric observations of CO emission in z ∼ 1 − 2 star-forming disks, confirming that it is a property of the entire ISM and not just of the ionized gas layer (Tacconi et al 2013, e.g.,). These properties, along with trends of increasing central dynamical mass fraction and bulge-to-disk ratio with galaxy mass and evolutionary stage, and possibly older clump ages at smaller galactocentric distances (e.g., Genzel et al 2008Genzel et al , 2011Förster Schreiber et al 2011b;Wuyts et al 2012;Tacchella et al 2014), are consistent with theoretical arguments and numerical simulations of turbulent gas-rich disks in which giant star-forming clumps result from violent gravitational instabilities and bulges form via efficient secular processes on timescales < 1 Gyr (e.g., Bournaud et al 2007Bournaud et al , 2014Ceverino et al 2012;Dekel & Burkert 2014). Bulge growth in high-z disks can lead to "morphological quenching" as the central stellar spheroidal component stabilizes the gas-rich disk against fragmentation (e.g., Martig et al 2009).…”

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.…”