Manuel Behrendt

2015MNRAS.448.1007B Behrendt, M.; Burkert, A.; Schartmann, M. 

Structure formation in gas-rich galactic discs with finite thickness: 

from discs to rings


Revision of the axisymmetric instabilities for thick disks at Q < Qcrit under well-defined conditions:

- The Toomre length (dominant perturbation) is around two times larger for discs with finite thickness compared to the thin disc approximation.

- Toomre’s Q and the Toomre length are scale-hight independent for a disc in hydrostatic equilibrium.

- In a first phase, ring-like structures grow according to the local Toomre length for the thick disc approximation.

- In a second phase, rings collapse under their self-gravity and break-up into many clumps much smaller than the initial Toomre length and are dense enough to form stars (if sufficiently resolved).

Ring And Clump Fragmentation

Face-on view of the surface density

Clump Galaxy Simulation

Surface Density Seen Under Different

Observational Resolution

2016ApJ...819L...2B Behrendt, M.; Burkert, A.; Schartmann, M.

Clusters of Small Clumps Can Explain the Peculiar Properties of Giant Clumps in High-redshift Galaxies



Giant clumps are a characteristic feature of observed high-redshift disk galaxies. We propose that these kiloparsec-sized clumps have a complex substructure and are the result of many smaller clumps self-organizing themselves into clump clusters (CCs). This bottom-up scenario is in contrast to the common top-down view that these giant clumps form first and then sub-fragment. Using a high-resolution hydrodynamical simulation of an isolated, fragmented massive gas disk and mimicking the observations from Genzel et al. at z ~ 2, we find remarkable agreement in many details. The CCs appear as single entities of sizes R_HWHM ~0.9–1.4 kpc and masses ~(1.5–3) x 10^9 Msol, representative of high-z observations. They are organized in a ring around the center of the galaxy. The origin of the observed clumps' high intrinsic velocity dispersion 50–100 k /s  is fully explained by the internal irregular motions of their substructure in our simulation. No additional energy input, e.g., via stellar feedback, is necessary. Furthermore, in agreement with observations, we find a small velocity gradient  8–27 km/s/kpc along the CCs in the beam-smeared velocity residual maps, which corresponds to net prograde and retrograde rotation with respect to the rotation of the galactic disk. The CC scenario could have strong implications for the internal evolution, lifetimes, and the migration timescales of the observed giant clumps, bulge growth, and active galactic nucleus activity, stellar feedback, and the chemical enrichment history of galactic disks.

2018MNRAS.473..953S Schartmann, M.; Mould, J.; Wada, K.; Burkert, A.; Durré, M.; Behrendt, M.; Davies, R. I.; Burtscher, L. 

The life cycle of starbursting circumnuclear gas discs



High-resolution observations from the submm to the optical wavelength regime resolve the central few 100 pc region of nearby galaxies in great detail. They reveal a large diversity of features: thick gas and stellar discs, nuclear starbursts, inflows and outflows, central activity, jet interaction, etc. Concentrating on the role circumnuclear discs play in the life cycles of galactic nuclei, we employ 3D adaptive mesh refinement hydrodynamical simulations with the RAMSES code to self-consistently trace the evolution from a quasi-stable gas disc, undergoing gravitational (Toomre) instability, the formation of clumps and stars and the disc's subsequent, partial dispersal via stellar feedback. Our approach builds upon the observational finding that many nearby Seyfert galaxies have undergone intense nuclear starbursts in their recent past and in many nearby sources star formation is concentrated in a handful of clumps on a few 100 pc distant from the galactic centre. We show that such observations can be understood as the result of gravitational instabilities in dense circumnuclear discs. By comparing these simulations to available integral field unit observations of a sample of nearby galactic nuclei, we find consistent gas and stellar masses, kinematics, star formation and outflow properties. Important ingredients in the simulations are the self-consistent treatment of star formation and the dynamical evolution of the stellar distribution as well as the modelling of a delay time distribution for the supernova feedback. The knowledge of the resulting simulated density structure and kinematics on pc scale is vital for understanding inflow and feedback processes towards galactic scales.

Face-on view of the surface density