Deviations from the Schmidt-Kennicutt relations during early galaxy evolution

We utilize detailed time-varying models of the coupled evolution of stars and the HI, H_2, and CO-bright H_2 gas phases in galaxy-sized numerical simulations to explore the evolution of gas-rich and/or metal-poor systems, expected to be numerous in the Early Universe. The inclusion of the CO-bright H_2 gas phase, and the realistic rendering of star formation as an H_2-regulated process (and the new feedback processes that this entails) allows the most realistic tracking of strongly evolving galaxies, and much better comparison with observations. We find that while galaxies eventually settle into states conforming to Schmidt-Kennicutt (S-K) relations, significant and systematic deviations of their star formation rates (SFRs) from the latter occur, especially pronounced and prolonged for ... ...This indicates potentially serious limitations of (S-K)-type relations as reliable sub-grid elements of star formation physics in simulations of structure formation in the Early Universe. We anticipate that galaxies with marked deviations from the S-K relations will be found at high redshifts as unbiased inventories of total gas mass become possible with ALMA and the EVLA.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical Journa

Molecular gas, CO, and star formation in galaxies: emergent empirical relations, feedback, and the evolution of very gas-rich systems

We use time-varying models of the coupled evolution of the HI, H_2 gas phases and stars in galaxy-sized numerical simulations to: a) test for the emergence of the Kennicutt-Schmidt (K-S) and the H_2-pressure relation, b) explore a realistic H_2-regulated star formation recipe which brings forth a neglected and potentially significant SF-regulating factor, and c) go beyond typical galactic environments (for which these galactic empirical relations are deduced) to explore the early evolution of very gas-rich galaxies. In this work we model low mass galaxies (M_{\rm baryon} \le 10^9 \msun), while incorporating an independent treatment of CO formation and destruction, the most important tracer molecule of H2 in galaxies, along with that for the H2 gas itself. We find that both the K-S and the H_2-pressure empirical relations can robustly emerge in galaxies after a dynamic equilibrium sets in between the various ISM states, the stellar component and its feedback. (abridged)Comment: 32 pages, 9 figures, accepted for publication in Ap

N-body Integrators with Individual Time Steps from Hierarchical Splitting

We review the implementation of individual particle time-stepping for N-body dynamics. We present a class of integrators derived from second order Hamiltonian splitting. In contrast to the usual implementation of individual time-stepping, these integrators are momentum conserving and show excellent energy conservation in conjunction with a symmetrized time step criterion. We use an explicit but approximate formula for the time symmetrization that is compatible with the use of individual time steps. No iterative scheme is necessary. We implement these ideas in the HUAYNO (available online at www.amusecode.org) code and present tests of the integrators and show that the presented integration schemes shows good energy conservation, with little or no systematic drift, while conserving momentum and angular momentum to machine precision for long term integrations.Comment: 20 pages, 9 figures, accepted for publication in New Astronom

The Influence of Dense Gas Rings on the Dynamics of a Stellar Disk in the Galactic Center

The Galactic center hosts several hundred early-type stars, about 20% of which lie in the so-called clockwise disk, while the remaining 80% do not belong to any disks. The circumnuclear ring (CNR), a ring of molecular gas that orbits the supermassive black hole (SMBH) with a radius of similar to 1.5 pc, has been claimed to induce precession and Kozai-Lidov oscillations onto the orbits of stars in the innermost parsec. We investigate the perturbations exerted by a gas ring on a nearly Keplerian stellar disk orbiting an SMBH by means of combined direct N-body and smoothed particle hydrodynamics simulations. We simulate the formation of gas rings through the infall and disruption of a molecular gas cloud, adopting different inclinations between the infalling gas cloud and the stellar disk. We find that a CNR-like ring is not efficient in affecting the stellar disk on a timescale of 3 Myr. In contrast, a gas ring in the innermost 0.5 pc induces precession of the longitude of the ascending node Omega, which significantly affects the stellar disk inclination. Furthermore, the combined effect of two-body relaxation and Omega-precession drives the stellar disk dismembering, displacing the stars from the disk. The impact of precession on the star orbits is stronger when the stellar disk and the inner gas ring are nearly coplanar. We speculate that the warm gas in the inner cavity might have played a major role in the evolution of the clockwise disk

Preprint typeset using LATEX style emulateapj v. 6/22/04 HOW RAPIDLY DO SUPERMASSIVE BLACK HOLE “SEEDS ” GROW AT EARLY TIMES?

We investigate the physical conditions for the growth of intermediate mass seed black holes assumed to have formed from remnants of the first generation of massive stars. We follow the collapse of high-σ halos with Tvir> 10 4 K using cosmological, smooth-particle hydrodynamic (SPH) simulations in the standard ΛCDM model. During collapse of the parent halo the seed holes are incorporated through mergers into larger systems and accrete mass from the surrounding gas. We include a self-consistent treatment of star formation, black hole accretion and associated feedback processes. Even under optimistic assumptions for the seed black hole mass and for efficient merger rates, we find that seed holes in halos M ≤ 10 10 M ⊙ never reach the conditions for critical Eddington growth. Most of the black hole growth in this regime is determined by the initial mass and the merger rates. Critical accretion rates are reached, albeit only after a significant delay, at the time of collapse (z ∼ 7) for 3-4σ halos of M ∼ 10 11 M⊙. Our results imply MBH = 5 × 10 6 M⊙(Mhalo/10 11 M⊙) 0.78 at the time of collapse. The required conditions of Eddington growth to explain the build-up of supermassive black holes ( ∼ 10 9 M⊙), as implied by Sloan quasars at z> 6, are therefore hard to meet in such a scenario. Without a ’jump-start ’ these conditions may be only achieved in extremely rare halos with Mhalo> 10 13 that collapsed before z ∼ 6. The sub-Eddington regime in which black holes holes accrete at early time implies a small contribution to the reionization by miniquasar but still sufficient to cause appreciable heating of the IGM at z � 15 − 18