Jan Palous

Hydrodynamics of the Matter Reinserted within Super Stellar Clusters

We present semianalytic and numerical models that take into account the effect of radiative cooling on the hydrodynamics of the matter reinserted by strong stellar winds and supernovae within the volume occupied by young, massive, and compact super stellar clusters. We first corroborate the location of the threshold line in the plane of mechanical energy input rate versus cluster size found by Silich et al. This line separates clusters that are able to drive a quasi-adiabatic or strongly radiative wind from those in which catastrophic cooling occurs within the cluster volume. Then we show that clusters above the threshold line exhibit a bimodal behavior in which the central, densest zones cool rapidly and accumulate the injected matter to eventually feed further generations of star formation, while the outer zones are still able to drive a stationary wind. The results are presented as a series of universal dimensionless diagrams, from which one can infer the sizes of the two zones, the fraction of the deposited mass that goes into each, and the mechanical luminosity of the resultant winds for clusters of all sizes and energy input rates, regardless the assumed adiabatic terminal speed (VA,∞).

Continuous stellar mass-loss in N -body models of galaxies

We present an N-body computer code { aimed at studies of galactic dynamics { with a CPU-ecient algorithm for a continuous (i.e. time-dependent) stellar mass-loss. First, we summarize available data on stellar mass-loss and derive the long-term (20 Gyr) dependence of mass-loss rate of a coeval stellar population. We then implement it through a simple parametric form into a particle-mesh code with stellar and gaseous particles. We perform several tests of the algorithm reliability and show an illustrative application: a 2D simulation of a disk galaxy, starting as purely stellar but evolving as two-component due to gradual mass-loss from initial stars and to star formation. In a subsequent paper we will use the code to study changes that are induced in galactic disks by the continuous gas recycling compared to the instantaneous recycling approximation, especially the changes in star formation rate and radial inflow of matter.

HI shells in the outer Milky Way

We present results of a method for an automatic search for Hi shells in 3D data cubes and apply it to the Leiden-Dwingeloo Hi survey of the northern Milky Way. In the 2nd Galactic quadrant, where identifications of structures are not substantially influenced by overlapping, we find nearly 300 structures. The Galactic distribution of shells has an exponential profile in the radial direction with a scale length of σ gsh = 3 kpc. In the z direction, one half of the shells are found at distances smaller than 500 pc. We also calculate the energies necessary to create the shells: there are several structures with energies greater than 10E SN but only one with an energy exceeding 100E SN . Their size distribution, corrected for distance effects, is approximated by a power-law with an index α = 2.1. Our identifications provide a lower limit to the filling factor of shells in the outer Milky Way: f 2D = 0.4 and f 3D = 0.05.

Two-dimensional Hydrodynamic Models of Super Star Clusters with a Positive Star Formation Feedback

Using the hydrodynamic code ZEUS, we perform 2D simulations to determine the fate of the gas ejected by massive stars within super star clusters. It turns out that the outcome depends mainly on the mass and radius of the cluster. In the case of less massive clusters, a hot high-velocity (~1000 km s−1) stationary wind develops and the metals injected by supernovae are dispersed to large distances from the cluster. On the other hand, the density of the thermalized ejecta within massive and compact clusters is sufficiently large as to immediately provoke the onset of thermal instabilities. These deplete, particularly in the central densest regions, the pressure and the pressure gradient required to establish a stationary wind, and instead the thermally unstable parcels of gas are rapidly compressed, by a plethora of repressurizing shocks, into compact high-density condensations. Most of these are unable to leave the cluster volume and thus accumulate to eventually feed further generations of star formation. The simulations cover an important fraction of the parameter space, which allows us to estimate the fraction of the reinserted gas that accumulates within the cluster and the fraction that leaves the cluster as a function of the cluster mechanical luminosity, the cluster size, and heating efficiency.

Gas stripping in galaxy clusters: A new SPH simulation approach

Aims. The influence of a time-varying ram pressure on spiral galaxies in clusters is explored with a new simulation method based on the N-body SPH/tree code GADGET. Methods. We have adapted the code to describe the interaction of two different gas phases, the diffuse hot intracluster medium (ICM) and the denser and colder interstellar medium (ISM). Both the ICM and ISM components are introduced as SPH particles. As a galaxy arrives on a highly radial orbit from outskirts to cluster center, it crosses the ICM density peak and experiences a time-varying wind. Results. Depending on the duration and intensity of the ISM-ICM interaction, early and late type galaxies in galaxy clusters with either a large or small ICM distribution are found to show different stripping efficiencies, amounts of reaccretion of the extra-planar ISM, and final masses. We compare the numerical results with analytical approximations of different complexity and indicate the limits of the Gunn & Gott simple stripping formula. Conclusions. Our investigations emphasize the role of the galactic orbital history to the stripping amount. We discuss the contribution of ram pressure stripping to the origin of the ICM and its metallicity. We propose gas accumulations like tails, filaments, or ripples to be responsible for stripping in regions with low overall ICM occurrence.

The fragmentation of expanding shells – I. Limitations of the thin-shell approximation

We investigate the gravitational fragmentation of expanding shells in the context of the linear thin-shell analysis. We make use of two very different numerical schemes; the flash adaptive mesh refinement code and a version of the Benz smoothed particle hydrodynamics code. We find that the agreement between the two codes is excellent. We use our numerical results to test the thin-shell approximation and we find that the external pressure applied to the shell has a strong effect on the fragmentation process. In cases where shells are not pressure-confined, the shells thicken as they expand and hydrodynamic flows perpendicular to the plane of the shell suppress fragmentation at short wavelengths. If the shells are pressure-confined internally and externally, so that their thickness remains approximately constant during their expansion, the agreement with the analytical solution is better.

Environmental dependences for star formation triggered by expanding shell collapse

Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy discs were determined using three-dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of seven independent variables, and the resultant probabilities for triggering and unstable masses were determined as functions of eight dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disc scaleheight and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, QA, is large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of c 5/( GL ), for rms velocity dispersion csh in the swept-up matter and shell-driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ∼5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time-scale is ∼4(csh/2πGρ[ GL ] 0.2 ) 0.5 for ambient midplane density ρ, (2) the total fragment mass is ∼2 × 10 7 M� , of which only a small fraction is likely to be molecular, (3) the triggering radius is ∼2 times the scaleheight, and the triggering probability is ∼0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas-rich galaxies, such as young galaxies or those with late Hubble types.

EVOLUTION OF SUPER STAR CLUSTER WINDS WITH STRONG COOLING

We study the evolution of super star cluster winds driven by stellar winds and supernova explosions. Time-dependent rates at which mass and energy are deposited into the cluster volume, as well as the time-dependent chemical composition of the re-inserted gas, are obtained from the population synthesis code Starburst99. These results are used as input for a semi-analytic code which determines the hydrodynamic properties of the cluster wind as a function of cluster age. Two types of winds are detected in the calculations. For the quasi-adiabatic solution, all of the inserted gas leaves the cluster in the form of a stationary wind. For the bimodal solution, some of the inserted gas becomes thermally unstable and forms dense warm clumps which accumulate inside the cluster. We calculate the evolution of the wind velocity and energy flux and integrate the amount of accumulated mass for clusters of different mass, radius, and initial metallicity. We also consider conditions with low heating efficiency of the re-inserted gas or mass loading of the hot thermalized plasma with the gas left over from star formation. We find that the bimodal regime and the related mass accumulation occur if at least one of the two conditions above is fulfilled.

Ram pressure stripping of tilted galaxies

Context. Ram pressure stripping of galaxies in clusters can yield gas deficient disks. Previous numerical simulations based on various approaches suggested that, except for near edge-on disk orientations, the amount of stripping depends very little on the inclination angle. Aims. Following our previous numerical and analytical study of face-on stripping, we extend the set of parameters with the disk tilt angle and explore in detail the effects of the ram pressure on the interstellar content (ISM) of tilted galaxies that orbit in various environments of clusters, with compact or extended distributions of the intra-cluster medium (ICM). We further study how results of numerical simulations could be estimated analytically. To isolate the effect of inclination, galaxies on strictly radial orbits are considered. Methods. A grid of numerical simulations with varying parameters is produced using the tree/SPH code GADGET with a modified method for calculating the ISM-ICM interaction. These SPH calculations extend the set of existing results obtained from different codes using various numerical techniques. Results. The simulations confirm the general trend of less stripping at orientations close to edge-on. The dependence on the disk tilt angle is more pronounced for compact ICM distributions, however it almost vanishes for strong ram pressure pulses. Although various hydrodynamical effects are present in the ISM-ICM interaction, the main quantitative stripping results appear to be roughly consistent with a simple scenario of momentum transfer from the encountered ICM. This behavior can also be found in previous simulations. To reproduce the numerical results we propose a fitting formula depending on the disk tilt angle and on the column density of the encountered ICM. Such a dependence is superior to that on the peak ram pressure used in previous simple estimates.

The fragmentation of expanding shells – II. Thickness matters

We study analytically the development of gravitational instability in an expanding shell having finite thickness. We consider three models for the radial density profile of the shell: (i) an analytic uniform-density model, (ii) a semi-analytic model obtained by numerical solution of the hydrostatic equilibrium equation and (iii) a 3D hydrodynamic simulation. We show that all three profiles are in close agreement, and this allows us to use the first model to describe fragments in the radial direction of the shell. We then use non-linear equations describing the time-evolution of a uniform oblate spheroid to derive the growth rates of shell fragments having different sizes. This yields a dispersion relation which depends on the shell thickness, and hence on the pressure confining the shell. We compare this dispersion relation with the dispersion relation obtained using the standard thin-shell analysis, and show that, if the confining pressure is low, only large fragments are unstable. On the other hand, if the confining pressure is high, fragments smaller than predicted by the thin-shell analysis become unstable. Finally, we compare the new dispersion relation with the results of 3D hydrodynamic simulations, and show that the two are in good agreement.