Juan C. Muzzio

On the use and abuse of Newton's second law for variable mass problems

We clarify some misunderstandings currently found in the literature that arise from improper application of Newtons second law to variable mass problems. In the particular case of isotropic mass loss, for example, several authors introduce a force that actually does not exist.

Models of cuspy triaxial stellar systems – I. Stability and chaoticity

We used the N-body code of Hernquist & Ostriker to build a dozen cuspy (γ � 1) triaxial models of stellar systems through dissipationless collapses of initially spherical distributions of 10 6 particles. We chose four sets of initial conditions that resulted in models morphologically resembling E2, E3, E4 and E5 galaxies, respectively. Within each set, three different seed numbers were selected for the random number generator used to create the initial conditions, so that the three models of each set are statistically equivalent. We checked the stability of our models using the values of their central densities and of their moments of inertia, which turned out to be very constant indeed. The changes of those values were all less than 3 per cent over one Hubble time and, moreover, we show that the most likely cause of those changes are relaxation effects in the numerical code. We computed the six Lyapunov exponents of nearly 5000 orbits in each model in order to recognize regular, partially and fully chaotic orbits. All the models turned out to be highly chaotic, with less than 25 per cent of their orbits being regular. We conclude that it is quite possible to obtain cuspy triaxial stellar models that contain large fractions of chaotic orbits and are highly stable. The difficulty in building such models with the method of Schwarzschild should be attributed to the method itself and not to physical causes.

Chaotic Orbits in Galactic Satellites

In several previous papers we had investigated the orbits of the stars that make up galactic satellites and found that many of those orbits were chaotic. In those investigations we made extensive use of the frequency analysis method of Carpintero and Aguilar (1998) to classify the orbits, because that method is much faster than the use of Lyapunov exponents, allows the classification of the regular orbits and our initial comparison of both methods had shown excellent agreement between their results. More recently, we have found some problems with the use of frequency analysis in rotating systems, so that here we present a new investigation of orbits inside galactic satellites using exclusively Lyapunov exponents. Some of our previous conclusions are confirmed, while others are altered. Besides, the Lyapunov times that are now obtained show that the time scales of the chaotic processes are shorter than, or comparable to, other time scales characteristic of galactic satellites.

Models of cuspy triaxial stellar systems – III. The effect of velocity anisotropy on chaoticity

Fil: Carpintero, Daniel Diego. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Astrofisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas. Instituto de Astrofisica la Plata; Argentina. Universidad Nacional de La Plata; Argentina

On the Effect of Chaotic Orbits on Dynamical Friction

Chaotic orbits suffer significant changes as a result of small perturbations. One can thus wonder whether the dynamical friction suffered by a satellite on a regular orbit, and interacting with the stars of a galaxy, will be different if the bulk of the stars of the galaxy are in regular or chaotic orbits. In order to check that idea, we investigated the orbital decay (caused by dynamical friction) of a rigid satellite moving within a larger stellar system (a galaxy) whose potential is nonintegrable. We performed numerical experiments using two kinds of triaxial galaxy models: (1) the triaxial generalization of Dehnens spherical mass model (Dehnen; Merritt & Fridman); (2) a modified Satoh model (Satoh; Carpintero, Muzzio, & Wachlin). The percentages of chaotic orbits present in these models were increased by perturbing them. In the first case, a central compact object (black hole) was introduced; in the second case, the perturbation was produced by allowing the galaxy to move on a circular orbit in a logarithmic potential. The equations of motion were integrated with a non-self-consistent code. Our results show that the presence of chaotic orbits does not affect significantly the orbital decay of the satellite.

Dwarf galaxies captured by giants in clusters

We performed simple numerical simulations to investigate the capture of dwarf galaxies by larger ones in the environment of a cluster of galaxies. The number of captures is small compared to the total number of dwarfs, but it is significant because these galaxies are the most abundant members of galaxy clusters. Our models show that, having a more massive central galaxy or a less massive background halo in the cluster, increases the number of captures. A high proportion of the orbits of the captured dwarfs are very eccentric.

Boltzmann equation approach to the N-body problem with masses varying according to the Eddington-Jeans law

We study special aspects of the N-body problem with masses varying according to the Eddington-Jeans law with powers n=2 and 3. Our main result is that a particular set of variables can be found that allows one to write the pertinent Boltzmann and Poisson equations in a fashion similar to that corresponding to the usual fixed mass situation.

Testing Galactic Oscillations

Recent numerical simulations using an N-body code suggest that galaxies may oscillate in a very regular and long lasting way. Here we investigate galactic oscillations using a different approach: the perturbation particle method. Our results confirm the computational results given by Miller and Smith (1994).

Tidal stripping and accretion in clusters of galaxies with smoothly distributed missing mass

Results are reported from simulations of the alterations in tidal stripping and tidal accretion caused by the presence of smoothly distributed missing mass (background) in Virgo-like clusters of galaxies. The simulations considered the collisional interactions of two galaxies treated as Schusters spheres, one containing 170 test particles. The spheres were surrounded with either a mass equal to the total galactic mass or to nine times the galactic mass, or with no background exerting gravitational force. The presence of the background significantly decreased mass exchanges and losses. The implications the results have for simulations of collisions between galaxies are discussed. 30 references.

Models of cuspy triaxial stellar systems – IV. Rotating systems

Fil: Carpintero, Daniel Diego. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Astrofisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas. Instituto de Astrofisica la Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina