Simulations of Spiral Galaxies

The outer parts of the rotation curves of external galaxies provide the best evidence for the existence of dark matter. In almost all of them the rotation curve is flat or slowly rising out to the last measured point. Very few galaxies show falling rotation curves, and the ones that do, either fall less than Keplerian or have nearby companions that perturb the velocity field. The simplest interpretation of these results is that spiral galaxies possess massive dark haloes that extend to larger radii than the optical disks.

The inner part of the rotation curve is crucial in determining the nature of the dark matter. The shape of the dark spherical halo is determined by the central density and the core radius. Haloes built up by hierarchical merging in dark matter cosmogonies are cusped and dominated by dark matter at the very centre. Simulations by Navarro, Frenk & White (1997) found a nearly universal profile which rises as r^{-1} to the centre. These characteristics seem to disagree with a number of observations. The number of subhaloes around typical galaxies, as identified by satellite galaxies, is an order of magnitude smaller than predicted by cold dark matter (CDM). The observed rotation curves of dwarf and low surface brightness galaxies seem to indicate that their dark matter haloes have a constant density core instead of steep cusps. For high surface brightness galaxies, the situation is not clear and there is not consensus to whether bright galaxies are dark matter dominated at their very centres or not.

In order to clarify the problem of the mass distribution in the inner parts of bright galaxies and constrain the galaxy formation scenario, we are carrying out a project to test whether the luminous mass in the inner parts of spiral galaxies can account for their observed gas kinematics or whether an additional dark matter component with a distinct mass distribution is required. For this purpose we are planning to model the galaxy dynamics of a significant sample of barred spiral galaxies by running a serial 3-D composite N-body/hydrocode based on a code developed by the Geneva Observatory galactic dynamics group, on the luminous underlying matter distribution of the sample galaxies and the compare it to the observe gas dynamics.

Principal Investigator

Ken Freeman
Astronomy, RSAA
Australian National University

Project

x42

Co-Investigators

Isabel Perez
Kapteyn Institute
Groningen

Roger Fux
Geneva Observatory
Ch. des Maillettes 51

RFCD Codes

240101

Significant Achievements, Anticipated Outcomes and Future Work

The SPH models using the mass models obtained directly from the H-band light distribution give a good representation of the gas distribution and dynamics of the modelled galaxies, supporting the maximum disk assumption. This result indicates that the gravitational field in the inner parts is mostly provided by the stellar luminous component. When 40% of the total mass is transferred to a dark halo, the modelled kinematics departs from the observed kinematics, whereas the departures are negligible for smaller contributions. This result sets a lower limit for the contribution of the luminous component in agreement with the maximum disk definition of the stellar contribution to the rotation. Fast bars give the best fit to the observed kinematics in agreement with the few direct measurements of bar pattern speed previously carried out.

We have recently obtained 2-D velocity field observations for a sample of late type and low surface brightness spiral galaxies. We will compare these velocity fields to the simulated position-velocity diagrams. In this way, will be able to conclude whether the results found for early type spirals also hold for different morphological types or whether their dark mass distribution and bar pattern speed is intrinsically different.

The next step is to do the same type of simulations for galaxies at higher redshifts. From cosmological simulations we know that galaxies are strongly evolving at redshifts of ~1. Until now, it was not possible to obtain kinematic information for galaxies at these redshifts. In the last years, with the development of very large aperture telescopes, it has become possible and rotation curves for high redshift galaxies have already been obtained. These rotation curves show the same trends as the ones found in the nearby universe. However, we still do not know whether these rotation curves reflect the same dark matter distribution as the ones at low redshift. Detailed simulation of their velocity fields and mass distribution are required. We have obtained a sample of suitable candidates at high redshifts in order to test whether the pattern speed and dark halo distribution are also evolving as a function of time. We expect during this coming year to obtain rotation curves for all of them and apply the same methodology as for the low redshift galaxies.

 

Computational Techniques Used

We are running a serial 3-D composite N-body/hydrocode based on a code developed by the Geneva Observatory galactic dynamics group. At each time step, it computes gravitational forces on all particles using a particle mesh with fast Fourier transform method. Secondly, it evaluates pressure and viscous forces on the gas particles, it uses a Lagrangian numerical method to solve Euler's equations of motions for a fluid. Finally, it move all particles one time step further by integrating the equations of motion and an adaptive time-step algorithm. The maximum resolution used in our simulation was a grid of 147456 cells and 500000 gas particles. Given the number of galaxies to simulate and the high resolution required it would be impossible to achieve this without the APAC National Facility capabilities.

 

Publications, Awards and External Funding

External Funding and Awards

None

Publications

I. Perez, R. Fux and K. Freeman, 'Gas flow and dark matter in the inner parts of early-type barred galaxies' A&A, 2004, submitted.
I. Perez Martin, 'Dark matter in the inner parts of barred galaxies'. 2003, Ph.D. thesis, Australian National University.