IJUP08 - Universidade do Porto

The dynamical state of galaxies: from 2D to 6D
P. Mondim 1 , J. Brinchmann 2
1 Department of Applied Mathematics, Faculty of Science, University of Porto, Portugal.
Centre for Astrophysics of University of Porto, Portugal
2 Leiden University, The Netherlands.
Centre for Astrophysics of University of Porto, Portugal
The goal of this work is to investigate the relationship between the dynamical state of a
galaxy (which belongs to a 6D phase-space, and can only be known with information
regarding the velocity and position of every galaxy's particles) and the information
available from observations: it's 2D velocity field.
This goal is pursued by numerically calculating the Tensor Virial Theorem [1] quantities to
study the phase-space structure of galaxies mergers' simulations and contrasting them with
synthetic observations of those galaxies.
An effective way of calculating the kinetic energy associated with movement that can be
regarded as “ordered” and the kinetic energy associated with movement that can be
regarded as “random” has been devised. Using this tool to study the phase-space structure
of merging galaxies, we were able to verify how ordered- and random-motion evolve
during the interaction of the galaxies. The ratio of ordered-motion to random-motion of the
system's particles has been found to peak early after the beginning of the interaction,
presenting a strong decrease thereafter and, finally, reaching a steady level, lower than the
initial value. Galaxies of different morphological types have been found to provide
somewhat different evolutions for the quantities that trace the ordered- and randommotion.
Work is now underway to create synthetic observations of those same mergers, in order to
make possible a comparison of the “observed” and “real” physical quantities. This may
lead, ultimately, to the establishment of innovative methods of non-parametric
classification of velocity fields. Such methods would be specially valuable for highredshift
galaxies, for which the known methods fail for requiring high signal-to-noise
ratios.
Thus, so far, the main conclusions for the current work are the establishment of typical
evolutions for the ordered- and random-motion of a galaxy's particles during a merger
event and the establishment of different trends for the evolution of those quantities in
galaxies of different morphological types. Further work is still underway.
References:
[1] Binney, J. and Tremaine, S. (1994), Galactic Dynamics, Princeton University Press.
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