Bibcode
Sylos Labini, F.; Benhaiem, D.; Comerón, S.; López-Corredoira, M.
Bibliographical reference
Astronomy and Astrophysics, Volume 622, id.A58, 24 pp.
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2
2019
Journal
Citations
8
Refereed citations
7
Description
Galaxy velocity mapsoften show the typical pattern of a rotating disk,
consistent with the dynamical model where emitters rotate in circular
orbits around the galactic center. The simplest template used to fit
these maps consists in the rotating disk model (RDM) where the amplitude
of circular velocities is fixed by the observed velocity profile along
the kinematic axis. A more sophisticated template is the rotating
tilted-ring model (RTRM) that takes into account the presence of warps
and allows a radius-dependent orientation of the kinematic axis. In both
cases, axisymmetry is assumed and residuals between the observed and the
model velocity fields are interpreted as noncircular motions. We show
that if a galaxy is not axisymmetric, there is an intrinsic degeneracy
between a rotational and a radial velocity field. We then introduce a
new galaxy template, the radial ellipse model (REM), that is not
axisymmetric and has a purely radial velocity field with an amplitude
that is correlated with the major axis of the ellipse. We show that best
fits to the observed two-dimensional velocity fields of 28 galaxies
extracted from the THINGS sample with both the REM and the RDM give
residuals with similar amplitudes, where the REM residuals trace
nonradial motions. Best fits obtained with the RTRM, because of its
larger number of free parameters, give the smallest residuals: however,
we argue that this does not necessarily imply that the RTRM gives the
most accurate representation of a galaxy velocity field. Instead, we
show that this method is not able to disentangle between circular and
radial motions for the case of nonaxisymmetric systems. We then discuss
a refinement of the REM, able to describe the properties of a more
heterogeneous velocity field where circular and radial motions are
respectively predominant at small and large distances from the galaxy
center. Finally, we consider the physical motivation of the REM, and
discuss how the interpretation of galactic dynamics changes if one
assumes that the main component of a galaxy velocity field is modeled as
a RDM/RTRM or as a REM.
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