Kinematics and dynamics of the luminous infrared galaxy pair NGC 5257/58
  (Arp 240)
  Encounters between galaxies modify their morphology, kinematics, and star
formation (SF) history. The relation between these changes and external
perturbations is not straightforward. The great number of parameters involved
requires both the study of large samples and individual encounters where
particular features, motions, and perturbations can be traced and analysed in
detail. We analysed the morphology, kinematics, and dynamics of two luminous
infrared spiral galaxies, NGC 5257 and NGC 5258, in which SF is mostly confined
to the spiral arms, in order to understand interactions between galaxies of
equivalent masses and SF processes during the encounter. Using scanning
Fabry-Perot interferometry, we studied the contribution of circular and
non-circular motions and the response of the ionized gas to external
perturbations. We compared the kinematics with direct images of the pair and
traced the SF processes and gravitational effects due to the presence of the
other galaxy. The SED of each member of the pair was fitted. A mass model was
fitted to the rotation curve of each galaxy. Large, non-circular motions
detected in both galaxies are associated with a bar, spiral arms, and HII
regions for the inner parts of the galaxies, and with the tidal interaction for
the outer parts of the discs. Bifurcations in the rotation curves indicate that
the galaxies have recently undergone their pericentric passage. The pattern
speed of a perturbation of one of the galaxies is computed. Location of a
possible corotation seems to indicate that the gravitational response of the
ionized gas in the outer parts of the disc is related to the regions where
ongoing SF is confined. The SED fit indicates a slightly different star
formation history for each member of the pair. For both galaxies, a
pseudo-isothermal halo better fits the global mass distribution.
