Kinematics of spiral galaxies: an overview

The kinematics of spiral galaxies are reviewed with special emphasis on rotation curves, the flat maxima and undulations of these curves in late types spirals. The two dimensional velocity fields from the HI 21-cm line as well as from optical lines support the finding that the maxima and minima of the rotation correspond reasonably well to arm and interarm regions respectively. It is argued that all galaxies show nuclear activity of some sort. The energetics of the activity cover a very wide range; from the most active QSO's down to ''normal'' galaxies. NGC 4736 is given as an example having mild nuclear activity at its nucleus which gives rise to a tight nuclear spiral. Such cases fall into a class which I call MAGN as against AGN (M stands for mildly). Finally, it is suggested that the morphological sequence of galaxies is essentially fixed by the global parameters with which a galaxy gets started. It appears that two independent initial parameters underlie the morphological sequence of galaxies. One such parameter is the total mass which doubtless is the most important one. The second parameter, unspecified as yet, may be the total kinetic energy or any other parameter. The remaining parameters will be derivable from the other two independent ones. This writer believes that the main features both morphological and dynamical are essentially determined by global initial conditions although consequences of interaction between galaxies in some cases cannot be ruled out. (author)