Authors: I. Trujillo, A. Asensio Ramos, J.A. Rubino-Martin, A. Graham, J.A.L. Aguerri, J. Cepa, C.M. Gutierrez
Abstract: We have investigated the structural and dynamical properties of triaxial stellar systems whose surface brightness profiles follow the r1/n luminosity law — extending the analysis of Ciotti (1991) who explored the properties of spherical r1/n systems. A new `analytical' expression that accurately reproduces the spatial (i.e. deprojected) luminosity density profiles (error < 0.1%) is presented for detailed modelling of the Sérsic family of luminosity profiles. We evaluate both the symmetric and the non-axisymmetric components of the gravitational potential and force, and compute the torques as a function of position. For a given triaxiality, stellar systems with smaller values of n have a greater non-axisymmetric gravitational field component. We also explore the strength of the non-axisymmetric forces produced by bulges with differing n and triaxiality on systems having a range of bulge-to-disc ratios. The increasing disc-to-bulge ratio with increasing galaxy type (decreasing n) is found to heavily reduce the amplitude of the non-axisymmetric terms, and therefore reduce the possibility that triaxial bulges in late-type systems may be the mechanism or perturbation for non-symmetric structures in the disc.
Using seeing-convolved r1/n-bulge plus exponential-disc fits to the K-band data from a sample of 80 nearby disc galaxies, we probe the relations between galaxy type, Sérsic index n and the bulge-to-disc luminosity ratio. These relations are shown to be primarily a consequence of the relation between n and the total bulge luminosity. In the K-band, the trend of decreasing bulge-to-disc luminosity ratio along the spiral Hubble sequence is predominantly, although not entirely, a consequence of the change in the total bulge luminosity; the trend between the total disc luminosity and Hubble type is much weaker.