Exponential bulges in late-type spirals: An improved description of the light distribution

Abstract

In many cases the modelling of spiral galaxies by an exponential disc and an r1/4-law bulge does not satisfactorily describe the mean radial distribution of light. This is most evident in non-linear least-squares fitting techniques in which the resulting effective radius and surface brightness of the bulge are characterized by large uncertainties and are scattered over large ranges, in sharp contrast to their disc counterparts. We attempt to decompose the major-axis profiles of 34 late-type spirals in terms of an alternative model consisting of an exponential disc and an exponential bulge, using seeing-convolved models. The results of this decomposition show that this model is superior in the statistical aspects of the fitting procedure, in the sense that the various goodness-of-fit indicators are better and the residuals are smaller. The fact that it also confines the parameters of the bulge to a range whose narrowness is comparable to that of the parameters of the disc indicates that this model has the potential to give a better and more consistent description of the bulges of late-type spirals.