2D modelling of the light distribution of early-type galaxies in a volume-limited sample - II. Results for real galaxies

In this paper we analyse the results of the two-dimensional (2D) fit of the light distribution of 73 early-type galaxies belonging to the Virgo and Fornax clusters, a sam le volume- and magnitude-limited down to M-B = -17.3, and highly homogeneous. In our previous paper (Paper I) we have presented the adopted 2D models of the surface-brightness distribution namely the r(1/n) and (r(1/n) + exp) models - we have discussed the main sources of error affecting the structural parameters, and we have tested the ability of the chosen minimization algorithm (MINUIT) in determining the fitting parameters using a sample of artificial galaxies. We show that, with the exception of 11 low-luminosity E galaxies, the best fit of the real galaxy sample is always achieved with the two-component (r(1/n) + exp) model. The improvement in the chi (2) due to the addition of the exponential component is found to be statistically significant. The best fit is obtained with the exponent n of the generalized r(1/n) Sersic law different from the classical de Vaucouleurs value of 4. Nearly 42 per cent of the sample have n < 2, suggesting the presence of exponential 'bulges' also in early-type galaxies. 20 luminous E galaxies are fitted by the two-component model, with a small central exponential structure ('disc') and an outer big spheroid with n > 4. We believe that this is probably due to their resolved core. The resulting scalelengths R-h and R-e of each component peak approximately at similar to1 and similar to2 kpc, respectively, although with different variances in their distributions. The ratio R-e/R-h peaks at similar to0.5, a value typical for normal lenticular galaxies. The first component, represented by the r(1/n) law, is probably made of two distinct families,,ordinary' and 'bright', on the basis of their distribution in the mu (e)-log(R-e) plane, a result already suggested by Capaccioli, Caon and D'Onofrio. The bulges of spirals and SO galaxies belong to the 'ordinary' family, while the large spheroids of luminous E galaxies form the 'bright' family. The second component, represented by the exponential law, also shows a wide distribution in the mu (c)(0)-1og(R-h) plane. Small discs (or cores) have short scalelengths and high central surface brightness, while normal lenticulars and spiral galaxies generally have scalelengths higher than 0.5 kpc and central surface brightness brighter than 20 mag arcsec(-2) (in the B band). The scalelengths R-e and R-h of the 'bulge' and 'disc' components are probably correlated, indicating that a self-regulating mechanism of galaxy formation may be at work. Alternatively, two regions of the R-e-R-h plane are avoided by galaxies due to dynamical instability effects. The bulge-to-disc (B/D) ratio seems to vary uniformly along the Hubble sequence, going from late-type spirals to E galaxies. At the end of the sequence the ratio between the large spheroidal component and the small inner core can reach B/D similar to 100.