2002A&A...384..812S

As a result of a systematic NIR H-band (1.65µm) imaging survey of normal galaxies in the local universe that includes objects both in the Virgo cluster and in the ``Great Wall'' (including A1367, A1656 as well as the ``isolated'' population in the bridge between the two clusters), we are able to measure in a highly homogeneous way photometric and structural properties for a sample of 1143 galaxies. We base our analysis on a quantitative structural parameter, the concentration index C₃₁ (defined as the ratio between the radii that enclose 75% and 25% of the total luminosity), instead of relying on the galaxies' morphological classification. The C₃₁ parameter provides a model independent, quantitative and continuous characterization of the light distribution within galaxies, and it is thus to be preferred to either the Hubble type or a parameter like the bulge-to-disk or bulge-to-total light ratio. Low C₃₁ objects are typically found among disk galaxies, while high C₃₁ describes bulge-dominated systems. We confirm our previous claim that C₃₁ correlates strongly and non-linearly with the galaxy total luminosity. C₃₁>5 values are found only at L_H>10¹⁰L_☉ (giant ellipticals mixed with early-type spirals), while at L_H<10¹⁰L_☉ galaxies have C₃₁<3 (dwarf Irregulars mixed with ellipticals). At high luminosity, low C₃₁ are allowed (bulge-less giant Scs). Thus C₃₁ and the total luminosity are not sufficient to fully characterize the family of galaxies. However we find that galaxies can be completely described by three parameters, namely: a scale parameter (the H-band luminosity), a shape parameter (the concentration index C₃₁) and a colour parameter (e.g. the B-H colour). At low luminosity dEs and dIs, having similar C₃₁, are colour-discriminated, while at very high luminosity different C₃₁ discriminate S0s from Scs, otherwise undistinguishable on the basis of their colour. A single, monotonic relation exists between luminosity and µ_e in the H-band, as opposed to the two separate regimes that are generally observed in the B-band. As NIR luminosity traces quite accurately the galaxy mass distribution, this relation re-enforces the indication in favour of a scale-dependent mass collapse mechanism which produces higher surface-brightness and more centrally peaked galaxies with increasing mass. However, the presence of high-luminosity low-C₃₁ galaxies hints at other machanisms and physical properties (such as angular momentum) playing an important role in galaxy formation.