ASTRONOMY AND ASTROPHYSICS 1 . 65 μ m ( H - band ) surface photometry of galaxies . IX : photometric and structural properties of galaxies

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 the galaxy total luminosity. C31 > 5 values are found only at LH > 10 L⊙ (giant ellipticals mixed with early-type spirals), while at LH < 10 L⊙ 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 mue in the H-band, as opposed to the two separate regimes that are generally observed in the B-band. As NIR luSend offprint requests to: M. Scodeggio Correspondence to: [email protected] minosity 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.