1.65  (H-band) surface photometry of galaxies

IX. Photometric and structural properties of galaxies

¹ Istituto di Fisica Cosmica “G. Occhialini”, CNR, via Bassini 15, 20133, Milano, Italy
² Università degli Studi di Milano – Bicocca, P.zza dell'Ateneo Nuovo 1, 20126 Milano, Italy
³ Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, 13376 Marseille Cedex 12, France
⁴ Dept. of Physics and Astronomy, University of Toledo, 2801 W. Bancroft, 43606, Toledo, Ohio, USA

Corresponding author: M. Scodeggio, marcos@ifctr.mi.cnr.it

Received: 31 July 2001
Accepted: 8 January 2002

Abstract

As a result of a systematic NIR H-band (m) 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. values are found only at (giant ellipticals mixed with early-type spirals), while at galaxies have (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 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 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.