The phenomenology of disk galaxies

Abstract

Multifrequency observations from the radio (21 cm line and 115 GHz CO line) to the UV (λ 2000 Å) of 928 nearby (z<0.03) late-type (giant+dwarf-Irr) galaxies are analyzed. The metric properties (radius, surface brightness, concentration index) of these objects have been determined in the visible and, for the first time, in the near-infrared (NIR) by means of an extensive imaging campaign carried out at H and K' bands. We revise the phenomenology of late-type galaxies in the local Universe as follows: 1) M/L in the NIR is constant (M/LH=4.6, solar units); 2) population I indicators, namely: the atomic and molecular hydrogen content, the current star formation rate, the luminosity and the surface brightness of the young stellar population, traced by the blue and UV light are anticorrelated with the mass of galaxies; 3) conversely, the luminosity and the surface brightness of the old stellar population, traced by NIR light, increase with mass; 4) the fraction of centrally peaked NIR structures (bulges, nuclei) and their surface brightness increase non-linearly with mass; 5) the above properties are independent of the detailed morphological type and on the environmental conditions. Bearing in mind that the above evidences are limited to late-type (disk) galaxies, and cannot be generalized to the elliptical and dwarf-elliptical population, they provide observational constraints to models of disk galaxy evolution. They find a natural interpretation if it is assumed that the efficiency of protogalaxy collapse is governed primarily by the system initial mass.