Downsizing among disk galaxies and the role of the environment

The study of PopI and PopII indicators in galaxies has a profound impact on our understanding of galaxy evolution. Their present (z = 0) ratio suggests that the star formation history of galaxies was primarily dictated by their global mass. Since 1989 Luis Carrasco and I spent most of our sleepless nights gathering Hα and near infrared surface photometry of galaxies in the local Universe and focused most of our scientific career on these two indicators trying to convince the community that the mass was the key parameter to their evolution. We were unsuccessful, until in 2004 the Sloan team rediscovered this phenomenon and named it ”downsizing”. Subject headings: Galaxies: evolution – Galaxies: fundamental parameters The collaboration with Luis Carrasco dates back to 1989, when we joined our efforts to rejuvenate the 2.1m telescope at San Pedro Martir (SPM) by dressing it up with an early CCD by Photometrics, equipped with narrow-band filters for redshifted Hα and with the Boller & Chievens spectrograph, that is still among the instrumentation offered at SPM. Since then our fruitful collaboration brought us to work together on many projects that eventually gave birth to 13 publications. The first (yet most important) discovery we made quite serendipitously at SPM was that the galaxy CGCG 224-038, which was known as a starburst, is probably so because it is under the action of ”harassment” (see fig. 1). It was therefore quite natural to baptize it as the ”Playboy galaxy”, for the glory of Ben Moore (et al. 1999). Later on we started some multifrequency surveying of significant samples of galaxies in the local Universe, following one of the preferred and most productive activities of Luis. We concentrated at obtaining surface photometry in Hα and NIR: the former was aimed at quantifying the massive, ongoing formation of young stars; the latter the oldest stellar population. The survey is still in progress. Departimento di Fisica, Universita’ degli studi di Milano–Bicocca, Milano, Italy. Since the pioneering work of Recillas et al. (1990, 1991) and of Gavazzi et al. (1996) it resulted, among other things, that the NIR band H was providing us with a very robust tool to infer the dynamical mass of galaxies, up to their optical radii. In other words M/L of spirals was found roughly constant in the infrared, as opposite to bluer bands. Following the seminal work of Gavazzi et al. (1996), of Boselli et al. (2001) and Scodeggio et al. (2002) and exploiting the data-set that we collected in the past years of collaboration with Luis Carrasco, in the present paper I wish to put together all possible evidences of ”downsizing” among disk galaxies . In other words I will present and discuss six independent lines of evidence, e.g. 6 scaling laws, all suggesting that the galaxy mass is the principal parameter governing the evolution of these systems. These ideas are receiving some late recognition, as the November 2008 issue of Nature reports two papers by Disney et al. (2008) and by van den Bergh (2008a) that are focused on these issues and confirm our earlier findings. The very existence of such correlations provides a challenge for hierarchical models of galaxy evolution (Cole at al. 2000) since, quoting van den Bergh (2008a), ”... if galaxies were Let me remind that the word ”downsizing” was first introduced by Cowie et al. (1996)