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1 Introduction - the $\mathit{z} = 0$ galaxy sample

The physical properties observed in galaxies at different redshifts, pose strong observational constraints to models of galaxy formation and galaxy evolution. Hierachical and monolithic models predict different evolutions of star formation activity and hydrogen content of the Universe (Steidel et al. 1999; Pei & Fall 1995), and of the properties of individual galaxies and their inter-relationships. In this framework, the study of galaxies at z=0 naturally plays a principal role since the small distance of the targets makes the aquisition and the interpretation of spectro-photometric data more accurate and easier than for high-redshift objects.

An exhaustive study of the phenomenology of galaxies at z=0 can be done only through the use of a multifrequency analysis (UV, visible, H$\alpha$ infrared, H I and CO), which is crucial for understanding the physical processes responsible for the emission at different wavelengths, as well as through the establishment of a sufficiently large, "unbiased'' sample, crucial once the general properties of galaxies have been derived from the statistical analysis of a given sample, since different selection criteria can bring about opposite results. Statistical analyses can only yield strong observational constraints when the sample selection biases are known and understood.

The importance of acquiring a well-defined local reference sample of galaxies, which is both complete at various wavelengths as well as of sufficiently large size is evident. It enables statistical analyses which study the phenomenology of galaxies of different type and luminosity; construct a reference sample for studies at higher redshifts; test the prediction of chemo-spectrophotometric evolution models; study environmental effects; understand the properties of the ISM such as the gas-to-dust ratio; and study the relationship between the activity of star formation and the gas content in galaxies.

  \begin{figure}\psfig{,width=18cm}\end{figure} Figure 1: Nançay 21-cm H I line spectra of all detections and marginal detections. Velocity resolution is 15.8 kms-1, radial velocities are according to the radio convention

It is evident that H I line data are crucial for the analysis of all these points; not only because they can be used to determine the H I gas mass, but also since they can provide us with a dynamical measurement through the linewidth.

Working towards the acquisition of a full reference sample of galaxies, priority was given to two well-defined regions of the sky, the Virgo Cluster and the Coma/A1367 supercluster region, since:

- A number of surveys at other wavelengths are either completed or under way: UV at 2000 Å (Donas et al. 1995, and references therein), visible (Binggeli et al. 1985; Gavazzi & Boselli 1996; Boselli et al. in preparation), near-infrared (Gavazzi et al. 1996a,b, 2000; Boselli et al. 1997a, 2000), mid-infrared (Boselli et al. 1998), far-infrared (IRAS catalogues), radio continuum (Gavazzi & Boselli 1999a,b), CO line data (Casoli et al. 1991; Boselli et al. 1995, 1997b; Kenney & Young 1988), H$\alpha$ data (Gavazzi et al. 1998; Boselli et al. in preparation);

- Their proximity allows the determination of accurate morphological classifications, expecially in Virgo, and the observation of low-luminosity galaxies (Mb=-16) at most of the above mentioned wavelengths;

- They span a large range in local galaxy density since they include rich clusters (Coma, A1367, Virgo), compact and loose groups, pairs and interacting galaxies and several isolated objects, and thus present ideal reference for environmental studies (Gavazzi et al. 1999a);

- Distance biases are minimized since isolated and cluster galaxies in the Coma/A1367 supercluster are all at the same distance, while Virgo galaxies are at the cluster distance (Gavazzi et al. 1999b);

- These two regions will be surveyed by the GALEX far-UV satellite (Martin et al. 1999). Given their proximity, the UV detection rate among the star forming objects belonging to the Virgo or the Coma/A1367 supercluster is expected to be high enough to construct a significant UV-selected sample, suitable for statistical analyses.

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