1 Introduction

An ideal tool for constraining observationally models of galaxy evolution would consist of a multi-dimensional "data-cube'': $D_{\rm imag}(\lambda, Type, Lum, z, Env)$containing imaging data of complete samples of galaxies, spanning the broadest possible wavelength ($\lambda$), redshift (z), morphological type (Type) and luminosity (Lum) ranges. Moreover, all environmental conditions (Env) should be equally represented, from the coarsest "field" to the densest cluster's cores.

Such an ideal data-base is irrealistic. First of all, multifrequency images hardly exist, at suitable resolution, even for galaxies in the Local Group. The requirement that the data-cube consists of "imaging" data must then be relaxed for the more realistic requirement that it should contain "integrated" data, as more commonly available from aperture/CCD photometry. Even with these reduced characteristics, very few such data-sets exist either for high z, or for local galaxies. The presently available samples cover a small wavelength window, such as those of Connolly et al. (1995) or Kinney et al. (1993), or they are biased towards starburst and active galaxies (Schmitt et al. 1997), thus they are not representative of "normal'' galaxies.

Within few years from now, however, when SLOAN will reach completion and the space missions GALEX (UV) and ASTRO-F (FIR) will perform their all-sky surveys, large data-sets meeting the above requirements will be at hands.

There is yet a sample which approaches the ideal requirements. The data-cube we are referring to is an optically selected (complete) one, representative of galaxy in a broad luminosity range and it is truly multifrequency (from the far-UV to the radio domains). It suffers from three limitations: it is local (z=0) and it represents only late-type galaxies in the densest environment, being composed of galaxies in the the Virgo cluster: $D_{\rm phot}(UV-radio, Late, Lum, z=0, Virgo)$. It is on this data-base that the present paper is focused.

Skipping through the details of the sample selection and of the available data that can be found in Sects. 2 and 3 of this paper respectively, it is worth spending some words on what scientific purpouses such data-base is aimed at.

Individual galaxies are represented in the data-base under the form of Spectral Energy Distributions (SEDs), such as those presented in Fig. 2. SEDs are powerful diagnostic tools for studying the energy balance between the principal constituents of galaxies. From 0.1 to 5 $\rm\mu m$ (UV, Visible, Near-IR) SEDs are dominated by the stellar thermal radiation, (but include most of the measurable recombination lines providing the diagnostics of the ISM). From 5 to 25 $\rm\mu m$ (Mid-IR) the dominant source is the radiation from very small grains of dust, but the contribution of emission lines (PAH) is relevant. From 25 to 1000 $\rm\mu m$ (Far-IR, sub-mm radio) the flux of SEDs is due to the thermal radiation from cold dust (10-100 K). Important diagnostic lines such as the [CII] ( $\lambda 158~ \mu$m) and CO are found in this interval. It is here that dust-rich objects peak their flux distributions. At wavelengths longer than 1 cm (radio) the radiation is non-thermal (synchrotron) by relativistic cosmic ray electrons and magnetic fields, but the most important ISM diagnostic line, the 21 cm line of the neutral hydrogen, lies in this domain. All these components and their complex feedback relations can be studied at once using the SEDs. First an estimate of the relative fraction of stars in the various age (temperature) classes can be obtained by fitting populations synthesis models (Bruzual & Charlot 1993) to the stellar continua (see e.g., Gavazzi et al. 2002a). Once the stellar populations are determined, by studying the ISM emission line properties (e.g. the H$\alpha$) one can learn about the ionization processes in HII regions. From the FIR properties we can study the dust heating mechanisms. Finally from the luminosity of the synchrotron radiation one can study the contribution of the various stellar populations to the cosmic ray acceleration.

Before energy balances can be quantitatively derived, however, the observed SEDs must be properly corrected for a number of effects that introduce wavelength dependent distortions to their shape. Primarily the SEDs must be rest-framed. Galaxies at large redshift require important K corrections. Their cosmic evolution can be studied by comparing their rest-frame SEDs with those of normal local galaxies. Hence the importance of obtaining template SEDs representative of normal galaxies, unlike those of starburst galaxies such as M 82 or Arp 220 (see Fig. 3), often used for such a purpouse.

Secondly comes the internal extinction correction. Stellar light is absorbed and scattered by the dust in a wavelength dependent way. Corrected SEDs can be derived if the proper amount of extinction is estimated. The amount of stellar light absorbed in the blue should equal that thermally re-emitted in the FIR by the dust. Thus the difference of the integral under the stellar continua in the SEDs before and after the extinction correction gives the energy radiated in the FIR. By reversing the argument Buat et al. (2002) derive a robust estimate of the internal extinction in normal galaxies.

Finally the comparison of SEDs of isolated and cluster galaxies can shed light on influences of the environment on the various components of galaxies. Our Virgo sample, spanning a large interval of galactocentric projected distance from M 87 (up to 6 degrees), provides a clue also on this issue.

Matter in the present paper is organized as follows The sample is described in Sect. 2; in Sect. 3 we give a new prescription for the determination of the UV, optical and near-IR internal extinction based on the FIR/UV flux ratio. The adopted extinction law is checked in Sect. 5.3 using considerations on the energy balance between the emitted far-IR radiation and the absorbed stellar light. The SEDs of the sample galaxies are presented in Sect. 4, and analyzed in Sect. 5. We construct template SEDs in bins of equal morphological type and luminosity and compare them to those of starburst galaxies (Sect. 5.1). The stellar contribution to the mid-IR emission of galaxies (Sect. 5.2) and the properties of the nonthermal radiation (Sect. 5.4) are also analyzed. The bolometric properties of the observed sample are described in Sect. 5.5.

New optical observations obtained using the 1.2 m telescope of the Observatoire de Haute Provence (OHP), the 0.9 m telescope at Kitt Peak and the 2.5 m INT telescope at el Roques de los Muchachos (La Palma) are given in the appendix.

All observations analyzed in the present paper are contained in a database that has been made available to the international community via the Word Wide Web site GOLDMine (http://goldmine.mib.infn.it) described in Gavazzi et al. (2003).