6 Summary and conclusions

We present a multifrequency dataset comprising an optically-selected, volume-limited, complete sample of 118 galaxies in the Virgo cluster. The sample includes all late-type ($\geq$S0a) Virgo A members in the core of the cluster, with projected distance $\Theta< 2$ degrees from M 87, or at the peryphery of the cluster ($\Theta> 4$ degrees from the position of maximum projected galaxy density).

The database includes UV, visible, near-IR, mid-IR, far-IR, radio continuum photometric data as well as spectroscopic data on the H$\alpha$, CO and HI lines.

Spectral energy distributions (SEDs) of the individual galaxies, as well as templates SEDs in bins of morphological type and luminosity are derived. The SEDs are fitted with stellar population synthesis models providing an estimate of the total stellar radiation, with modified black-bodies fitted to the far-IR data giving the energy re-emitted by dust, and with power laws representing the synchrotron emission.

Assuming the energy balance between the absorbed stellar light and the energy radiated in the IR by dust, we calibrate an empirical attenuation law suitable for correcting photometric and spectroscopic data of normal galaxies.

The analysis of the SED show that low-luminosity, dwarf galaxies have on average bluer stellar continua and higher far-IR luminosities (per unit galaxy mass) than giant, early-type spirals. Normal spirals have relatively similar observed stellar spectra but 10 to 100 times lower IR luminosities than nearby starburst galaxies such as M 82 and Arp 220. The temperature of the cold dust component increases with the far-IR luminosity, from giant spirals to dwarf irregulars and to an higher extent in starburst galaxies. SEDs of starburst galaxies should not be used as templates of normal high redshift galaxies.

We show that the contribution of the stellar emission to the 6.75 $\mu$m mid-IR flux is generally important, from $\sim$80% in Sa to $\sim$20% in Sc.

Acknowledgements

We thank J. Donas and D. Pierini for providing us with unpublished UV and near-IR data. We thank S. Arnouts, V. Buat and M. Sauvage for stimulating discussions, and D. Elbaz for providing us with the SED of M 82 and Arp 220. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. most of the data presented in this work are available through the WEB page http://goldmine.mib.infn.it