Mars: Mapping surface units by means of statistical analysis of TES spectra

We present a multifrequency dataset for an optically-selected, volume-limited, complete sample of 118 late-type galaxies ( > =S0a) in the Virgo cluster. The database includes UV, visible, near-IR, mid-IR, far-IR, radio continuum photometric data as well as spectroscopic data of H(alpha), CO and HI lines, homogeneously reduced, obtained from our own observations or compiled from the literature. Assuming the energy balance between the absorbed stellar light and that radiated in the IR by dust, we calibarte an empirical attenuation law suitable for correcting photometric and spectroscopic data of normal galaxies. The data, corrected for internal extinction, are used to construct the spectral energy distribution (SED) of each individual galaxy, and combined to trace the median SED of galaxies in various classes of morphological type and luminosity. Lowluminosity, dwarf galaxies have on average bluer stellar continua and higher far-IR luminosities per unit galaxy mass than giant, early-type spirals. If compared to nearby starburst galaxies such as M 82 and Arp 220, normal spirals have relatively similar observed stellar spectra but 10-100 times lower IR luminosities. The temperature of the cold dust component increases with the far-IR luminosity, from giant spirals to dwarf irregulars. The SED are used to separate the stellar emission from the dust emission in the mid-IR regime. We show that the contribution of the stellar emission at 6.75 (mu)m to the total emission of galaxies is generally important, from ~80% in Sa to ~20% in Sc.