Cold Dust in Late-Type Virgo Cluster Galaxies

We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo Cluster galaxies measured using the Infrared Space Observatory by Tuffs and coworkers in bands centered on 60, 100, and 170 μm. Thirty of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spiral galaxies to blue compact dwarfs (BCDs) and which could not have been recognized by IRAS. We fitted the data with a superposition of two modified blackbody functions, physically identified with a localized warm dust emission component associated with H II regions (whose temperature was constrained to be 47 K), and a diffuse emission component of cold dust. The cold dust temperatures were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust mass is correspondingly increased by factors of typically 6-13. A good linear correlation is found between the "warm FIR" luminosities and the Hα equivalent widths (EWs), supporting the assumptions of our constrained spectral energy distribution fit procedure. We also found a good nonlinear correlation between the "cold FIR" luminosities and the Hα EWs, consistent with the prediction of Popescu and coworkers that the FIR-submillimeter emission should mainly be due to diffuse nonionizing UV photons. Both the "warm" and the "cold" FIR luminosity components are nonlinearly correlated with the (predominantly nonthermal) radio luminosities. There is a tendency for the temperatures of the cold dust component to become colder and for the cold dust surface densities (normalized to optical area) to increase for later morphological types. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems.