Survey of the ISM in early-type galaxies - IV. The hot dust component

We present mid-IR photometric properties for a sample of 28 early-type galaxies observed at 6.75, 9.63 and 15 mum with the ISOCAM instrument on board the ISO satellite. We find total mid-IR luminosities in the range (3 42) x 10(8) L.. The spectral energy distributions (SED) of the galaxies were derived using the mid-IR data together with previously published UV, optical and near-IR data. These SEDs clearly show a mid-IR emission coming from dust heated at T similar or equal to 260 K. Dust grains properties are inferred from the mid-IR colors. The masses of the hot dust component are in the range 10-400 M.. The relationship between the masses derived from mid-IR observations and those derived from visual extinction are discussed. The possible common heating source for the gas and dust is investigated through the correlations between Halpha and mid-IR luminosities.