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

Abstract

We present mid-IR photometric properties for a sample of 28 early-type galaxies observed at 6.75, 9.63 and 15 mu m with the ISOCAM instrument on board the ISO satellite. We find total mid-IR luminosities in the range (3-42) x 10⁸ L_sun. 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 =~ 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_sun. 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 Hα and mid-IR luminosities. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the UK) and with participation of ISAS and NASA.