Mid and Far IR properties of late-type galaxies in the Coma and A1367 clusters: ISOCAM and ISOPHOT observations

Abstract

We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belonging to the Coma and A1367 clusters, carried out with the CAM and PHOT instruments on board the ISO satellite. Complementary photometry from the UV to the Near Infrared (NIR) together with Hα imaging, HI and ^(12)CO line measurements allow us to study the relationships between the IR emission and the star formation properties of these galaxies. Most of the resolved galaxies show extended MIR emission throughout their disks even where no Hα emission is present. This suggests that the Aromatic carriers can be excited by the general interstellar radiation field (ISRF), i.e. by visible photons. Only close to HII regions the UV photons are the principal sources of Aromatic carrier excitation. However, when the UV radiation field becomes intense enough these carriers can be destroyed. The average integrated 15/6.75µ_m ratio of the observed galaxies is ∼1, i.e. the typical value for the photodissociation regions (PDRs). This suggests that, despite the high star formation rate (SFR) and the very luminous HII regions of these galaxies, their integrated MIR emission is dominated by PDR-like regions rather than HII-like regions. A cold dust component with average temperature ∼22 K exists in most of the target galaxies, probably arising from big dust grains (BGs) in thermal equilibrium with the ISRF. The contribution to the BGs heating from the ionizing stars decreases with increasing wavelength. A warmer dust component whose emission dominates the spectrum between 20 and 100µ_m is likely to exist. This is probably due to both Very Small Grains (VSGs) and warm BGs emission. The dust to gas ratio of the target galaxies is comparable to that of the solar neighborhood. There is a weak trend between the dust total mass and both the atomic and molecular gas content. The MIR and FIR properties of the analyzed galaxies do not seem to be affected by the environment despite the fact that most of the targets are interacting with the Intra-Cluster-Medium.

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