Laurent Vigroux , M. Sauvage , D. Tran , H. Roussel , S. Madden
DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
ISOCAM provides a unique tool to probe the interstellar medium in nearby galaxies. Its wavelength domain, 3 to 17 microns, encompasses a large number of emission mechanisms: ionic lines, broad bands due to carbonaceous material (the so-called Unidentified Infrared Bands, UIB), and thermal continuum due to hot (150 K) very small grains. Large spiral galaxies, barred spirals, irregular, dwarf, and early type galaxies have been observed with ISOCAM in direct imaging mode with broad band filters, as well as in a low resolution imaging spectrophotometric mode with Circular Variable Filters.
Compared to a simple two component model, with a warm ISM linked to star-forming regions and a diffuse cold cirrus-like component which had been predicted from IRAS observations, a much more complex view emerges from the ISOCAM observations. While UIB have been detected in the diffuse component, they are brighter in the spiral arms of spiral galaxies. They can be used to trace the star forming regions in external galaxies. Compared to the observations of galactic HII regions where UIB are associated with the surrounding photodissociation regions, and are absent in their core, external galaxy observations demonstrate that there, at the spatial resolution of ISOCAM (6 arcs.), the mid-IR emission, is dominated by the PDR rather than by the core of star forming regions.
At the opposite of the Hubble sequence, some early type galaxies exhibits very extended UIB disks which are related to the HI disks. In these galaxies, the UIBs are excited by visible photons. This gives some insights on the nature of the species and the heating mechanisms which are responsible for the emission of the UIBs.
Hot very small grains can be detected in the most active star forming regions like the center of barred spirals or starburst galaxies, but not in normal star forming complexes. The presence of hot grains is not linked with the hardness of the UV interstellar radiation field which can be measured with infrared ionic lines, e.g. NeIII/NeII line ratios. It seems to be associated with the energy density of the interstellar radiation field. Below a given threshold, there is not enough energy to heat the very small grains. Above this threshold, the energy density is large enough to heat theses grains producing a strong continuum at 15 microns. The 15 microns to 7 microns surface brightness ratio appears as a very good indicator of star formation efficiency in external galaxies.
The purpose of this presentation is to give an overview of the mid infrared properties of nearby galaxies as derived from ISOCAM. It is an introduction to more specific presentations which will be presented elsewhere during this colloquium.