study of the diffuse pah/vsg component in external galaxies is an almost completely open question: a) the ir bands at 3.3, 6.2, 7.7, 8.6 and 11.3 microns have indeed been observed in external galaxies, but in practically all cases this is in starburst galaxies where the emission is intense and comes from very dusty regions, irradiated by the strong uv fluxes of very young stars; b) on the other hand, the arome experiment detected a diffuse emission from the milky way in the 3.3 and 6.2 microns lines, showing that the carriers of the bands were also an important component of the diffuse interstellar medium of our galaxy. such a study is however strongly limited by the small column density of carriers as long as the line of sight is not within the plane of the studied galaxy: this is why edge-on galaxies offer the most favourable cases (more than 100 times larger optical path than face-on galaxies). we propose here to extend the results from arome in our galaxy to a small sample of external nearby spiral edge-on galaxies by mapping them with isocam in the dedicated pah filters and with phot-p in a few long wavelength bands and finally by using phot-s on the central region. beyond the proof of an universal presence of a pah/vsg component in the diffuse interstellar medium of spiral galaxies, those observations, combined with maps at radio wavelengths (hi, hii, co) and near-ir (stellar population), will allow us to study : a) the dependance of the ionization state, size and relative abundance of the ir bands carriers with the conditions of excitation and density at the scale of the galactic structure (nuclear region, bulge, disk, giant complexes); this will help to disentangle the complex problem of the competition between photo-destruction, shielding by dust from exciting photons, sticking/ejection of pah on/from grain mantle etc; b) the ism at large galactic longitudes and latitudes; this will be totally new, and may give significant inputs to more general problems such as the hidden mass or the metallicity gradient.
Instrument
CAM01 , PHT03 , PHT40
Temporal Coverage
1996-02-23T15:15:52Z/1997-07-10T15:20:23Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
