THE LARGE-SCALE DISTRIBUTION OF PAHS IN THE GALAXY

We present new data obtained with the French balloon-borne instrument AROME, concerning the general emission of the Galaxy at 3.3 mum: conti nuum and emission feature. Maps of the galactic plane are presented fo r galactic longitudes -60-degrees < LII < 60-degrees and latitudes -5- degrees < BII < 5-degrees. The continuum shows the exponential disk pl us bulge stellar distributions, and the feature shows the thin dust di sk. We show that, at large scale, the variations of the 3.3 mum featur e/100 mum and 12 mum/100 mum ratios can be interpreted in terms of ext inction along the line of sight. We derive the following generic galac tic dust colors for the diffuse medium: DELTAlambdaepsilon(lambda)(3.3 mum feature)/lambdaepsilon(lambda)(100) = 2.5 +/- 0.4 10(-3), and MBD Aepsilon(lambda)(12)/LAMBDAepsilon(lambda)(100) = 0.31 +/- 0.03. We sh ow that there is no evidence for large scale variations of these color s, neither with the galactocentric distance, nor with the nature of th e gas in which the emitting dust is located, neutral or molecular. Thi s is interpreted as a strong argument in favor of a universal galactic mixture of dust grains. We show that the value of the 12/100 ratio ca n only be explained with a dust model including large molecules a few Angstroms size. The observation of the 3.3 mum feature indicates that a significant fraction of these molecules may be Polycyclic Aromatic H ydrocarbons (PAHs). In this hypothesis, our results imply a homogeneou s mixing of the PAHs into the general interstellar medium and a mass a bundance of 3 to 8 % of the cosmic carbon. We show that these observat ional facts are marginally compatible with the hypothesis of mass-losi ng carbon stars being the main sites of PAH production in the Galaxy. We suggest that direct formation of PAHs in interstellar clouds may al so occur.