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F. Galliano (NRC at NASA GSFC), E. Dwek (NASA GSFC), P. Chanial (NRC at NASA GSFC)
Recent Spitzer and ISO observations (Engelbracht et al. 2005, ApJ, 628, L29; Madden et al. 2005, A&A, astro-ph/0510086) have confirmed the correlation between the metallicity, Z, of a galaxy's ISM and the relative intensity of the thermal emission by its Polycyclic Aromatic Hydrocarbons (PAHs). One explanation that has been proposed to this phenomenon is that the low dust content of low-metallicity environments allows the hard radiation field to penetrate deeply in the ISM and photoevaporate the PAHs. However, this explanation supposes the existence of PAHs in low-metallicity environments. Since PAHs are produced in AGB stars, an alternative explanation is that their paucity in low-metallicity environments is the result of their delayed injection (~~300~Myr, a typical main sequence lifetime) into the ISM, compared to the prompt (~~10~Myr) metal enrichment by supernovae (Dwek 2005, AIP Conf. Series No. 761, page 103).
To test this hypothesis, we calculated the abundance of PAHs in a sample of 37 nearby galaxies of various morphologies, with Z values ranging from 1/50 to 3\:Z\odot, observed by ISO/CAM or Spitzer/IRS. Our results show that the PAH/H~{\sc i} abundance ratio is clearly correlated with metallicity, in good agreement with the prediction of our dust evolution model (Dwek 1998, ApJ, 501, 643). These results are the first observational evidence of the delayed injection of carbon dust by AGB stars, providing an estimate of their formation efficiency in these stars. Furthermore, this work can be used to convert the PAH contribution to galaxies’ SEDs into PAH/H~{\sc i} abundances as a function of ISM metallicity.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.