George Helou here (Spitzer Science Center, where I will be for the next two weeks) showed me beautiful data on the mid-infrared properties of nearby galaxies, including NGC 1377, which is a luminous lenticular, but it has a tiny knot of star formation that looks like an enormously powerful, low-metallicity starburst (perhaps a bit of gas that was accreted?). The starburst knot shows enormous silicate absorption and effective extinction, but no PAH emission at all. The more normal galaxies he showed me (primarily from the Spitzer SINGS program) generally show PAH (polycyclic aromatic hydrocarbon) emission when there is star formation, with some variation depending on the contribution of AGN to the radiation field. On the other hand, the galaxies show a huge variation in the hot and warm dust underlying the PAH emission, making analysis of the PAH features non-trivial. Fortunately his team is working on some generally useful tools. Although most investigators seem to think that PAH emission is related to metallicity, this is not at all clear from the currently available data.
Lee Armus here agreed to help Ronin Wu and me understand our Spitzer spectra of low-luminosity galaxies, for which the galaxies Helou showed me will serve as context
. We might be able to shed light on the PAH relationship to metallicity and other possible things (such as radiation field, dust temperature, and geometry).
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