The Vanishing Act: PAHs and Heavy Element Abundance in M101

One of Spitzer's hallmark extragalactic discoveries was the sensitive dependence of PAH emission on metallicity. PAHs seem simply to vanish in galaxies below approximately one-quarter of the solar abundance. Understanding the physical origin of this unexpected phenomenon is urgent, and will have major implications for all upcoming JWST proposals using the mid-IR PAH bands to trace star formation at high redshift.

We outline a modest imaging-only program to map the 3.3µm PAH feature along a radial strip in the galaxy M101, beyond 1/5th solar metallicity. M101 is the ideal laboratory for this simple experiment. Thanks to impressively deep 8m-class "auroral-line" spectroscopy from the CHAOS program, M101's steep radial abundance gradient is the most accurately known of any galaxy in the Universe. Additionally, ~50 hours of ultra-deep Spitzer/IRS mapping spectroscopy probe a strip extending 20kpc from its center. Spitzer provided no access to the shortest and highest excitation 3.3µm PAH feature. This band provides a crucial missing piece of the puzzle since it is highly diagnostic of the very smallest PAH grains which are most likely to be affected by changes in low-metallicity environments. NIRCAM's high surface brightness sensitivity and perfectly matched filter set enable a deep measurement of 3.3µm PAH emission at ~4pc resolution, simultaneously tracing signs of mechanical heating with H_2 2.12µm narrowband imaging. Armed with new models for PAH band variations driven by grain destruction, inhibited formation, and changes in the starlight spectrum, we can definitively answer: how & why do PAH grains respond so sensitively to the varying heavy metal content of galaxies?