16513

The NIR-MIR SED of galaxies is dominated by spectral features from polycyclic aromatic hydrocarbon (PAH) molecules fluorescing in regions illuminated by FUV photons. In low metallicity systems (Z < 0.2 Zo) comma previous studies with Spitzer have revealed a substantial deficiency in the emission and abundance of PAHs comma the origin of which remains unexplained due to the lack of deep comma resolved comma multi-band photometry or spectroscopy. We will obtain NIRCam and MIRI imaging to observe the 3.3 comma 7.7 comma and 11.3 micron PAH features in dusty star-forming regions in two low metallicity galaxies comma IC 1613 (15% solar) and Sextans A (8% solar). Additionally comma we will obtain HST-WFC3 imaging with F275W comma F336W comma F475W comma and F814W to complement the JWST observations. The HST observations will allow us to derive extinction maps comma which will be used to normalize the PAH abundance to that of big grains. To obtain the extinction maps comma we will fit the NUV-NIR SED of thousands of stars in the HST and JWST images with the BEAST code (Gordon+2016) in order to constrain the stellar mass comma age and dust extinction (A_V) toward each sight-line. A_V measurements toward individual stars will be combined on the sky to produce the extinction maps. The observations will allow us to 1) estimate the PAH properties comma particularly their abundance comma ionized fraction comma and size distribution and 2) correlate those properties with other tracers of the ISM comma such as the star formation rate density comma the distance and UV spectrum of massive stars comma the distance to PAH-producing stellar sources comma the ISM column density of gas and dust comma and gas-phase abundances. By observing how the PAH properties and abundance vary with environment comma we will constrain the mechanisms responsible for the formation comma destruction comma excitation comma and low abundance of PAHs in those poorly shielded low metallicity environments. Since JWST will measure star formation rates as low as 10 Mo-yr from PAH emission at the peak of cosmic star formation at redshift zvirgul2 when the average metallicity of the universe was about 10% solar comma it is crucial that we deepen our physical understanding of PAHs at low metallicity.