Hot dust in Panchromatic SED Fitting: Identification of AGN and improved galaxy properties [GA]

Forward modeling of the full galaxy SED is a powerful technique, providing self-consistent constraints on stellar ages, star formation rates, dust properties, and metallicities. However, the accuracy of these results is contingent on the accuracy of the model. One significant source of uncertainty is the contribution of obscured AGN to the SED, as they are relatively common and can produce substantial mid-IR (MIR) emission. Here we include emission from dusty AGN torii in the Prospector SED-fitting framework, and fit the UV-IR broadband photometry of 129 nearby galaxies. We find that 10% of the fitted galaxies host an AGN contributing at least 10% of the total observed MIR luminosity. We demonstrate the necessity of this AGN component in the following ways. First, we compare observed spectral features to spectral features predicted from our model fit to the photometry. We find that the AGN component greatly improves predictions for observed H$\alpha$ and H$\beta$ luminosities, as well as MIR $Akari$ and $Spitzer$/IRS spectra. Second, we show that inclusion of the AGN component changes stellar ages and SFRs by up to a factor of 10, and dust attenuations by up to a factor of 2.5. Finally, we show that the strength of the AGN component in our model correlates with independent AGN indicators, including X-ray fluxes, MIR color gradients, emission line ratios, and other published AGN diagnostics. This evidence strongly implies that these galaxies truly host AGN, and that the full SED modeling approach is able to detect AGN down to contrasts of of L${\mathrm{MIR,AGN}}$/L${\mathrm{MIR,tot}} \sim 0.1$. Notably, only 46% of the SED-detected AGN would be detected with a simple MIR color selection. Based on these results, we conclude that SED models which do not include AGN when fitting MIR data are vulnerable to substantial bias in their derived stellar populations parameters.

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J. Leja, B. Johnson, C. Conroy, et. al.
Fri, 15 Sep 17

Comments: Submitted to ApJ. 25 pages, 16 figures