The Impact of an Active Galactic Nucleus on Polycyclic Aromatic Hydrocarbon Emission in Galaxies: The Case of Ring Galaxy NGC 4138

Creators: Donnelly, G. P.; Smith, J. D. T.; Draine, B. T.; Togi, A.; Lai, T. S.-Y.; Armus, L.¹; Dale, D. A.; Charmandaris, V.

1. California Institute of Technology

Abstract

We present a focused study of radially resolved varying PAH emission in the low-luminosity active galactic nucleus (AGN)-host NGC 4138 using deep Spitzer/infrared spectrograph spectral maps. Using new model PAH spectra, we investigate whether these variations could be associated with changes to the PAH grain size distribution due to photodestruction by the AGN. Separately, we model the effects of the varying radiation field within NGC 4138, and we use this model to predict the corresponding changes in the PAH emission spectrum. We find that PAH band ratios are strongly variable with radius in this galaxy with short-to-long wavelength band ratios peaking in the starburst ring. The changing mix of starlight appears to have a considerable effect on the trends in these band ratios, and our radiation model predicts the shapes of these trends. However, the amplitude of observed variation is ∼2.5 × larger than predicted for some ratios. A cutoff of small grains in the PAH size distribution, as has been suggested for AGN, together with changes in PAH ionization fraction could explain the behavior of the shorter bands, but this model fails to reproduce longer band behaviors. Additionally, we find that short-to-long wavelength PAH band ratios increase slightly within ∼270 pc of the center, suggesting that the AGN may directly influence PAH emission there.

Copyright and License

© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the members of R. Chandar's Paper Writing Class (2022–2023) for the constructive feedback over many iterations. We also thank B. Hensley and the attendees of the 2023 PAH Fest for helpful suggestions and conversations. We thank the anonymous referee for the insightful comments, which improved this work. We gratefully acknowledge support for this project from the Research Corporation for Science Advancement through Cottrell SEED award No. 27852. This research was supported in part by NSF grant AST-1908123. This research has made use of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology. J.D.S. gratefully acknowledges support for this project from the Research Corporation for Science Advancement through Cottrell SEED award No. 27852. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

Data Availability

This research made use of Spitzer/IRS (AORKEY: 18240768), the Spitzer/IRAC and MIPS Super Mosaics (Spitzer Science Center and IRSA 2020), and GALEX (STScI 2013).

Facilities

Spitzer - Spitzer Space Telescope satellite (IRAC, MIPS, IRS), GALEX

Software References

Astropy (Astropy Collaboration et al. 2013, 2018, 2022), Matplotlib (Hunter 2007), NumPy (Harris et al. 2020), SciPy (Virtanen et al. 2020)

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