Resolving the extended interstellar medium of infrared galaxies: I. Polycyclic aromatic hydrocarbon emission
August 2026 • 2026A&A...712A..84L
Abstract • Context. Luminous infrared galaxies (LIRGs) represent a key phase of galaxy evolution that is characterized by intense star formation, frequent mergers, and in some cases significant active galactic nucleus (AGN) activity. Polycyclic aromatic hydrocarbons (PAHs) and far-infrared (FIR) cooling lines provide powerful diagnostics of the physical conditions of the interstellar medium (ISM) and the efficiency of photoelectric heating of the gas, but most previous studies have relied on nuclear or galaxy-integrated measurements. Aims. We aim to investigate how PAH properties and photoelectric heating efficiency vary spatially across LIRGs, and to assess the relative roles of star formation and AGN activity in regulating gas heating and dust grain processing on kiloparsec scales. Methods. We used spatially resolved mid-infrared (MIR) spectroscopy from Spitzer/IRS and FIR line maps from Herschel/PACS for a sample of 67 nearby LIRGs from the Great Observatories All-sky LIRG Survey (GOALS) sample. Spectra were extracted in concentric annuli and matched square-aperture grids to probe regions from the nuclei out to ∼14 kpc. We measured MIR PAH features and emission lines using the CAFE spectral decomposition tool, while FIR emission lines were measured through Gaussian profile fitting. We then traced the photoelectric heating efficiency by combining PAH emission with FIR cooling lines after isolating the neutral-gas contribution. Results. In starburst galaxies, the PAH 11.3/7.7 ratio increases from the nuclear regions out to ∼5 kpc, while the PAH 6.2/7.7 ratio remains approximately constant, indicating neutral and large PAH populations. At intermediate galactocentric distances (5─8 kpc), the PAH 11.3/7.7 ratio continues to rise as PAH sizes begin to shift toward smaller grains, marking a transition regime. Beyond ∼8 kpc, the PAH 11.3/7.7 ratio decreases and the PAH 6.2/7.7 ratio increases, signaling increasingly ionized and smaller PAHs, which is consistent with a photo-destruction scenario driven by a strong radiation field close to the central starburst. Nuclear and inner extended regions in strong AGN systems are dominated by more neutral PAH populations and harder radiation fields, whereas at larger radii (> 5 kpc) PAHs become increasingly ionized and converge toward the starburst (SB) locus with a declining radiation field intensity, which indicates the influence of the AGN is diminishing. Our results also show that powerful AGNs can influence the average PAH ionization states at large radii (6─14 kpc) with the same effectiveness that moderate AGNs achieve at smaller scales (1─6 kpc). The photoelectric efficiency increases with galactocentric distance in both SB and AGN-hosting galaxies, due to the decreasing intensity of the radiation field at progressively larger radii. In SB, the increase in photoelectric efficiency is accompanied by a progressive shift of the PAH population toward more neutral charge states, whereas in AGN-dominated systems this correlation appears to be inverted. Conclusions. Our results demonstrate that both star formation and AGN activity regulate PAH properties over kiloparsec scales (up to ∼5 kpc) in LIRGs, with the influence of the AGN weakening with galactocentric distance. This work provides the first quantitative, spatially resolved constraints on the kiloparsec-scale extent over which AGN activity impacts MIR dust grain properties in luminous dusty galaxies as a population.
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