GOALS-JWST: Small Neutral Grains and Enhanced 3.3 μm PAH Emission in the Seyfert Galaxy NGC 7469

Abstract

We present James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) integral field spectroscopy of the nearby luminous infrared galaxy NGC 7469. We take advantage of the high spatial/spectral resolution and wavelength coverage of JWST/NIRSpec to study the 3.3 μm neutral polycyclic aromatic hydrocarbon (PAH) grain emission on ∼200 pc scales. A clear change in the average grain properties between the star-forming ring and the central AGN is found. Regions in the vicinity of the AGN, with [Ne iii]/[Ne ii] > 0.25, tend to have larger grain sizes and lower aliphatic-to-aromatic (3.4/3.3) ratios, indicating that smaller grains are preferentially removed by photodestruction in the vicinity of the AGN. PAH emission at the nucleus is weak and shows a low 11.3/3.3 PAH ratio. We find an overall suppression of the total PAH emission relative to the ionized gas in the central 1 kpc region of the AGN in NGC 7469 compared to what has been observed with Spitzer on 3 kpc scales. However, the fractional 3.3 μm–to–total PAH power is enhanced in the starburst ring, possibly due to a variety of physical effects on subkiloparsec scales, including recurrent fluorescence of small grains or multiple photon absorption by large grains. Finally, the IFU data show that while the 3.3 μm PAH-derived star formation rate (SFR) in the ring is 27% higher than that inferred from the [Ne ii] and [Ne iii] emission lines, the integrated SFR derived from the 3.3 μm feature would be underestimated by a factor of 2 due to the deficit of PAHs around the AGN, as might occur if a composite system like NGC 7469 were to be observed at high redshift.

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

“…Intriguingly, they too detect a much larger L(6.2 μm)/L(7.7 μm) near the nucleus analogous to the upturn in our ratio profile. In the same galaxy, Lai et al (2023) detected an enhancement in L(7.7 μm)/L (11.3 μm) near the nucleus, another short-to-long ratio. García-Bernete et al (2022b) also reports a larger relative L(6.2 μm)/L (7.7 μm) in the nucleus using the same observations.…”

“…Sales et al (2010), García-Bernete et al (2022a) saw similar suppression of both L(6.2 μm)/L(7.7 μm) and L(7.7 μm)/L(11.3 μm) in the Spitzer spectra for the nuclei of AGN hosts and noted that their results are consistent with the logic of the selective destruction by AGN hypothesis. Recently, Lai et al (2023) showed using JWST spectra that L(3.3 μm)/L(11.3 μm), a sensitive tracer of PAH size, is suppressed in the nucleus of the Seyfert galaxy NGC 7469 compared to other regions in the same galaxy. However, in a Spitzer study of 35 Seyfert galaxies, Diamond-Stanic & Rieke (2010) found significant suppression of L(7.7 μm)/L(11.3 μm) but not of L(6.2 μm)/L(7.7 μm), which is another tracer for PAH size.…”

GOALS-JWST: Gas Dynamics and Excitation in NGC 7469 Revealed by NIRSpec

“…Only the lines with at least a 3σ detection in each extraction are included. The fluxes of the 3.3 μm PAH and aliphatic feature are part of our companion paper (Lai et al 2023), which discusses the dust grain distribution in NGC 7469 in detail.…”

“…Our final data cubes have a spatial resolution of 0 14 at 1.12 μm, as obtained from the FWHM of the O I 1.12 μm emission line image, allowing us to resolve structures down to ∼50 pc at this wavelength. The spatial resolution is poorer at longer wavelengths, as demonstrated by Lai et al (2023), who measure a spatial resolution of 0 19 for dust features at 3.3 μm.…”

JWST Observations of Starbursts: Polycyclic Aromatic Hydrocarbon Emission at the Base of the M82 Galactic Wind

“…PAHs are extraordinarily resilient, being observed in close proximity to active galactic nuclei (e.g., García-Bernete et al 2022). They are destroyed by shocks (Micelotta et al 2010a) or X-rays associated with luminous active galactic nuclei (Xie & Ho 2022;Lai et al 2023) and T Tauri stars (Siebenmorgen & Krügel 2010), however, and are rapidly destroyed in hot gas (e.g., Micelotta et al 2010b); therefore, they trace the cooler phases of the ISM and their spectrum provides key information on the physical conditions in those phases.…”