GOALS-JWST: Small neutral grains and enhanced 3.3 micron PAH emission in the Seyfert galaxy NGC 7469

TL;DR

This study uses JWST/NIRSpec spectroscopy to analyze 3.3 micron PAH emission in NGC 7469, revealing variations in grain properties near the AGN and starburst ring, with implications for star formation rate estimates.

Contribution

First high-resolution JWST observations of PAH emission in NGC 7469, showing how AGN activity affects grain properties and PAH emission on sub-kiloparsec scales.

Findings

Smaller grains are destroyed near the AGN due to photo-destruction.

PAH emission is suppressed in the AGN core compared to star-forming regions.

Star formation rate estimates from PAH features can be underestimated in AGN-hosting galaxies.

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 um neutral polycyclic aromatic hydrocarbon (PAH) grain emission on ~60 pc scales. We find a clear change in the average grain properties between the star-forming ring and the central AGN. Regions in the vicinity of the AGN, with [NeIII]/[NeII]>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 photo-destruction in the vicinity of the AGN. 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 um to total PAH power is enhanced in the starburst ring, possibly due to a variety of physical effects on sub-kpc scales, including recurrent fluorescence of small grains or multiple photon absorption by large grains. Finally, the IFU data show that while the 3.3 um PAH-derived star formation rate (SFR) in the ring is 8% higher than that inferred from the [NeII] and [NeIII] emission lines, the integrated SFR derived from the 3.3 um feature would be underestimated by a factor of two 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.