James Webb Space Telescope Feed Post

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

Left: the 3.3 µm PAH map of NGC 7469. This image was created by subtracting a linear fit to the continuum in the unsmoothed NIRSpec cube (see Section 3), and it has a pixel size of 0farcs1, providing us with the highest spatial view of the dust distribution in NGC 7469 using JWST IFU. The spectra extracted in this study are based on the overlaid square grid, with each cell having a width of 0farcs6. Grids in white are cells with full NIRSpec (G395H)+MIRI coverage, while the ones in dashed blue have only partial coverage in MIRI. The central grid is masked due to noise in the linear continuum subtraction at the nucleus, where the PAH emission is very weak but has no effect on the extracted spectra analyzed in this paper. Middle: the [Ne ii] 12.81 µm MIRI/MRS map for comparison, which has a pixel size of 0farcs2 and traces the ionized atomic gas in the starburst ring. The same spectral grid is overlaid here. In general, there is good agreement between the 3.3 PAH and [Ne II] maps, but the 3.3 PAH map shows finer detail owing to its higher intrinsic spatial resolution, and differences are evident in the ratio of PAH to ionized gas emission around the ring. Right: emission line flux ratio map of [Ne iii]/[Ne ii], a measure of the radiation field hardness. Here the [Ne ii] map was smoothed to match the PSF of [Ne iii] before generating the line ratio map. As expected, the [Ne iii]/[Ne ii] ratio is highly peaked toward the center. These maps were created by applying a linear local continuum subtraction using the QFitsView tool (Ott 2012). 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.