MICONIC: JWST/MIRI-MRS reveals heavily reprocessed PAH emission in the circum-nuclear disc of Centaurus A

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are key dust components in galaxies and play a fundamental role in the physics of the interstellar medium (ISM), yet their response to AGN feedback remains debated. We present a spatially resolved analysis of PAHs in the central $7<sup>{\prime\prime}\times12<sup>{\prime\prime}$ ($\sim100\times200$ pc$<sup>2$) of Centaurus A. We use JWST/MIRI-MRS observations at 5-28 $μ$m from the MIRI European consortium GTO program MICONIC, with angular resolution of $0.35<sup>{\prime\prime}-1<sup>{\prime\prime}$ (about 6-17 pc). We derive PAH moment-0 maps via local continuum subtraction and extract one-dimensional spectra from five regions of interest, including the nucleus, the circumnuclear disc, and a PAH-deficient region. The spectra are decomposed into continuum, emission lines, and PAHs to measure feature intensities and equivalent widths (EWs). PAH emission is primarily distributed in a ring-like structure with localized enhancements at $\sim40$ pc from the nucleus. A distinct PAH-deficient region is observed to the north-west, roughly perpendicular to the jet axis, and coincident with enhanced ionized-gas velocity dispersion and inflowing molecular streamers. The 11.3/7.7 $μ$m and 6.2/7.7 $μ$m ratios exceed model predictions for pericondensed PAHs, indicating processed populations with more open structures. The 11.3/12.7 $μ$m ratio suggests a dominance of solo hydrogen sites and partial dehydrogenation, particularly in the PAH-deficient region, where shocks likely drive erosion. The largest EWs are found in the ring, while reduced values in the deficient region point to partial destruction; in the nucleus, low EWs are mainly due to continuum dilution.

• The paper presents the first sub-arcsecond maps of Polycyclic Aromatic Hydrocarbon (PAH) emission in the ICND of Centaurus A at unprecedented resolution.
• It reveals PAH populations dominated catacondensed molecules indicating shock-driven restructuring and hydrogenation.
• Author analysis shows a unique PAH ring-like structure, a central cavity, and hydrogenation patterns that map to jet interactions it also indicates shock-driven erosion of PAH along the jet-inflated bubbles.

The innermost circum-nuclear disc (ICND) of Centaurus A (NGC 5128) has been mapped in polycyclic aromatic hydrocarbon (PAH) emission at sub-arcsecond resolution using JWST/MIRI-MRS, as part of the MIRI European consortium GTO program MICONIC (2603.23674). The observations cover the full MRS spectral range of $4.9-27.9\,\mu$m over a $7^{\prime\prime}\times12^{\prime\prime}$ ($\sim100\times200$ pc$^2$) mosaic centred on the nucleus, with angular resolutions between $0.35^{\prime\prime}$ and $1^{\prime\prime}$ ($\sim6-17$ pc). At the distance of Cen A ($D_L = 3.5$ Mpc), this resolution is comparable to the outer radius of AGN tori and allows nuclear and circum-nuclear emission components to be separated for the first time in this system. The analysis targets how AGN-driven shocks and radiation fields modify PAH populations within $\sim10$ pc scales.

Observations and data reduction

The data were obtained under Cycle 1 GTO program ID 1269 using a $1\times2$ mosaic with a 4-point extended-source dither pattern, 600 s of on-source integration per grating setting, and dedicated off-source backgrounds. Reduction used the JWST pipeline v1.14.1 with lowered cosmic-ray jump thresholds, pixel-by-pixel background subtraction, residual fringe correction, and bad-pixel self-calibration; master background subtraction and sky matching were disabled due to artefact introduction. Cubes were astrometrically registered to Gaia DR3 to better than $7^{\prime\prime}\times12^{\prime\prime}$0. No band stitching was required beyond a minor additive offset of 0.025 Jy between Ch 1A and Ch 2A–B, reflecting the improved spectrophotometric calibration.

The resulting spectrum shows prominent PAH features at 6.2, 7.7, 8.6, 11.3, 12.0, and 16.5 $7^{\prime\prime}\times12^{\prime\prime}$1m, weaker features at 12.7, 13.5, and 14.2 $7^{\prime\prime}\times12^{\prime\prime}$2m partially obscured by the strong AGN torus continuum and the 9.8 $7^{\prime\prime}\times12^{\prime\prime}$3m silicate absorption, together with H$7^{\prime\prime}\times12^{\prime\prime}$4 rotational lines S(1)–S(8) and ionized-gas fine-structure lines.

Methods

Two complementary extraction strategies were adopted. For two-dimensional mapping, the four MRS channels were merged into a "super-cube" resampled to a common spaxel scale of $7^{\prime\prime}\times12^{\prime\prime}$5 ($7^{\prime\prime}\times12^{\prime\prime}$6 pc). Continuum was removed via linear fits over narrow windows immediately adjacent to each PAH band, and fluxes integrated over fixed intervals while excluding overlapping gas lines (e.g., Pfund $7^{\prime\prime}\times12^{\prime\prime}$7, [Ne II], H$7^{\prime\prime}\times12^{\prime\prime}$8 S(4) blended with the 7.7 $7^{\prime\prime}\times12^{\prime\prime}$9m complex). The authors caution that local continuum subtraction may suppress broad PAH wings and that residual line contamination cannot be fully excluded; consequently, mom0 maps are not used for intensity ratios.

For quantitative measurements, one-dimensional spectra were extracted from five regions: the full Ch 1A mosaic, the nucleus (a conical aperture of twice the wavelength-dependent FWHM), the circum-nuclear region (mosaic minus nucleus), the PAH ring (pixels above $\sim100\times200$0 in the 16.5 $\sim100\times200$1m map), and the PAH-deficient region (its complement, excluding the nucleus). Spectra over $\sim100\times200$2m were decomposed with SPIRIT, which fits non-parametric modified blackbody continua with differential extinction, Drude-profile PAHs with asymmetric subcomponents, and stellar templates, while masking emission lines. An independent decomposition with CAFE was performed as a robustness check.

Spatial distribution of PAH emission

All PAH features share nearly identical morphologies (with the caveat that the 7.7 $\sim100\times200$3m map may be contaminated by blended emission lines), following a ring-like structure at a radius of $\sim100\times200$4 pc from the active nucleus, with clumpy intensity enhancements offset by tens of parsecs from the brightest warm H$\sim100\times200$5 peaks. Emission is deficient within the inner $\sim100\times200$6 pc, from which the jet is launched, although features buried beneath the strong MIR continuum cannot be ruled out at smaller radii.

A distinct PAH-deficient area is found toward the North-West, oriented approximately perpendicular to the jet axis. This region is spatially coincident with enhanced ionized-gas velocity dispersion ([O IV]; Alonso et al.), interpreted as a jet-inflated expanding bubble, and with the innermost inflowing branch of cold molecular gas traced by CO(3–2) (Espada et al.). The co-location of PAH depletion, shock-tracing kinematics, and multi-phase molecular streamers is a central result: it directly links PAH modification to jet–ISM interaction rather than to isotropic AGN irradiation.

Integrated spectra and fit quality

SPIRIT fits reproduce the off-nuclear regions well, with reduced $\sim100\times200$7 of 2 (PAH ring) and 1.8 (PAH-deficient region) and residuals within 10–15%. The nuclear spectrum yields $\sim100\times200$8, with the largest residuals across $\sim100\times200$9m where silicate absorption and continuum dilution dominate. An F-test comparing the default fit against a continuum-only model ($^2$0) confirms that PAH emission is statistically present even in the nucleus (p-value $^2$1), but the strong AGN continuum prevents reliable quantification; nuclear PAH intensities are therefore treated as potentially biased and likely underestimated. Bootstrap uncertainties from SPIRIT (below a few percent) were judged unrealistically small and replaced by Median Absolute Deviation-based errors.

PAH size, charge, and structure

The measured intensity ratios place all five regions at $^2$2 and $^2$3 (except the poorly constrained nucleus), lying beyond the predictions of pericondensed PAH models by Draine & Li, and only marginally consistent with the neutral-PAH grids of Rigopoulou et al. for small species ($^2$4). Large neutral PAHs could raise 11.3/7.7 but would predict low 6.2/7.7, contrary to observation. The authors argue the ratios are naturally explained if the population is dominated by catacondensed PAHs — open, irregular structures (e.g., naphthalene, pentacene) with higher hydrogen-to-carbon ratios than pericondensed species — an interpretation consistent with shock-driven structural processing. They explicitly note two caveats: diagnostics based on 6.2/7.7 may bias conclusions if the population is neutral-dominated (the 3.3 $^2$5m feature would provide an independent test, but lies outside MRS coverage and existing NIRSpec data cover only the central $^2$6 pc$^2$7), and extreme ratios in the nucleus and full mosaic may partly reflect imperfect continuum fitting.

Comparison with literature samples strengthens the interpretation: Seyfert nuclei and outflows from Diamond-Stanic et al., García-Bernete et al., and Zhang et al. occupy the same locus beyond the Draine & Li predictions, generally consistent with neutral PAHs (ionized fraction below 25%) and elevated H$^2$8/PAH ratios indicative of shocks. The pattern implies preferential destruction of ionized PAHs in AGN-driven shocks, with neutral species surviving.

Hydrogenation state

The 11.3/12.7 ratio traces solo versus duo/trio peripheral hydrogen sites. All circum-nuclear regions show ratios $^2$9, indicating dominance of solo sites and partial dehydrogenation, consistent with values reported for other Seyfert nuclei. The most extreme dehydrogenation occurs in the PAH-deficient region, supporting shock-driven erosion of PAH hydrogen mantles there.

Equivalent widths and excitation mechanisms

PAH equivalent widths vary strongly across the five regions: lowest in the nucleus, full mosaic, and circum-nuclear region; largest in the PAH ring (e.g., EW of $0.35^{\prime\prime}$0 nm for the 11.3 $0.35^{\prime\prime}$1m complex); and markedly reduced in the PAH-deficient region. Two regimes emerge when EWs are plotted against the [Ne V]/[Ne II] hardness tracer:

Region	Dominant mechanism reducing EW
Nucleus, Ch 1A mosaic, circum-nuclear	AGN continuum dilution (steep decline with [Ne V]/[Ne II])
PAH ring, PAH-deficient region	Shocks (shallow trend; high [Ne V]/[Ne II])

For the first group, the steep EW decline tracks the strong MIR continua and poor fit quality, so dilution is the natural explanation, though photo-erosion in the nucleus cannot be excluded. For the second group, continuum dilution appears negligible given weak continua and good fits, and the reduced EWs in the PAH-deficient region relative to the ring are attributed to shock-induced partial destruction. Plotting EWs against H$0.35^{\prime\prime}$2 S(1)/PAH luminosity reinforces this dichotomy: the first group's EWs increase with H$0.35^{\prime\prime}$3/PAH (ruling out shocks as the driver there), whereas the PAH-deficient region combines the highest H$0.35^{\prime\prime}$4/PAH ratio with suppressed EWs.

Shock diagnostics

All five regions exceed the threshold $0.35^{\prime\prime}$5 above which non-radiative heating is required, with the maximum value of 0.20 in the PAH-deficient region. The H$0.35^{\prime\prime}$6 S(3)-to-PAH ratios anti-correlate with 7.7/11.3, and the Cen A regions follow the same trend as the shock-dominated central kiloparsec of ESO 137-G034. Neon fine-structure line ratios ([Ne V]/[Ne II] versus [Ne III]/[Ne II]) are inconsistent with pure AGN photoionization models (Feltre et al.) and require a non-negligible shock contribution. Given Micelotta et al.'s results that PAHs with $0.35^{\prime\prime}$7 are severely modified in $0.35^{\prime\prime}$8 km s$0.35^{\prime\prime}$9 shocks and destroyed above $1^{\prime\prime}$0 km s$1^{\prime\prime}$1, the surviving population plausibly comprises small, neutral, catacondensed molecules. One mitigating factor noted is that some prominent PAH emission may arise from clumps shielded by cold H$1^{\prime\prime}$2: the warm H$1^{\prime\prime}$3 column ($1^{\prime\prime}$4 cm$1^{\prime\prime}$5) is too low to protect them, but the cold H$1^{\prime\prime}$6 column of $1^{\prime\prime}$7 cm$1^{\prime\prime}$8 (averaged over $1^{\prime\prime}$9 pc) would suffice — an assumption limited by its large-aperture averaging.

Limitations and open questions

Several limitations qualify the conclusions. Nuclear PAH properties remain effectively unconstrained owing to continuum dilution and silicate absorption. The absence of the 3.3 $\sim6-17$0m feature prevents independent verification of the catacondensed-PAH hypothesis through neutral-species size diagnostics. Residual contamination from gas lines blended with the 7.7 $\sim6-17$1m complex may affect both maps and ratios. Local continuum subtraction may underestimate total PAH fluxes, and the theoretical grids available for comparison do not yet include catacondensed species, so the proposed structural interpretation awaits dedicated modelling. Finally, the shielding argument relies on a column density averaged over scales much larger than the regions of interest.

Conclusion

This work delivers the first sub-arcsecond-resolved census of PAH emission in the ICND of Cen A, resolving a $\sim6-17$2 pc PAH ring, a nuclear cavity, and a jet-perpendicular PAH-deficient zone coincident with shock signatures in the ionized and molecular gas. The combination of extreme 11.3/7.7 and 6.2/7.7 ratios, high 11.3/12.7 ratios, suppressed equivalent widths, and elevated H$\sim6-17$3/PAH luminosity ratios collectively indicates a heavily reprocessed, predominantly neutral, partially dehydrogenated PAH population whose structures are more open and irregular than standard pericondensed models assume, with shock-driven erosion strongest where the jet-inflated bubble interacts with the disc. Extending PAH models to catacondensed species and obtaining observations of the 3.3 $\sim6-17$4m feature are identified as the specific requirements for testing this picture further.