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 $7^{\prime\prime}\times12^{\prime\prime}$ ($\sim100\times200$ pc$^2$) 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 $0.35^{\prime\prime}-1^{\prime\prime}$ (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.

Figures (13)

• Figure 1: Panel A: RGB composite image of Cen A. Red colour shows the 5.8 $\upmu$m emission from Cen A as observed by Spitzer/IRAC Quillen2006_diskQuillen2006_shell; green colour corresponds to X-ray emission observed with Chandra/ACIS Hardcastle2007ApJ...670L..81H; and blue colour traces the radio jet structure imaged with the VLA Hardcastle2003ApJ...593..169H. In black we show the footprint of MIRI-MRS observations. Panel B: Zoom-in on the central few hundred parsecs of Cen A, showing cold molecular gas traced by CO(6--5) and CO(3--2) in blue and green colours, and the VLT/SINFONI H$_2$ 1-0 S(1) intensity map by Neumayer2007ApJ...671.1329N in red Espada2017ApJ...843..136E. White rectangles mark the each MIRI-MRS mosaic of $1\times2$ FoV Evangelista2026. North is up, East is to the left.
• Figure 2: MIRI-MRS spectrum of the central $7.2^{\prime\prime}\times3.6^{\prime\prime}$ region of Cen A, integrated over the Channel 1A mosaic, shown up to $\lambda_{\rm rest}=21$$\upmu$m. The four MRS channels are colour-coded as follows: yellow (Ch 1), cyan (Ch 2), violet (Ch 3), and pink (Ch 4). PAH features commonly observed in galactic and extragalactic environments are highlighted in gray. Ionized gas emission lines and warm molecular hydrogen transitions are marked in black and blue, respectively. Strong silicate absorption feature is evident at 9.8 $\upmu$m.
• Figure 3: Left: mom0 map of the PAH 16.5 $\upmu$m feature in the nuclear region of Cen A at $0.65^{\prime\prime}$ resolution ($\sim11$ pc), obtained after subtraction of the local continuum. Violet contours show the PAH emission at levels of $(0.2,0.25)\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$. Cyan contours trace the H$_2$ 0-0 S(1) 17.03 $\upmu$m rotational line at levels of $(0.19,0.23)\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$. The black arrows indicate the position angle of the jet (i.e., PA$_{\rm jet}=51\,$deg). Bright pixels near the AGN position are artifacts resulting from the local continuum subtraction. Right: 1D extraction regions defined within the Ch 1A mosaic (red filled area), overlaid on the PAH 16.5 $\upmu$m surface density map (greyscale): nucleus (concentric circular areas in black), PAH ring (yellow), PAH-deficient region (green). The circum-nuclear region is defined as the full Ch 1A mosaic minus the nuclear emission. In both panels, the star marks the peak of the continuum, corresponding to the position of the AGN. The [0,0] point on the axes (pixel [11,12]) denotes the centre of the sub-channel array (Ch 3C). North is up, and East is to the left.
• Figure 4: MIRI–MRS spectra extracted from five regions (top to bottom): full Ch 1A mosaic (red), nucleus (magenta), circum-nuclear region (violet), PAH ring (yellow), and PAH-deficient region (green). Extraction regions are shown in the right panel of Fig. \ref{['Fig:PAH16p5']}. Black lines indicate the total best-fit models from the MIR decomposition tool Donnan2024MNRAS.529.1386D; blue and gray lines show the fitted continua (stellar + AGN) and PAH components, respectively. Fit residuals are displayed below each panel; the reduced $\chi^2$ is reported next to the legend.
• Figure 5: PAH 11.3/7.7 $\upmu$m versus PAH 6.2/7.7 $\upmu$m diagnostic plot for the five regions of interest: full Ch 1A mosaic (red circle), nucleus (magenta diamond), circum-nuclear region (violet hexagon), PAH ring (yellow cross), and PAH-deficient region (green star). The large uncertainty on the nuclear PAH 6.2/7.7 $\upmu$m ratio and the and the upper limit on 11.3/7.7 $\upmu$m result from partial detection of the 6.2 and 7.7 $\upmu$m features (Appendix \ref{['App:nucleus']}). Grey symbols show CAFE-derived ratios (Appendix \ref{['App:cafe']}). Uncertainties on the CAFE ratios, which span nearly the full x-range of the figure while remaining close to the SPIRIT y-axis values, were omitted for clarity. We overlay the grids of Rigopoulou2021MNRAS.504.5287R for neutral PAHs (grey; illuminated by $0.5-10^3$ ISRF), and partially ionized PAHs ($25-75$%; blue; 1 ISRF). PAH size increases along the x-axis (right to left) with the number of carbon atoms (N$_C$) while the ionized fraction increases along the y-axis (top to bottom). The dashed black and blue curves mark the neutral and ionized PAH limits from DraineLi2001ApJ...551..807D, for 1 ISRF. For comparison, we include PAH ratios for Seyfert nuclei from Diamond-Stanic2010ApJ...724..140D; and from Garcia-Bernete2022AA...666L...5GGarcia-Bernete2024AA...691A.162G, shown as squares. Triangles denote outflow regions in the same systems. Plus symbols represent the three regions in the central kpc of the Seyfert 2 ESO 137-G034 ZhangLulu2024ApJ...975L...2Z. For clarity, we omit the uncertainties associated with literature ratios.