Evidence of Feedback Effects in Low-luminosity Active Galactic Nuclei Revealed by JWST Spectroscopy

TL;DR

This study uses JWST NIRSpec/IFU and MIRI/MRS spectroscopy to examine the nuclear regions (r < 150 pc) of four low-luminosity AGN, revealing that their ionized gas is predominantly excited by fast radiative shocks rather than AGN photoionization, and that their PAH populations show a bias toward larger, often neutral PAHs, consistent with selective destruction of smaller PAHs by feedback. The warm H2 gas is not fully thermalized and appears to be excited mainly by slow, jet-driven molecular shocks, with non-thermal UV and X-ray heating unable to account for the observed H2 emission; jet coupling efficiencies of roughly 0.1–4% can reproduce the H2 fluxes. Together with literature results, the findings indicate that AGN feedback operates across luminosities, but its manifestation depends on luminosity, with kinetic-mode effects being particularly evident in LLAGN. The work demonstrates the diagnostic power of PAH band ratios to distinguish between kinetic- and radiative-mode feedback and motivates larger, spatially resolved JWST studies to map feedback across the AGN population.

Abstract

This letter presents an analysis of the infrared ($\sim 3-28\,μm$) spectra extracted from the nuclear ($r < 150$ pc) regions of four low-luminosity active galactic nuclei (AGN), observed by JWST NIRSpec/IFU and MIRI/MRS as an extension of the Galaxy Activity, Torus, and Outflow Survey (GATOS). We find that, compared to higher-luminosity AGN, these low-luminosity AGN exhibit distinct properties in their emission of ionized gas, polycyclic aromatic hydrocarbons (PAHs), and molecular hydrogen (H$_2$). Specifically, the low-luminosity AGN exhibit relatively weak high-ionization potential lines (e.g., [Ne V] and [O IV]), and the line ratios suggest that fast radiative shocks (with $v_{\rm s}$ of $\sim \rm 100s\,km\,s^{-1}$) are the primary excitation source of ionized gas therein. Under the low-excitation conditions of their nuclear regions, these low-luminosity AGN generally exhibit a higher fraction of PAHs with large size ($N_{\rm C} \gtrsim 200$), reflecting the preferential destruction of smaller PAH molecules by AGN feedback. Furthermore, the H$_2$ transitions in these low-luminosity AGN are not fully thermalized, with slow, plausibly jet-driven molecular shocks (with $v_{\rm s} \leq \rm 10\,km\,s^{-1}$) likely being the extra excitation source. Taken together with results from the literature, these findings indicate that feedback operates in both low- and high-luminosity AGN, albeit its impact varies with AGN luminosity. In particular, systematic variations in PAH band ratios are found across AGN, demonstrating the differing influence of feedback in AGN of varying luminosities and highlighting the potential of PAH band ratios as diagnostics for distinguishing kinetic- and radiative-mode AGN feedback.

Figures (11)

• Figure 1: Top panel: The residual emission line spectrum obtained by subtracting the best-fit model (red curve in the middle panel) from the nuclear $\sim 3-28\,{\rm \mu m}$ spectrum. The positions of key emission lines (not necessarily all detected) are indicated by short-colored lines at the top. Middle panel: Illustrations of the multi-component fitting for PAH feature decomposition (see details in Section \ref{['sec2.3']}), with the positions of the two silicate absorption features and the adopted ice absorption features for each target indicated by short-gray lines at the top. The gray curve is the nuclear NIRSpec/IFU + MIRI/MRS spectra, after masking the emission lines, as well as the CO$_2$ and CO features marked in panel b. The red curve shows the best-fit model, t he blue curve shows the sum of all continuum components (i.e., stellar and dust continuum), and these green curves are Drude profiles representing individual PAH features. Bottom panel: The residual PAH spectrum (black curve) obtained by subtracting all continuum components from the masked nuclear NIRSpec/IFU + MIRI/MRS spectrum. The positions of prominent PAH features around 3.3, 6.2, 7.7, 8.6, 11.3, 12.7, and 17.0 ${\rm \mu m}$ are indicated by short-black lines at the top. Same as in the middle panel, these green curves are Drude profiles representing individual PAH features, and the magenta curve shows the sum of all green curves, i.e., the modeled PAH spectrum. All x-axes and the y-axis of the middle panel are in logarithmic scale, while the other y-axes are in linear scale, and all spectra are in the rest frame. See Figures \ref{['DeSpec_II']}, \ref{['DeSpec_III']}, and \ref{['DeSpec_IV']} in Appendix \ref{['secA0']} for the same plots for NGC 1266, NGC 3190, and NGC 4736.
• Figure 2: Normalized nuclear emission line profiles of [Fe II], [Ar II], [Ne II], and [Ne III], shown in order of increasing ionization potential from lighter to darker colors, along with H$_2\,S(7)$, H$_2\,S(5)$, H$_2\,S(2)$, and H$_2\,S(1)$, whose wavelengths are close to those of the ionized emission lines with the same colors, respectively. The gray shaded regions in each panel indicate the range of instrumental broadening (FWHM) at the wavelengths of these lines. All line profiles are shown in the rest frame.
• Figure 3: Diagnostic diagrams of ionized emission line ratios: (a) [Ne V]/[Ne III] versus [O IV]/[Ne III], and (b) [Ne V]/[Fe II] versus [O IV]/[Fe II]. The reddish and greenish contours show the distributions of model results computed by Zhang.etal.2025 for AGN and fast radiative shocks (including the shock precursor), respectively, with each contour enclosing 30%, 60%, and 90% of the model results from inside to outside. The colored and gray data points represent measurements for the nuclear regions of the four low-luminosity AGN studied here and the six higher-luminosity Seyferts studied by Zhang.etal.2024a, respectively. The line ratios of NGC 1266 are marked as upper limits because [Ne V] and [O IV] flux measurements of this galaxy are below three times of the standard deviation noise of local continuum.
• Figure 4: Diagnostic diagrams of PAH band ratios: (a) 11.3 ${\rm \mu m}$/7.7 ${\rm \mu m}$ versus 6.2 ${\rm \mu m}$/7.7 ${\rm \mu m}$ and (b) 11.3 ${\rm \mu m}$/7.7 ${\rm \mu m}$ versus 11.3 ${\rm \mu m}$/3.3 ${\rm \mu m}$, with the colored data points representing measurements in the nuclear regions of the four low-luminosity AGN. The gray contours represent the PAH band ratio distribution of spatially resolved regions in SINGS SFGs (Zhang.etal.2022). The gray grids represent model results of PAH band ratios by Rigopoulou.etal.2024 for neutral (top grid), 30% ionized (middle grid), and 70% ionized (bottom grid) PAHs of different sizes (with carbon number $N_{\rm C} = 20 - 400$ as marked in order), in the interstellar radiation field (ISRF; the top boundary of each grid) and the $10^3\times$ ISRF (the bottom boundary of each grid). Note that the 3.3 ${\rm \mu m}$ PAH feature in the nuclear region of NGC 4736 is not detected and the corresponding data point in panel (b) is shown as a faint upper limit with an arbitrary value for illustration purposes.
• Figure 5: Diagnostic diagrams of H$_2$ transitions: H$_2\,S(3)$/H$_2\,S(1)$ versus H$_2$ 1-0 $O(5)$/H$_2\,S(3)$, sensitive to H$_2$ rotational and vibrational temperatures, respectively. The colored data points represent measurements in the nuclear regions of the four low-luminosity AGN, while the gray contours show the distribution of the best-matched C-type molecular shock model results taken from Kristensen.etal.2023. Additionally, the solid black curve represents the predicted trend of fully thermalized warm H$_2$ gas at different temperatures, with the corresponding $T_{\rm rot}$ and $T_{\rm vib}$ labeled on the right and upper axes, respectively.