Shocked, Heated, and Now Resolved: H$_2$ excitation in the low-luminosity AGN at M58 core with JWST

TL;DR

Using JWST NIRSpec and MIRI MRS, the study maps the inner ~1 kpc of M58 and detects 44 H$_2$ lines, enabling a detailed reconstruction of warm molecular gas excitation and kinematics near a LLAGN. The H$_2$ rotational ladder is consistent with low-velocity ($\sim$10–40 km s$^{-1}$) C-type shocks driven by the jet, described by a continuous power-law temperature distribution with a soft cutoff $T_{\mathrm{cut}}=3500$ K and $T_{\max}=5000$ K, while rovibrational lines show sub-thermal excitation due to low densities and a modest nonthermal component (\sim10–30%) likely powered by X-ray irradiation from the ADAF. The nucleus and lobes exhibit elevated OPR depressions (\text{OPR} $\sim$1.5–2) and complex kinematics, including turbulence and outflow-like features within ~200 pc, but the large-scale disk remains largely intact and thermally heated rather than expelled. The results demonstrate that even low-power jets can subtly reshape the molecular ISM, driving localized shocks and turbulence that regulate nuclear gas reservoirs and star formation, a mode of feedback that JWST is uniquely poised to uncover. The findings highlight the importance of multiwavelength, high-resolution observations to capture the stratified, long-term impact of LLAGN on their hosts.

Abstract

We present JWST NIRSpec and MIRI MRS observations of the central kiloparsec of M58 (NGC 4579), a nearby LINER galaxy hosting a low-luminosity AGN (LLAGN; $L_\mathrm{bol} \sim 10^{42}$ erg s$^{-1}$) with a low-power jet. These data provide an unprecedented view of the warm molecular gas phase and reveal clear signatures of feedback. We detect 44 H$_2$ lines, including bright pure rotational lines (S(1)-S(18)) and rovibrational lines up to $ν=2$, probing a wide range of excitation conditions. Excitation diagrams show that rotational lines follow a power-law temperature distribution with an exponential cutoff, consistent with heating by low-velocity shocks. H$_2$ rovibrational lines deviate from thermal models primarily because of sub-thermal excitation at low density. Additionally, there may be a 10% contribution powered by AGN X-ray photons in the nucleus. The dust lanes associated with the spiral inflow appear dynamically undisturbed but show signs of shock heating, while the inner $\sim$200 pc exhibits turbulent kinematics produced by outflowing molecular gas. These results reveal the subtle yet measurable impact of LLAGN feedback on the interstellar medium, demonstrating that even weak, vertically oriented jets and low radiative accretion rates can perturb molecular gas and regulate nuclear reservoirs. This study highlights JWST's transformative ability to uncover hidden modes of AGN feedback.

Figures (14)

• Figure 1: Left: Composite image showing F200W (blue; stellar continuum), continuum-subtracted H$_2$ 1–0 S(1) from F212N (green), and F770W (red; tracing PAH 7.7 $\mu$m emission). The LOFAR 144 MHz radio contours are overlaid. The NIRSpec and MIRI Channel 4 fields of view are shown with cyan and yellow dashed lines, respectively. Right: Zoom-in highlighting warm molecular gas traced by H$_2$ S(9) (green), S(1) (red), and CO 2-1 (blue). Overlaid contours are C band from VLA (white) and MERLIN (black). Key regions are labeled, including the northern and southern dust lanes as well as shocked regions to the northeast and southwest. Physical scale bars are shown in both panels.
• Figure 2: JWST spectra for the nucleus (blue) and for a circumnuclear region (red). Ionic emission lines are indicated by vertical blue lines. The PAH features and H$_2$ lines are marked along the bottom of each panel. The circumnuclear region exhibits strong PAH emission, pure rotational H$_2$, and low-excitation ionized lines. In contrast, the nuclear spectrum shows prominent high-ionization lines, relatively stronger H$_2$ rovibrational lines, and two silicate features at 10 $\mu$m and 17 $\mu$m. Shaded-gray regions indicate gaps in the NIRSpec instrument.
• Figure 3: Moment maps showing H$_2$, low-ionization [Ar ii], and high-ionization lines [Ne v]. Only pixels with detections above 5$\sigma$ are shown. The dashed and dotted gray lines indicate the jet axis and the galaxy’s major axis, respectively. A black x marker indicates the AGN radio core.
• Figure 4: NIRSpec channel maps, illustrating both rotational and rovibrational kinematics. Top row: H$_2$ 1–0 S(1); Bottom row: H$_2$ S(9). Each column corresponds to a velocity bin (from –500 to +500 km s$^{-1}$). A dashed white line represents the jet axis. Both transitions trace the dust lanes between –300 to +200 km s$^{-1}$, compatible with rotation. The NE feature, strongest at –500 km s$^{-1}$ and persisting to +200 km s$^{-1}$, is identified as the forward shock front. The overall lack of alignment with the jet axis suggests the jet passes outside the disk plane.
• Figure 5: Maps of pure H$_2$ rotational line ratios. Top: S(3)/S(1) show elevated values in the dust lanes and in two-lobed structures near the AGN. White contours represent the OPR; the outermost contour corresponds to 3 (expected in LTE at $T$$\gtrsim$$200$ K), with inner contours decreasing in steps of 0.25 down to 1.5. Middle: S(9)/S(5). Bottom: S(13)/S(7). These ratios reveal that the higher-$J$ lines trace arc-like structures in both lobes. The dashed gray line indicates the jet axis.