Shock-driven heating in the circumnuclear star-forming regions of NGC 7582: Insights from JWST NIRSpec and MIRI/MRS spectroscopy

TL;DR

This study uses JWST NIRSpec and MIRI/MRS integral-field spectroscopy to map warm molecular gas in the circumnuclear regions of NGC 7582. Through H$_2$ rotational line analysis, excitation diagrams, and complementary mid-IR tracers, the authors show that the southern star-forming regions SF 1 and SF 2 host molecular gas temperatures near those of the nucleus, best explained by slow $v_s \,\sim 10$ km s$^{-1}$ C-type shocks in dense gas with $n_H \sim 10^{4}$–$10^{5.5}$ cm$^{-3}$ and $G_0 \sim 10^{3}$ Habing. PDRToolbox constraints and Paris-Durham shock modelling, complemented by archival SINFONI rovibrational lines, indicate that starburst-driven shocks, rather than direct AGN photoionisation, dominate heating in SF 1 and SF 2, with possible contributions from an AGN jet not ruled out. The results demonstrate JWST’s capability to resolve multi-phase ISM and feedback processes in AGN environments at ~100 pc scales, advancing our understanding of how star formation and shocks shape circumnuclear gas dynamics. Overall, the work provides a cohesive, multi-tracer view that links H$_2$ excitation, mid-IR diagnostics, and shock modelling to quantify ISM conditions and heating mechanisms near an obscured AGN.

Abstract

We present combined JWST NIRSpec and MIRI/MRS integral field spectroscopy data of the nuclear and circumnuclear regions of the highly dust obscured Seyfert 2 galaxy NGC 7582, which is part of the sample of AGN in the Galaxy Activity, Torus and Outflow Survey (GATOS). Spatially resolved analysis of the pure rotational H$_2$ lines (S(1)-S(7)) reveals a characteristic power-law temperature distribution in different apertures, with the two prominent southern star-forming regions exhibiting unexpectedly high molecular gas temperatures, comparable to those in the AGN powered nuclear region. We investigate potential heating mechanisms including direct AGN photoionisation, UV fluorescent excitation from young star clusters, and shock excitation. We find that shock heating gives the most plausible explanation, consistent with multiple near- and mid-IR tracers and diagnostics. Using photoionisation models from the PhotoDissociation Region Toolbox, we quantify the ISM conditions in the different regions, determining that the southern star-forming regions have a high density ($n_H \sim 10^{5}$ cm$^{-3}$) and are irradiated by a moderate UV radiation field ($G_0 \sim 10^{3}$ Habing). Fitting a suite of Paris-Durham shock models to the rotational H$_2$ lines, as well as rovibrational 1-0 S(1), 1-0 S(2), and 2-1 S(1) H$_2$ emission lines, we find that a slow ($v_s \sim 10$ km/s) C-type shock is likely responsible for the elevated temperatures. Our analysis loosely favours local starburst activity as the driver of the shocks and circumnuclear gas dynamics in NGC 7582, though the possibility of an AGN jet contribution cannot be excluded.

Figures (13)

• Figure 1: Pf$\beta$ (7-5) hydrogen recombination flux map from NIRSpec-IFS, highlighting regions of strong nuclear activity and/or star-formation. Circles denote apertures for which spectra were extracted. Red lines show the edges of the well established ionisation cones morris1985velocity. Dark blue arrows show the proposed direction of the bipolar jets from ricci2018optical. Solid lines show orientation towards us and dashed show orientation away from us. The dotted black ellipse shows the approximate position of the star forming ring. North is up, East is to the left.
• Figure 2: Observed spectrum (rest-frame) for each aperture after applying PSF corrections, but before any extinction corrections. Each line shows the total flux within each aperture at every wavelength bin available across the combined NIRSpec and MIRI/MRS wavelength range. Grey dotted lines show the locations of rotational H$_2$ lines S(1) to S(7). Many atomic emission lines are also clearly visible in each aperture but are not labelled for clarity.
• Figure 3: Flux emission, velocity and velocity dispersion maps for H$_2$ 0-0 S(1), S(3) and S(5). Contours show the respective flux line emission. As $J$ increases, the emission lines are tracing warmer molecular gas. This shows that the nucleus, SF 1, and SF 2 contain more warmer molecular gas than any other region.
• Figure 4: Velocity fit for each spaxel along a pseudoslit of width 3 pixels along PA = $0^\circ$ for the rotational lines S(1), S(3), S(5).
• Figure 5: H$_2$ excitation diagrams for each aperture with three model fits. Top: Power-law temperature distribution with a free OPR. When including rotational lines down to S(1), a best-fit $T_l \sim 300$ K approximately corresponds to an OPR of 3:1. Middle: Power-law temperature distribution with the OPR fixed to 3:1, leaving $n$ as the sole free parameter. Bottom: Single-temperature model, where the only free parameters are the excitation temperature, $T$, and the total H$_2$ column density. A small portion of the Cone 1 aperture extends beyond the MIRI/MRS FOV for S(5), S(6), and S(7), hence these line fluxes are lower limits which we show as triangular markers.