JWST/MIRI-MRS view of the metal-poor galaxy CGCG 007-025: the spatial location of PAHs and very highly ionized gas

TL;DR

This study uses JWST/MIRI‑MRS observations of the metal‑poor dwarf CGCG 007‑025 to map the spatial distribution of the 11.3 μm PAH feature and high‑IP mid‑IR gas. A dual approach—spaxel‑scale line fitting for extended gas and integrated spectrum modeling with pahfit—reveals a compact (~50 pc) PAH region coincident with strong [NeV] and other high‑ionization lines, indicating a mix of star formation and a modest AGN contribution (~4–8%). The 11.3 μm PAH is detected while other bright PAHs (e.g., 6.2 μm) are not, implying a population of large, neutral PAHs resilient to hard radiation. The results challenge purely SF models at low metallicity and illustrate that PAH diagnostics and mid‑IR line ratios in metal‑poor environments require incorporating high‑energy ionizing sources, providing a valuable benchmark for ISM studies in the early universe. JWST/MIRI’s capabilities enable spatially resolving faint PAHs in such environments, refining our understanding of dust, star formation, and AGN‑like processes in the first galaxies.

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are key diagnostics of the physical conditions in the interstellar medium and are widely used to trace star formation in the mid-infrared (mid-IR). The relative strengths of mid-IR PAH emission features (e.g., 6.2, 7.7, 11.3 um) are sensitive to both the size and ionization state of the molecules and can be strongly influenced by the local radiation field. However, at low metallicities ( Z < 0.2 Zsun), detecting PAHs remains notoriously difficult, likely reflecting a combination of suppressed formation and enhanced destruction mechanisms. We present new JWST/MIRI MRS observations of the metal-poor (Z = 0.1 Zsun) dwarf galaxy CGCG 007-025. We confirm the tentative PAH detection previously reported from Spitzer data and, for the first time, identify a compact (approx. 50 pc) PAH-emitting region nearly co-spatial with the newly detected [NeV](I.P. = 97 eV) emission and the galaxy's most metal-poor, strongly star-forming region. The 11.3 um PAH feature is clearly detected, while no emission is found from the other typically brighter features, suggesting a PAH population dominated by large, neutral molecules resilient to hard ionizing fields. When compared with models, mid-IR line ratios involving [NeIII], [OIV], and [NeV] can only be reproduced by a combination of star formation and AGN ionization, with the latter contributing 4--8%. The [OIV] and [NeV] luminosities exceed what massive stars or shocks can produce, highlighting a puzzling scenario in line with recent JWST observations of similar galaxies. This work provides a crucial reference for studying the physical conditions of the dust and star formation in low-metallicity starburst regions, environments typical of the early universe.

Figures (6)

• Figure 1: SUBARU/HSC $giy$ color image of the high-$z$ local analog CGCG 007-025, where g, i and y filters correspond to the colors blue, green and red, respectively. Overlaid are the MIRI/MRS channel coverages displayed in blue (channel 1), green (channel 2), yellow (channel 3) and red (channel 4). The slit position from the previous Spitzer/IRS observations is shown in light gray. Inset panel: [Ne$\;$] $\lambda$15.55${\rm\mu m}$ integrated flux (channel 3, PSF FWHM represented in the bottom left of the inset), showing a PSF-like structure as well as more extended diffuse emission. In both images, the white contours trace the H$\alpha$ emission as seen by MUSE valleespinosa2023MNRAS.522.2089D, with the outermost corresponding to H$\alpha$ emission down to $\sim 1\times 10^{-16}$ erg/s/cm$^2$.
• Figure 2: Left panel: Location of the 11.3 ${\rm \mu m}$ PAH feature. The solid pink contour encompasses the region were the 11.3 ${\rm \mu m}$ PAH feature is detected above $3\sigma$ on the smooth spectra. The [Ne$\;$] extended emission, detected at spaxel level with ${\rm S/N > 3}$, is delimited by the solid light green contour. The location of the PSF-like structure is marked with a black cross. The background image corresponds to the F814W HST/WFC3 filter, with the white contours tracing the H$\alpha$ emission as seen by MUSE valleespinosa2023MNRAS.522.2089D. The dark green square displays the FoV coverage of MIRI/MRS channel 2, where the PAH feature is present. Right panel: pahfit modeling of the PAH emitting region in CGCG 007-025. The spectrum is shown in solid-gray, with the best fit modeling displayed in solid-orange and the residuals in light purple in the bottom panel. The dotted-lines show parts of the spectrum not used during the fit. The derived values for the attenuation curve (dashed line) are represented in the right hand side axis of the top plot. In the axis inset, we zoom around the 11.3 ${\rm \mu m}$ PAH feature. The theoretical location of other PAH features such as the 6.2 and the 7.7 ${\rm \mu m}$, not present in the data, are highlighted with vertical green lines.
• Figure 3: Top panel: Integrated spectrum of CGCG 007-025 as seen by MIRI/MRS channel 1. The position of multiple lines are marked with vertical orange-dashed lines, highlighting the richness of emission in this region of the galaxy. In maroon (offset for visualization purposes), the MIRI convolved and resampled spectrum to match the Spitzer/IRS dataset from hunt2010ApJ...712..164H. At lower resolution most of the faint emission lines are undetectable, while the sensitivity of MIRI/MRS allows to resolve individual features in the mid-IR. Bottom panel: Comparison between the Spitzer/IRS spectrum (dark blue line) and the MIRI/MRS spectrum (maroon line) of CGCG 007-025. To guide the eye, the theoretical position of the PAH features are marked with vertical light green lines in the top of the panel, with the darker green being the PAH features reported in hunt2010ApJ...712..164H. Only the bump around 11.3 ${\rm \mu m}$ is visible in the MIRI/MRS data, with the small peak close to 7.42 ${\rm \mu m}$ arising from the Pfund$\alpha$ and Humphrey$\beta$ lines. Note that only [S$\;$] was clearly detected in hunt2010ApJ...712..164H in the shown wavelength range, with the redder wavelengths of the spectrum largely dominated by noise.
• Figure 4: The 6.2 over 11.3 ${\rm \mu m}$ PAH ratios versus [Ne$\;$] $\lambda$15.55${\rm \mu m}$/[Ne$\;$] $\lambda$12.81${\rm \mu m}$. The location of CGCG 007-025 in the diagram is highlighted with an orange arrow. Comparison samples, using Spitzer/IRS, from the literature are shown for context: star-forming galaxies (light green star), LIRGs (dark green squares), and Seyfert galaxies (lime circles) from garciabernete2022MNRAS.509.4256G. Blue compact dwarfs (BCDs) are shown as diamonds (pink for hunt2010ApJ...712..164H and purple for xie2019ApJ...884..136X), with open symbol indicating upper limits on the PAH ratios. The range of typical [Ne$\;$]/[Ne$\;$] ratios for other BCDs is shaded in pink cormier2015AA...578A..53C. Additional Seyfert samples, using MIRI/MRS data, are plotted as lime circles with black edges zhang2024ApJ...975L...2Zramosalmeida2025AA...698A.194R. The recent results from lai2025arXiv250904662L on the BCD IIZw40 using MIRI/MRS are shown as light pink diamonds with black edges. The horizontal dashed lines represent the typical PAH ratios for star-forming regions, AGNs and radio galaxies (see text for references).
• Figure 5: Excitation diagrams [Ne$\;$]/[Ne$\;$] (top panel) and [O$\;$]/[Ne$\;$] (bottom panel) vs. [Ne$\;$]/[Ne$\;$]. Grids color-coded in purple-pink-white show the IMBH models from richardson2022ApJ...927..165R as a function of the AGN fraction. In green, the grid for a 2.5 Myr SSP from martinezparedes2023MNRAS.525.2916M, and in black the shock models from flury2025MNRAS.tmp.1541F. The star corresponds to the integrated line ratios for CGCG 007-025 within the PAH aperture, while the individual spaxels are shown as dots. Literature points for SBS 0335-052E are shown in light blue mingozzi2025ApJ...985..253M. The dashed gray in the top panel is the demarcation line from inami2013ApJ...777..156I, while the dashed gray lines in the bottom panel are the demarcation lines from richardson2025arXiv250507749R. The line ratios of CGCG 007-025 overlap with the 4%-8% AGN fraction grids.