JWST Observations of Starbursts: PAHs Closely Trace the Cool Phase of M82's Galactic Wind

TL;DR

This study leverages JWST/MIRI PAH maps at 7.7 and 11.3 μm to trace the cold phase of M82's galactic wind and compares them with high-resolution CO(1–0), Hα, and X-ray data. The authors quantify how PAH emission correlates with different gas phases, finding a strong PAH–CO relationship in the wind that resembles disk-scale trends, while the PAH–X-ray correlation weakens when distance effects are accounted for, revealing decoupling from hot gas at large radii. Hα traces cloud surfaces and correlates with PAH morphology, consistent with shocks ionizing outer cloud layers; ratio maps show PAHs inhabit regions around and within the wind cone, often at interfaces between hot and cold gas. The PAH-to-neutral gas ratio remains approximately constant up to ~2–2.5 kpc, suggesting the product of PAH abundance and dust-to-gas ratio does not vary significantly along the inner outflow, making PAHs a robust, high-resolution proxy for mapping entrained cold material in galactic winds and informing the cold gas lifecycle in feedback processes.

Abstract

Stellar feedback drives multiphase gas outflows from starburst galaxies, but the interpretation of dust emission in these winds remains uncertain. To investigate this, we analyze new JWST mid-infrared images tracing polycyclic aromatic hydrocarbon (PAH) emission at 7.7 and 11.3~$μ$m from the outflow of the prototypical starburst M82 out to $3.2$ kpc. We find that PAH emission shows significant correlations with CO, H$α$, and X-ray emission within the outflow, though the strengths and behaviors of these correlations vary with gas phase and distance from the starburst. PAH emission correlates strongly with cold molecular gas, with PAH--CO scaling relations in the wind nearly identical to those in galaxy disks despite the very different conditions. The H$α$--PAH correlation indicates that H$α$ traces the surfaces of PAH-bearing clouds, consistent with arising from ionized layers produced by shocks. Meanwhile the PAH--X-ray correlation disappears once distance effects are controlled for past 2~kpc, suggesting that PAHs are decoupled from the hot gas and the global correlation merely reflects the large-scale structure of the outflow. The PAH-to-neutral gas ratio remains nearly flat to 2~kpc, with variations following changes in the radiation field. This implies that the product of PAH abundance and dust-to-gas ratio does not change significantly over the inner portion of the outflow. Together, these results demonstrate that PAHs robustly trace the cold phase of M82's wind, surviving well beyond the starburst and providing a powerful, high-resolution proxy for mapping the life cycle of entrained cold material in galactic outflows.

Figures (7)

• Figure 1: Left: Log-stretched JWST F770W image of M82 with stellar continuum and background subtracted, capturing the strength of the 7.7$\mu$m PAH feature. Blank pixels in the center are where the detector is saturated. Right: Same as left but for the F1130W data tracing the 11.3$\mu$m PAH feature.
• Figure 2: Log-stretch images of M82's multiphase outflow. Top Left: Spitzer 8 $\mu$m image from Engelbracht2006 reflecting mostly PAH 7.7 $\mu$m emission. Top Right: IRAM NOEMA+30-m CO(1--0) emission from Krieger2021 tracing cold molecular gas. Bottom Left: HST H$\alpha$ map tracing $\sim 10^4$ K ionized gas from Lopez2025. Bottom Right: Chandra X-ray emission from Lopez2020 tracing hot $\sim 10^7$ K gas. The contours in all panels show the JWST 7.7 $\mu$m intensity as well as the field of view of the JWST data.
• Figure 3: Correlations between PAH emission and gas tracers at $\sim38$ pc resolution. The left column shows the relationship between JWST F770W and gas tracers, and the right shows the same for F1130W. The color of points indicates their vertical distance from the M82 center. Overplotted are the median ratio of the bands along with the maximum and minimum of the colorbar in Figure \ref{['fig:ratios']}. The top row shows the relation between PAH and CO(1--0) emission, the middle row compares to H$\alpha$, and the bottom to X-ray emission. Black points in each panel show median $y$ values after binning the data by $x$, with error bars showing the 16--84% range in each bin. The best-fit power law describing the bins is shown by the solid black line. Black contours enclose 15, 25, 50, 75, and 95% of the data points, and light-blue triangles in the top panels show the absolute value of CO data where the background-subtracted intensity is negative due to noise fluctuations. Dark blue points and dashed lines in the top row show the PAH-CO(2--1) relationships found for 70 galaxy disks by Chown2025. In each panel we report Spearman rank coefficients ($\rho$) and scatter ($\sigma$) of the data (in dex) about the best fit at distances above and below 750 pc (marked by the black line in the colorbar). All gas tracers show good agreement with PAH emission, and the CO-PAH relationship closely resembles that found in galaxy disks.
• Figure 4: Ratio maps of JWST F770W to other wavelengths. All images were convolved to 2.2$^{\prime\prime}$ and reprojected onto the same grid, and the ratio is shown on a log stretch centered on the median value (light blue), and the colorbar bounds are one dex below and above the median. Black contours mark the JWST F770W intensity. Within the central starburst, PAH emission appears enhanced relative to all other tracers. For the PAH-to-CO ($\mathrm{MJy/sr\;/\;K\;km/s}$), the red patches are where the CO emission is negative; thus they are set to upper limits. Elsewhere the CO and PAHs show a fairly constant ratio (light blue). Both PAH-to-H$\alpha$ ($\mathrm{MJy/sr\;/\;erg/cm^2/s/sr}$) and PAH-to-X-ray ($\mathrm{MJy/sr\;/\;Counts}$) ratios appear enhanced in the galaxy disk (due to extinction) and along the image edges (red), with patchy ratios (white and salmon) visible in the interior. This appears consistent with a picture where the PAH emission arises from cool material surrounding the hot wind or patchy, filamentary cool material is mixed into the wind.
• Figure 5: Profiles as a function of distance from the disk toward the north (left) and south (right). These profiles were constructed at a resolution of 27$^{\prime\prime}$ ($\approx 500$ pc) to match the lowest resolution datasets. The profiles span the area covered by the JWST mapping (3.2$^\prime$$\times$2.2$^\prime$ in the north and 2.4$^\prime$$\times$2.0$^\prime$ in the south). We calculate the total gas surface density of the neutral phase as $\rm{\Sigma_{HI+H_2}}=\Sigma_{HI}+I_{CO(2-1)}\alpha_{CO}^{(2-1)}$ assuming $\rm{\alpha_{CO}^{(2-1)}=1\;M_\odot pc^{-2}}(K\;km\;s^{-1})^{-1}$ and an $R_{21}=0.6$ we derived from the IRAM CO(2--1) and CO(1--0) maps from Leroy2015 . For reference we add a vertical line at 750 pc since below that distance the HI goes into absorption and becomes unreliable. The top row shows the mass surface densities of the HI, H$_2$, and the total neutral gas. Also shown is the fall off in intensity for the 8$\mu$m PAH emission from Spitzer. The shaded areas around the H$_2$ and total gas mass are to mark the uncertainty in the $\alpha_{CO}$ used ($0.5<\alpha_{CO}<2$). The middle row shows ratios that trace the coupling between PAH emission and the cold gas phases: 8 $\mu$m/H$_2$, 8 $\mu$m/HI, and 8 $\mu$m/${\mathrm{M_{HI+H_2}}}$. The bottom row show different radiation field estimates: an analytic $1/r^2$ decline (dashed pink), $\langle U \rangle$ from dust SED modeling (solid violet), and $\mathrm{G/G_0}$ from [C II] PDR modeling (brown squares). The radiation field and PAH/$M_{\mathrm{HI+H_2}}$ ratio behave similarly, showing how the radiation field affects the PAH intensity.