Title: JWST Observations of Starbursts: PAHs Closely Trace the Cool Phase of M82’s Galactic Wind
Authors: Sebastian Lopez, Colton Ring, Adam K. Leroy, Serena A. Cronin, Alberto D. Bolatto, Laura A. Lopez, Vicente Villanueva, Deanne B. Fisher, Todd A. Thompson, Lee Armus, Torsten Boeker, Leindert A. Boogaard, Martha L. Boyer, Ryan Chown, Daniel A. Dale, Keaton Donaghue, Kimberly Emig, Simon C. O. Glover, Rodrigo Herrera-Camus, Ralf S. Klessen, Thomas S.-Y. Lai, Laura Lenkic, Rebecca C. Levy, David S. Meier, Elisabeth Mills, Juergen Ott, Evan D. Skillman, J.D. T. Smith, Elizabeth J. Tarantino, Sylvain Veilleux, Fabian Walter, Paul P. van der Werf
First Author’s Institution: Ohio State University, Ohio, USA
Status: Submitted to ApJ Letters
Messier 82 (M82) is often considered the prototypical starburst galaxy. Located relatively close to us (a mere 12 million light years away) and undergoing rapid star formation, M82 is characterized by a beautiful outflow, driven by stellar feedback: winds generated from newly formed stars and supernovae. This outflow is quite complex, consisting of gas phases with a wide range of temperatures and densities. There’s the hot phase, which is gas with temperatures around 10 million Kelvin, visible through X-ray emission. There’s the warm phase, down around 10 thousand Kelvin, seen through optical emission. Then there’s the cool phase, with temperatures down to 100 Kelvin, seen through emission from molecules such as CO (carbon monoxide). The interactions of these different phases are complicated and not fully understood. A significant fraction of the outflow’s mass lies in the cool phase, but we aren’t certain how it formed (whether in the interstellar medium or cooling out of the warm phase in the outflow itself) nor how it evolves and where it goes over time.
Tracing the distribution of this cool gas and understanding its behavior is important for constraining how starburst-driven outflows evolve, but it’s often expensive and difficult to observe due to the relatively faint signals. Today’s paper explores an easier way to observe this cool gas: not looking at the gas itself, but rather emission from dust! Previous work has shown that dust grains, specifically a kind called polycyclic aromatic hydrocarbons or PAHs, are correlated with cool molecular gas in the star-forming disks of spiral galaxies. This work seeks to understand if a similar correlation exists in the outflow of M82, thus providing a new way to explore the cool phase of galactic outflows.
PAHs emit in the infrared, making the James Webb Space Telescope (JWST) an excellent tool for seeing this dust. Figure 1 shows JWST observations of M82, using one specific filter to generate a map of dust emission. With the stars and background subtracted, we can see that JWST provides a wonderfully detailed view of the dust structure within the outflowing wind.
The authors of today’s paper compared this JWST-observed dust profile with other observations of M82 in a range of wavelengths that are sensitive to different gas temperatures. Figure 2 shows four of these observations, overlaid with the JWST emission mapped using white contours. It’s worth noting that, as seen in Figure 1, the JWST observations do not cover the entire region around M82, so comparisons between the data sets should only be drawn within the white contours. The top-left panel shows a Spitzer image, which also shows at the infrared dust emission, so it can be used to get a lower-resolution sense of the dust distribution beyond the JWST mosaic.
Of particular note in Figure 2 is the top-right panel, which shows the CO emission coming from the cool phase. Here we see a good correlation between the dust and the cool gas, particularly when looking at smaller structures on the right hand side of the outflow. There is also decent correlation with the warm phase traced by Hɑ emission, particularly in the structure of the outflow below the disk. The X-ray emission shows less structure than what is observed in the cooler phases. The dust emission visible using the Spitzer image also extends across a wider area than the Hɑ and X-ray emission. Overall, we see the dust aligning with the cool and warm phases much better than the hot.
Figure 3 further quantifies these correlations, specifically looking at the dust and cool gas observations. We see that the relationship from M82’s outflow (black points) is very similar to that found when looking in galaxy disks (blue points). This agreement is good news, as it means that PAH emission is still tracing cool gas even in a starburst wind, but also a bit surprising. Given how different of an environment the disks of spiral galaxies are to a starburst-driven outflow, we might expect the relationship between cool gas and dust to differ. But it seems like the ability of dust to trace the cold gas distribution is relatively independent of the underlying conditions. This work also suggests that there is a decent correlation between PAH emission and the warm gas as well, although the PAH emission extends wider than warm gas (Hɑ) observations. This suggests that both the PAH and Hɑ emission is coming from the edges of the outflow cone, or along the edges of smaller clouds of cold gas being carried by the hot outflow.
Overall, this work showcases the complex relationship between the different phases of M82’s wind. It suggests that we can use PAH emission to trace the cold gas, providing a novel way to explore multi-phase outflows in detail. We are still far from understanding all the dynamics of M82, but detailed observations of the dust from JWST are an exciting new resource.
Astrobite edited by Anavi Uppal
Featured image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation)
