Webb observations of polycyclic aromatic hydrocarbon emission provide some of the deepest and highest resolution views of the cold interstellar medium in several nearby galaxies, including IC 5332, Messier 74, NGC 1365, and NGC 7496.
This Webb image shows NGC 7496, a barred spiral galaxy some 24 million light-years away in the constellation of Grus. Image credit: NASA / ESA / CSA / Webb / Judy Schmidt.
“With Webb, you can make incredible maps of nearby galaxies at very high resolution that provide amazingly detailed images of the interstellar medium,” said Dr. Karin Sandstrom, an astronomer at the University of California, San Diego.
In their research, Dr. Sandstrom and colleagues focused on a specific component of the interstellar medium called polycyclic aromatic hydrocarbons (PAHs).
PAHs are small particles of dust and it’s their small size that makes them so valuable to astronomers.
When PAHs absorb a photon from a star, they vibrate and produce emission features that can be detected in the mid-infrared electromagnetic spectrum — something that typically doesn’t happen with larger dust grains from the interstellar medium.
The vibrational features of PAHs allow researchers to observe many important characteristics including size, ionization and structure.
“NASA’s Spitzer Space Telescope looked at the mid-infrared and that’s what I used in my Ph.D. thesis,” Dr. Sandstrom said.
“Since Spitzer was retired, we haven’t had much access to the mid-infrared spectrum, but Webb is incredible.”
“Spitzer had a mirror that was 0.8 m; Webb’s mirror is 6.5 m. It’s a huge telescope and it has amazing instruments. I’ve been waiting a very long time for this.”
Even though PAHs are not by mass a big fraction of the overall interstellar medium, they’re important because they’re easily ionized — a process that can produce photoelectrons which heat the rest of the gas in the interstellar medium.
A better understanding of PAHs will lead to a better understanding of the physics of the interstellar medium and how it operates.
Astronomers are hopeful Webb can provide a view into how PAHs are formed, how they change and how they’re destroyed.
Because PAHs are evenly distributed throughout the interstellar medium, they allow researchers to see not just the PAHs themselves, but everything around them as well.
Previous maps, such as ones taken by Spitzer, contained much less detail — they essentially looked like galactic blobs.
With the clarity Webb provides, astronomers can now see gas filaments and even ‘bubbles’ blown by newly formed stars, whose intense radiation fields and resulting supernova evaporate the gas clouds around them.
“One of the things I’m most excited about is now that we have this high-resolution tracer of the interstellar medium, we can map all kinds of things, including the structure of the diffuse gas, which has to become denser and molecular for star formation to occur,” Dr. Sandstrom said.
“We can also map the gas surrounding newly formed stars where there is a lot of ‘feedback’ such as from supernova explosions.”
“We really get to see the whole cycle of the interstellar medium in a lot of detail. That is the core of how a galaxy is going to form stars.”
The findings appear in a series of papers published in the Astrophysical Journal Letters.
