When the James Webb Space Telescope (JWST) launched, one of its jobs was studying galactic formation and evolution. When we look around the Universe, today’s galaxies take the shape of grand spirals like the Whirlpool galaxy and giant ellipticals like M60. But galaxies didn’t always look like this.
We don’t see these shapes when we look at the most distant and most ancient galaxies. Early galaxies are lumpy and misshapen and lack the structure of modern galaxies.
A new study based on JSWT observations looks at organic molecules near galactic centers. The researchers say observing these molecules can teach us a lot about galactic evolution.
The molecules in this study are polycyclic aromatic hydrocarbons (PAHs). They’re important building blocks for prebiotic compounds. Those compounds may have played a role in the early formation of life. But they’re not only attractive to scientists because of their connection to life. When PAHs are illuminated with optical and UV radiation from stars, they get excited and are very bright in infrared emission bands. So observing them tells astronomers a lot about conditions inside the galaxy.
The study is “A high angular resolution view of the PAH emission in Seyfert galaxies using JWST/MRS data,” published in the journal Astronomy and Astrophysics. The lead author is Dr. Ismael García-Bernete from Oxford University’s Department of Physics.
This study is based on data from the JWST’s Mid-Infrared Instrument (MIRI.) MIRI is tuned to observe in the 5 to 28-micron range of the electromagnetic spectrum and can provide wide-field imaging. It can also perform medium-resolution spectroscopy.
(Left) The Whirlpool Galaxy is a grand spiral. (Right) M60 is an elliptical galaxy. Modern galaxies have evolved structured forms, while ancient galaxies are misshapen. One of the JWST’s science objectives is to study galaxy evolution. Image Credit: Left: By NASA and European Space Agency – Public Domain, https://commons.wikimedia.org/w/index.php?curid=3863746. Right: By NASA, ESA, CXC, and J. Strader (Michigan State University) Public Domain, https://commons.wikimedia.org/w/index.php?curid=28850575.Galaxy evolution is a highly-active field of research with many papers published every year by researchers around the world. The structural evolution of galaxies is a key component of the overall effort. That’s because the structural evolution is dependent on so many other properties of a galaxy, like star formation, stellar mass, and metallicity. They’re all tied together and when we learn about one of them, it means we’re learning about all of them.
Throught the lifetime of a galaxy, it’ll undergo multiple processes that alter its structure and morphology. These include the formation of galactic bulges and disks, and the end of active star formation. They also include gas inflow, which drives the formation of spiral arms. The main event that drives galaxy evolution is probably mergers with other galaxies.
Since PAHs are widespread in space in different environments and objects, they lend themselves well to understanding galaxy evolution. Their ubiquitous nature makes them valuable to scientists because they can compare how different PAHs behave in different places and understand more about the environments they’re in. This study focused on PAHs in three different Seyfert Galaxies.
Because PAHs get excited by stars and become luminous in infrared, astronomers use them to trace activity inside galaxies. They’re excellent tracers for star formation in star-forming galaxies. They also trace activity in active galactic nuclei (AGN,) the luminous regions at the center of galaxies where the luminosity doesn’t come from stars. In this study the researchers compared the PAH emissions near the AGN with PAH emissions near star-forming regions.
Theory predicts how PAHs should behave. Previous research predicted that PAHs couldn’t survive near the centers of galaxies with active black holes. They should be destroyed in that environment, where energetic photons can shred them.
But thanks to the power of the JWST and this research team, we now know that’s not true. The researchers think the PAHs can survive near the black holes because there might be a large amount of molecular gas near the galactic nucleus.
But they don’t survive unscathed. After all, supermassive black holes (SMBH) are enormous strange and powerful objects that literally warp space time. They also emit powerful radiation, sometimes as high energy photons in x-ray and gamma-ray wavebands. So even though PAHs can survive near SMBHs, they don’t all survive. This study showed that smaller molecules and charged molecules were destroyed in this environment, but larger and more neutral molecules did survive. This places important limitations on using PAHs as tracers in the future.
This image from the study gives more detail. The panel on the left mainly traces PAHs in the 7.7 micron band. Black circles with S1 to S7 are circumnuclear zones in one of the galaxies, NGC 7469. The researchers measured those regions so they could compare PAHs in star formation regions vs AGN regions. Red and blue circles 01 to 06 are in the outflow region. The green bar is the nuclear molecular gas bar. The panel on the right shows PAHs in the 6.2 micron band. In both images, red indicates brighter IR emissions. The main takeaway is that PAH emissions generally trace star-forming regions. In the nuclear region near the black hole, emissions in 7.7 microns and 6.2 microns are lower. Image Credit: Bernete et. al. 2022.This is some of the first research to result from JWST observations. Some of the observations in this study come from the telescope’s initial commissioning phase. So there’s much more of this type of research coming in the future. For this team that’s important. In their conclusion they write, “In the near future, detailed studies such as this one – that take advantage of the unprecedented capabilities of JWST and are carried out not only with larger samples of galaxies but covering wider luminosity, hydrogen column density, and Eddington ratio ranges – are needed to improve the statistic of the results reported in this work.”
But while looking forward to research that builds on these results, lead author García-Bernete is also enthusiastic about what they’ve accomplished so far. The JWST’s observing power is turning PAHs into an even more accurate too.
“This research is of great interest to the wider astronomy community, particularly those focused on the formation of planets and stars in the most distant and faint galaxies,” said Dr. García-Bernete. “It is incredible to think that we can observe PAH molecules in the nuclear region of a galaxy and the next step is to analyse a larger sample of active galaxies with different properties. This will enable us to better understand how PAH molecules survive and which are their specific properties in the nuclear region. Such knowledge is key to using PAHs as an accurate tool for characterising the amount of star formation in galaxies, and thus, how galaxies evolve over time.’
