To understand how stars form, astronomers need to observe the process that occurs in galaxies. This simple fact is behind PHANGS, the “Physics at High Angular Resolution in Nearby GalaxieS” survey. It is a large-scale, multi-wavelength, multi-telescope survey of dozens of nearby spiral galaxies. Its targets are galaxies close enough that star formation features such as giant molecular clouds (GMCs), HII regions and star clusters can be resolved.
PHANGS began years ago with observations from telescopes such as ALMA and the Hubble. When the JWST was launched, it also took part. The core question PHANGS addresses is simple: How exactly do gases become stars and how does stellar feedback modulate the process?
PHANGS has created catalogs of data cited in more than 150 scientific papers. It was a major success for astronomers studying star formation and feedback. But a collection of beautiful images was also created, many of which were presented as “Image of the Week” (POTW), “Astronomy Image of the Day” (APOD) as well as other special images and even as an ESA/Hubble calendar. There is also a stamp with the JWST image of NGC 628.
The JWST image of the spiral galaxy NGC 628 is featured on a U.S. Postal Service stamp. Image source: NASA, ESA, Canadian Space Agency and Space Telescope Science Institute. US Postal Service.
The JWST has made an important contribution to PHANGS. It’s something of a missing link in the survey because it can see the dust better than other telescopes. This means it can detect earlier stages of star formation than its comrades.
But as Universe Today readers know, the telescope’s portraits of spiral galaxies are delicious as stand-alone images, even without the scientific context. We were all thrilled by the galactic portraits that JWST gave us in 2023. They placed the creative splendor of nature on a pedestal where it belongs.
This mosaic shows 19 galaxies imaged in near- and mid-infrared light by JWST as part of PHANGS. There is so much beauty and detail that it’s hard to digest it all. Image source: NASA, ESA, CSA, STScI, J. Lee (STScI), T. Williams (Oxford), PHANGS Team, E. Wheatley (STScI)
The latest ESA image of the month shows NGC 5134, a spiral galaxy about 65 million years away. The JWST was captured in both near-infrared and mid-infrared light. MIRI, the telescope’s mid-infrared instrument, captures the light emitted by warm dust in the galaxy. It shows the clumps and strands of gas interwoven throughout the galaxy. NIR, the near-infrared instrument, captures the light from the star clusters that populate the spiral arms.
Galaxies like NGC 5134 have a constant ebb and flow of gas. It’s almost like a giant circulatory system in which gas moves and is recycled through heating and cooling phases through galactic feedback. Individual stars with their stellar winds and supernove explosions play an important role.
The billowing gas clouds in the spiral arms are where most star formation occurs. The star populations differ in different parts of the arms. To understand this, we need to understand something crucial about spiral galaxies: the arms don’t rotate.
Although they look like giant rotating wind turbines, spiral galaxies are not. The arms don’t rotate, just density waves. The waves sweep through the galaxy, condensing the gas, and the arms respond by forming stars.
The inner edge of the arms is prestellar. There are still few stars here and the region is tracked by its CO emissions, recorded by ALMA and the JWST. Some of the interstellar medium is compressed and is visible as dark streaks.
Within each arm is the active star-forming region. The compressed gas collapses to form hot young stars, and the region also contains ionized nebulae, star clusters, protostars, and star clusters still embedded in thick dust, which were revealed by the JWST.
Star formation has dropped off at the trailing edge. Here we find older OB stars, stars drifting away from their birth clusters, and supernovae remnants and bubbles.
Outside the main arms we find intermediate stars such as F, G and K stars. It is also home to older red giants and AGB stars, as well as old open star clusters and diffuse gas. There are very few giant molecular star-forming clouds here.
*ESA’s Image of the Month comes from JWST and its efforts to understand the full complexity of star formation. In the nearby spiral galaxy NGC 5134, gas goes through hot and cold phases as it moves through the galaxy. The gas is compressed in the spiral arms, where hot young stars form. The spiral arms don’t actually move; rather, density waves spiral through the material of the galaxy. Image source: ESA/Webb, NASA & CSA, A. Leroy*
This JWST Image of the Month comes from the GO 3707 observation program. It focuses on the movement of gas in galaxies, which is clearly an important part of star formation. The JWST collected important information relevant to star formation, including detailed information about star clusters, the shape and form of the clouds in which stars form, the connections between gas and dust in the interstellar medium, and how energetic newly formed stars shape their surroundings.
Most galaxies lie beyond the reach of the JWST themselves. The telescope can take images of them, but extensive scientific details are only available for closer spirals such as NGC 5134 and the other spirals in PHANGS. What researchers learn from nearby galaxies can be applied to galaxies far out of reach, including the ones that fill the background of this Image of the Month.
What we learn from these galaxies also helps us understand our own Milky Way Galaxy. In some ways it’s harder to understand because we’re in it.
As far as we can tell, the Milky Way is also a spiral, although some details are unclear. The star formation process here is the same as elsewhere and is shaped by the spiral density waves. If we had a telescope far enough away, the Milky Way would probably appear as magnificent as NGC 5134.
Perhaps somewhere out there in the cosmic vastness, another intelligent species like us, lacking in wisdom but technologically advanced, is gazing upon our galaxy. Maybe they celebrate the Milky Way as an example of the creative power of nature.
Or maybe not.
