Universe Today
To understand how stars form, astronomers need to watch the process play out in galaxies. That simple fact is behind PHANGS, the Physics at High Angular resolution in Nearby GalaxieS survey. It's a large-scale, multiwavelength, multitelescope survey of dozens of nearby spiral galaxies. Its targets are galaxies close enough that star-forming features like giant molecular clouds (GMCs), HII regions, and stellar clusters can be resolved.
PHANGS started years ago with observations from telescopes like ALMA and the Hubble. When the JWST was launched, it participated as well. The core question that PHANGS is addressing is simple: How exactly does gas become stars, and how does stellar feedback modulate the process?
PHANGS has generated catalogs of data that's been cited in more than 150 scientific papers. It's been a huge success for astronomers who study stellar formation and feedback. But it's also generated a collection of gorgeous images, many of which have been featured as a Picture of the Week (POTW), Astronomy Picture of the Day (APOD), as well as other featured images, and even an ESA/Hubble calendar. There's also a postage stamp featuring the JWST's image of NGC 628.
The JWST's image of the spiral galaxy NGC 628 is featured in a US Postal Service stamp. Image Credit: NASA, ESA, Canadian Space Agency, and Space Telescope Science Institute. US Postal Service.
The JWST has made an important contribution to PHANGS. It's kind of like the missing link in the survey, because it can see inside dust better than other telescopes. That means it can see 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 excited by the galactic portraits the JWST gifted us in 2023. They placed Nature's creative glory on a pedestal where it belongs.
This mosaic shows 19 galaxies imaged in near- and mid-infrared light by the JWST as part of PHANGS. There's so much beauty and detail it's hard to digest it all. Image Credit: NASA, ESA, CSA, STScI, J. Lee (STScI), T. Williams (Oxford), PHANGS Team, E. Wheatley (STScI)
The latest ESA Picture of the Month features NGC 5134, a spiral galaxy about 65 million years away. The JWST 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 woven throughout the galaxy. NIR, the Near-Infrared Instrument, captures the light from the clusters of stars that populate the spiral arms.
Galaxies like NGC 5134 feature a constant ebb and flow of gas. It's almost like a vast circulatory system, where gas moves around and is recycled through heating and cooling phases by galactic feedback. Individual stars play an important role in this with their stellar winds and supernove explosions.
The billowing clouds of gas in the spiral arms is where most of the star forming action takes place. Populations of stars differ in different parts of the arms. To understand that, we have to understand something critical about spiral galaxies: the arms don't rotate.
Even though they look like giant rotating pinwheels, that's not what spiral galaxies are. The arms don't rotate, only density waves do. The waves sweep through the galaxy, compressing gas and the arms respond to this by forming stars.
The inner edge of the arms is pre-stellar. There are few stars here yet and the region is traced by their CO emissions, captured by ALMA and the JWST. Some of the interstellar medium is becoming compressed and is visible as dark streaks.
Within each arm is the active star formation region. The compressed gas collapses to form hot young stars, and the region also contains ionized nebulae, stellar clusters, protostars, and clusters still embedded in thick dust, made visible by the JWST.
The trailing edge is where star formation has fallen off. This is where we find older OB stars, stars that are drifting away from their birth clusters, and supernovae remnants and bubbles.
Outside of the main arms is where we find intermediate stars like F, G, and K stars. It's also home to older red giants and AGB stars, along with old open clusters and diffuse gas. There are very few giant molecular star-forming clouds here.
*The ESA's Picture of the Month comes from the JWST and its effort to understand all the complexity involved in star formation. In the nearby spiral galaxy NGC 5134, gas is recycled through hot and cold phases as it moves around the galaxy. The gas is compressed inside the spiral arms, where hot young stars form. The spiral arms don't actually move, rather density waves move through the galaxy's matter in a spiral pattern. Image Credit: ESA/Webb, NASA & CSA, A. Leroy*
This JWST Picture of the Month comes from observing program GO 3707. It's focused on how gas moves around in galaxies, which is clearly an important part of star formation. The JWST gathered important information relevant to star formation, including detailed information on star clusters, the shape and form of the clouds that stars form in, the links between gas and dust in the interstellar medium, and how energetic newly-formed stars shape their surroundings.
Most galaxies are beyond the reach of even the JWST. The telescope can capture images of them, but rich scientific detail is only available for closer ones like NGC 5134 and the other spirals in PHANGS. What researchers learn from nearby galaxies can be applied to galaxies well out of reach, including the ones that fill the background of this Picture of the Month.
What we learn from these galaxies also helps us understand our own Milky Way galaxy. In some ways, it's more challenging to understand because we're inside of it.
The Milky Way is also a spiral, as far as we can tell, though some of the details are fuzzy. The star formation process is the same here as it is elsewhere, and is shaped by the spiral density waves. If we had a telescope far enough away, the Milky Way would likely appear every bit as glorious as NGC 5134 does.
Maybe somewhere out there in the cosmic expanse, another intelligent species like us, lacking in wisdom but technologically advanced, is gazing at our galaxy right now. Maybe they're celebrating the Milky Way as an example of Nature's creative power.
Or maybe not.
