Recent observations of 19 nearby galaxies by NASA’s Webb Space Telescope confirm that star formation is not only rife within the spiral arms of these galaxies but is also taking place in so-called galactic spurs. These tendril-like spurs act somewhat like bicycle spokes that connect the spiral arms via extensions of gas and dust and molecular clouds that stretch over several hundred light years.
In a paper submitted to The Astrophysical Journal Supplement Series, the authors write that in nearby galaxies, Webb telescope observations “are revolutionizing our understanding of the first phases of star formation and the dusty interstellar medium.” The team credits Webb’s higher sensitivity and its ability to observe in the far infrared wavelength range with facilitating these filaments’ discovery.
The observations were made as part of an international ongoing effort known as PHANGS —- Physics at High Angular resolution in Nearby Galaxies and includes some 150 researchers based mostly in the U.S. and the European Union.
We know that most stars form in the disk of a galaxy, Thomas Williams, a postdoctoral researcher in astrophysics at the University of Oxford in the U.K. and the paper’s lead author, told me in his office. But the news from these latest observations is that star formation is taking place en masse outside these galaxies’ spiral structure, he says.
Galactic Spurs Are Pretty Universal
We’ve seen this in simulations of galaxies for a while, but we’ve never had the kind of sensitivity to see these things in actual galaxies, says Williams. This has really shown us is that spurs are ubiquitous in all kinds of galaxies, and it seems that there is star formation occurring in them all, he says.
How these spurs maintain their structure in a galaxy that is continuously rotating remains poorly understood.
Are these structures fixed?
The spurs probably don’t stick around very long and aren’t rigid over time, says Williams. They must be quite transient; they must move as fast as the galaxies move, he says.
The bottom line is that star formation and galaxy structure seem a bit messier than previously thought.
A classic case in point is the Triangulum Galaxy, a flocculant spiral, located some 2.7 million light years away in the northern constellation of Triangulum. Known by its Messier number M33, the Triangulum Galaxy is classified as flocculant because its spiral arms are not well-defined, and even look a bit fluffy.
Our own Milky Way’s four spiral arms are thought to be more well defined. And Williams points out that these galactic spurs have yet to be observed within our own Milky Way.
As for how stars form in these galaxies’ molecular clouds?
It Starts With Mere Dust
Dust is formed when massive stars explode as supernovae at the end of their lifetimes, says Williams. Dust provides a site for chemistry to take place; essentially, things land on the dust and chemistry happens. And that’s what creates these giant molecular clouds.
At Webb telescope wavelengths, you can actually pick up where the very youngest stars in the galaxies are being born, says Williams. These stars are less than a million years old, he says.
A Wealth Of New Data
We have data in two chunks, the first is 19 galaxies over the first year and then 55 in the next year (which we have about 10 done so far), says Williams. We can now start cataloging the spurs between the spirals, he says.
With an eventual observational treasury of about 90 galaxies, Williams and colleagues should be able to pin down the earliest phases of galactic star formation and learn how it varies across the nearby galaxy population.
Do we really understand how galaxies work?
There’s still work to be done on how galaxies structure themselves and how that affects ongoing star formation, says Williams. But we understand things a lot better now that we can check our simulations against these latest observations, he says.