Astronomers Find the Birthplaces of Stars in the Whirlpool Galaxy

Understanding how star-forming works at a galactic scale is challenging in our Milky Way. While we have a general understanding of the layout of our galaxy, we can’t see all of the details head-on like we would want to if we were exploring a single galaxy for details of star formation. Luckily, we have a pretty good view of the entirety of one of the most famous galaxies in all of astronomy – M51, the Whirlpool Galaxy. Now, a team of researchers from the Max Planck Institute for Astronomy has completed a survey of molecules throughout the galaxy and developed a map of potential star-forming regions.

Tracking star formation from far away is best done by monitoring cold clouds of gas and dust formed as part of the creation process. These clouds can span entire galaxies and are tracked by astronomers using two types of molecules – hydrogen cyanide (HCN) and diazenylium (N₂H+). 

Typically, these molecules interact with hydrogen floating in interstellar space and are spun up with some rotational speed. If that rotation is slowed down, say by interacting with other molecules, they emit a specific radio frequency signal at a three-millimeter wavelength.

So far, there haven’t been any telescopes sensitive enough to track HCN and diazenylium outside our galaxy carefully. However, the researchers found a tool that could do so – the Northern Extended Millimetre Array (NOEMA). Located in the French Alps, NOEMA uses a technique called interferometry to detect radio signals much fainter than a single-dished telescope would be able to. 

That sensitivity allowed the researchers to look at the HCN and diazenylium signals of the Whirlpool Galaxy in all regions for the first time. What they found was surprising. Even from a distance of 28 million light years, the researchers can see obvious patterns of gaseous clouds in the spiral arms, signified by signals for both identifying molecules.

However, things get trickier closer to the center of the galaxy. HCN jumps up in brightness compared to the brightness of diazenylium. The researchers think this might be caused by the supermassive black hole at the center of the galaxy pulling the HCN at much higher speeds than out in the spiral arms, which causes friction with other molecules, and again, the type of radio radiation that astronomers would rely on to track the gas clouds.

Diazenylium doesn’t appear to be affected by this phenomenon, so it remains a stable source of information for tracking gas clouds even close to the galaxy’s center. However, it has a very simple disadvantage – it’s up to five times fainter than the signal for HCN. That is where NOEMA comes in.

The researchers used 214 observational hours on the interferometer to watch the Whirlpool galaxy and supplemented it with another 70 hours on a smaller, single-dish radio telescope in Spain. Interferometry data is complicated, though, so it took the researchers over a year to collect, categorize, and analyze it to the point where it is now ready to publish in Astronomy & Astrophysics. 

That’s just a start, though – plenty of other galaxies with star-forming regions could be explored using this technique. However, the Whirlpool Galaxy seems unique in its signal strength for these two molecules in particular. The researchers think collecting data on other galaxies would require even more sensitive telescopes. Luckily, there are plenty of powerful radio telescopes on the horizon, including the next-generation Very Large Array, so hopefully, shortly, researchers will have even more robust tools to peer into the star-forming regions of nearby galaxies.

Learn More:
MPIA – The birthplaces of stars in the Whirlpool Galaxy
Stuber et al. – Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)
UT – Feast Your Eyes on this Star-Forming Region, Thanks to the JWST
UT – Star Formation in the Center of the Milky Way Started at the Core and Then Worked its way out

Lead Image:
This illustration depicts the distribution of diazenylium molecule radiation (false colors) in the Whirlpool Galaxy
Image Credit: Thomas Müller (HdA/MPIA), S. Stuber et al. (MPIA), NASA, ESA, S. Beckwith (STScI), and the Hubble Heritage Team (STScI/AURA)