Face-on View of Galaxy NGC 4303 Reveals its Arms are Filled with Active Star Formation

Galaxies fill a lot of roles in the universe. The most obvious one is star formation factories. Without that activity, the cosmos would be a very different place. The European Southern Observatory and the Atacama Large Millimeter Array recently zeroed in on the galaxy NGC 4303. Their goal: to take a multi-wavelength view of its star formation activity.

The object was to help astronomers understand how stars form in galactic environments. The resulting image shows a golden glow of molecular clouds of gas threading through the spiral arms and the existence of already-formed stars.

NGC 4303 is a beautiful spiral galaxy located more than 50 million light-years away, in the Virgo Cluster of galaxies. Astronomers rank it as a weakly barred spiral. It also looks like it may have a ring structure within its spiral arms. The arms sparkle with star formation, making it a starburst galaxy. There’s has an active nucleus there, too, likely hiding a supermassive black hole.

This galaxy is classified as a late-type spiral. That means it turned gas into stars more slowly in the past and still has a lot left today. Sure enough, based on this and other studies, it appears very rich in neutral hydrogen. That’s the building block of stars.

The galaxy has a massive collection of stars at its heart. In addition, it sports older star clusters. These all indicate starforming activity in the ancient past. There’s also ample evidence of more recent star-birth activity across the entire galaxy. Bright nebulae highlight places where newborn stars are forming (or about to be born). So, why is this “late-type” galaxy so active when it comes to making stars?

Studying NGC 4303

To answer that requires looking at the galaxy in more than one wavelength of light. Astronomers used the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope to study existing stars in the galaxy. It can image the galaxy in one observation. At the same time, it measures the intensity of light coming from various regions. In doing so, it provides a fascinating “3D” look at the galaxy and its components.

The Atacama Large Millimeter Array provided a different view using millimeter waves (close to radio waves). It specifically observes the clouds of hydrogen in the galaxy. The idea is to compare the amount of gas available for star formation to the populations of stars already formed. By using two different instruments, astronomers get a better idea of what triggers star birth. The joint studies also reveal processes and events that enhance the process.

In addition, they can also figure out what hampers the formation of stars in different regions. For example, the creation of supermassive stars can gobble up the available gas. That leaves very little to form smaller stars. In other places, the deaths of supermassive stars in supernova explosions send out shock waves. Those can trigger the process of star birth in nearby molecular clouds. For NGC 4303, astronomers will use data from this and other observations to figure out the history (and future) of its star formation activity.

Multiwavelength Studies of Galactic Star Formation: The Big Picture

This study of NGC 4303 is part of a larger effort called the Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) project. It uses ground-based telescopes, as well as space-based observatories, to make detailed observations of neighboring galaxies. The idea is to look at all aspects of a galaxy’s structure using as many different approaches in as many wavelengths as possible. In particular, the project wants to study the physics and interactions of gas and star formation against a backdrop of galaxy structure and evolution.

PHANGS is joined by a number of other projects doing similar studies of galaxy evolution and star birth at different wavelengths. These include MUSTANG—the Multi-scale Star Formation across Nascent Galaxies project, which looks at the lifecycle of clouds and starforming regions. Data from that program is important in galaxy formation simulations.

Ultimately, studies of galaxies like NGC 4303 and others will give a detailed understanding of just how galaxies and their stars form and evolve. In the case of NGC 4303, the extent of both past and future star formation looks quite impressive. The almost uniform distribution of neutral gas clouds across its spiral arms and core predicts a very bright future for this galaxy.

For More Information

A Hypnotising Galaxy

The PHANGS Survey