Unraveling the Mysteries of Star Formation in the Whirlpool Galaxy

For the first time, signatures of individual cold and dense star-forming clouds in a galaxy milky way are mapped over a wide area.

An international research team led by astronomers from the Max Planck Institute for Astronomy (MPIA) has meticulously mapped, in unprecedented detail, the extensive regions of cold and dense gas, future stellar nurseries, in a galaxy outside the Milky Way. Using the NOEMA interferometer, these observations cover a wide expanse within the galaxy, providing insight into the various conditions favorable to star formation. The data represent a breakthrough in this type of measurement, allowing researchers, for the first time, to study the early stages of star formation beyond the Milky Way to the minute as individual gas clouds give birth to stars.

Birthplaces of Stars in the Whirlpool Galaxy

Paradoxically, the evolution of hot stars begins in some of the coldest regions of the universe—thick clouds of gas and dust that permeate entire galaxies. „To explore the early stages of star formation, where gas gradually condenses to eventually form stars, we first need to identify these regions.” says Sophia Stuber, a PhD student at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. He is the lead author of a research paper scheduled for publication in Astronomy & Astrophysics. „To this end, we typically measure the radiation emitted by specific molecules that are particularly abundant in these very cold and dense regions.„

Molecules as chemical probes

Astronomers commonly use molecules such as HCN (hydrogen cyanide) and N₂H⁺ (diazenylium) chemical studies investigating star formation within the Milky Way. „But only now have we been able to measure these signatures in great detail over a wide range within a galaxy outside the Milky Way, covering many different regions with different conditions,” explains MPIA's Research Group Leader Eva Schinnerer. „Even at first glance, it is clear that while both molecules effectively represent dense gas, they also exhibit interesting differences.”

By colliding with a large number of hydrogen molecules, the challenge of detecting them is that other molecules are arranged in a ring. Following a decrease in rotational speed, they emit radiation with characteristic wavelengths, approximately three millimeters for the aforementioned molecules.

The measurements are part of a larger observational project called SWAN (Surveying the Hydrosphere at Arcseconds with NOMA), led by Schinnerer and Frank Bigiel of the University of Bonn. Using the North Extended Millimeter Array (NOEMA), a radio interferometer in the French Alps, the team aims to study the distribution of various molecules within the inner 20,000 light-years of the Whirlpool galaxy (Messier 51), including hydrogen cyanide and diacenium. In addition to the 214 hours of observations from this project, about 70 hours from other observing campaigns with the 30-meter single dish telescope in southern Spain complete the dataset.

„Because the data from the radio interferometers is more complex than the telescope images, it took almost a year to process and refine the data,” notes Jerome Petty of the Institute for Radio Astronomy Millimeter (IRAM), the company that operates the telescopes. Interferometric telescopes such as NOEMA consist of several individual antennas that collectively achieve a detail resolution comparable to that of a telescope with a primary mirror diameter equal to the spacing between the individual telescopes.

Gas properties depend on the environment

As we observe this galaxy from approximately 28 million light-years away, we can distinguish the signatures of individual gas clouds in different regions, such as the center and spiral arms. „We used this scenario to determine how well the two gases trace us the dense clouds in this galaxy and whether they are equally suitable,” Stuber explains.

The radiation intensity of hydrogen cyanide and diazennium continuously increases and decreases in the spiral arms, providing equally reliable results for determining the gas density, astronomers have found a significant deviation in the center of the galaxy. The brightness of the hydrogen cyanide emission increases significantly in this region compared to diasenlium. There appears to be a mechanism by which hydrogen cyanide induces the emission of additional light, which is not observed in diazenylium.

„We suspect that the active galactic nucleus in the Whirlpool Galaxy is responsible,” Schinnerer says. This region surrounds the central massif black hole. Before the gas falls into the black hole, it forms a spinning disk, accelerates to high speeds, heats up to thousands of degrees through friction, and emits intense radiation. This radiation can partially contribute to the additional emission of hydrogen cyanide molecules. „However, we need to investigate in more detail how the two gases behave differently,” Schinnerer adds.

A worthwhile challenge

Thus, at least in the central region of the Whirlpool Galaxy, diacenylium appears to be a more reliable density probe compared to hydrogen cyanide. Unfortunately, it shines five times dimmer on average for the same gas density, significantly increasing the measurement effort. The additional sensitivity required is achieved by significantly longer observation periods.

The prospect of studying the early stages in galaxies outside the Milky Way in greater detail gives scientists hope. A clear view of the Whirlpool Galaxy is not available for the Milky Way. When molecular clouds and star-forming regions are close together in the Milky Way, determining the exact structure and location of the spiral arms and clouds is more challenging.

„Although we can learn a lot from the extensive observing program with the Whirlpool Galaxy, this is in a sense a pilot project,” Steuber points out. „We want to explore many more such galaxies in the future.” However, this possibility currently faces limitations due to technical capabilities. The Whirlpool Galaxy shines exceptionally bright in light of those chemical studies. For other galaxies, telescopes and instruments must be more sensitive.

„The next-generation Very Large Array (ngVLA), currently in planning, will be powerful enough,” Schinnerer believes. If all goes well, it will be available in just ten years. Until then, the Whirlpool Galaxy serves as a rich laboratory for studying star formation at the galactic scale.

Reference: “Probing the Whirlpool at Arcseconds with NOEMA (SWAN) – I. Mapping HCN and N₂Sophia K. Stuber, Jerome Petty, Eva Schinnerer, Frank Bigiel, Antonio Yusro, Ivana Peslik, Miguel Querejeta, Maria J. Jiménez-Donaire, Adam Leroy, Jakob den Broek, Lucas Youmans Hsuan Teng, Ashley Barnes, Melanie Chevance, Dario Colombo, Daniel A. Dale, Simon CO Glover, Daizhong Liu and Hsi-An Pan, 20 December 2023, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202348205

MPIA researchers involved in this study are Sophia Stuber and Eva Schinnerer.

Other contributors are Jérôme Pety (IRAM and Observatoire de Paris/PSL, France [PSL]), Frank Bigiel (University of Bonn, Germany [UB]), Antonio Yuzo (National Astronomical Observatory/IGN, Madrid, Spain [OAN]), Ivana Peslik (PSL), Miguel Querejeta (OAN), J. Maria Jiménez-Donaire (OAN and Yebes Observatory/IGN, Guadalajara, Spain), Adam Leroy (Ohio State University, Columbus, USA), Jakob den Brok (Center for Astrophysics, Harvard & Smithsonian, Cambridge, USA, Lucas Neumann (UP), Cosima Eibensteiner (UP), Yu-Hsuan Deng (University of California San Diego, La Jolla, USA), Ashley Barnes (European Southern Laboratory, Garching). , Germany[[That]), Melanie Sevans (Center for Astronomy, University of Heidelberg, Germany [ZAH] and Cosmic Origins of Life Research DAO), Dario Colombo (UB), Daniel A. Dale (University of Wyoming, Laramie, USA), Simon CO Glover (ZAH), Daizhong Liu (Max Planck Institute for Extraterrestrial Physics, Garching, Germany), and Hsi-An Pan (Tamkang University, Taiwan).