Extragalactic star breeding grounds mapped

Special molecular radiation (highlighted in color) shows the regions with star-forming clouds in the Whirlpool Galaxy. © Thomas Müller (HdA/MPIA), S. Stuber et al. (MPIA), NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)

Where are the regions with particularly high star birth rates in galaxies? For the first time, astronomers have now recorded the characteristic signatures of star-forming clouds on a large scale in a neighboring galaxy of the Milky Way. This special mapping of the so-called Whirlpool Galaxy could now help to investigate the early phases of extragalactic star formation on the size scales of individual star-forming gas clouds, say the scientists.

What is at the beginning of developments? This question is particularly interesting in many areas of science - including astronomy. When it comes to the formation of stars like our Sun, it is assumed that they most often cluster together in cosmic regions with particularly high density of matter. However, calling the corresponding regions in galaxies hotspots would be flawed. As previous studies in our Milky Way show, the development of stars begins in dense galactic clouds made of rather cool gas and dust.
Their identification is therefore a challenge for astronomy. “To do this, we usually measure the radiation of certain molecules that are particularly common in these very cold and dense zones,” says first author Sophia Stuber from the Max Planck Institute for Astronomy (MPIA) in Heidelberg. When researching star formation in the Milky Way, two substances in particular serve as chemical probes: HCN (hydrogen cyanide) and N2H+ (diazenylium).

The Whirlpool Galaxy in sight

In their study, Stuber and her colleagues have now explored the extent to which the spectral signatures of these substances are also suitable for identifying star-forming clouds outside our own galaxy. One may initially ask oneself why one should look into the distance when researching star-forming regions. Surprisingly, however, the view of the galactic neighborhood can be better than that of our own homeland. Because the “worm’s eye view” means we don’t have an overview of the Milky Way: the molecular clouds and star formation regions are closer here, but it is difficult to grasp their structure and position.

That's why the researchers have now set their sights on one of our neighboring galaxies: the so-called Whirlpool Galaxy (Messier 51). It is “only” around 28 million light-years away and is visible to us. This makes it particularly amenable to detailed investigations. “We used this fact to find out how well the two gases can detect the dense clouds in this galaxy for us,” explains Stuber. The Northern Extended Millimeter Array (NOEMA) was used - a radio interferometer in the French Alps. The scientists explain that complex evaluation systems can reveal the radiation signatures of hydrogen cyanide and diazenylium in its data.

A galactic map – and a puzzle

As it turned out, this works in the case of the Whirlpool Galaxy: The team was now able to map the cold and dense gas typical of future star nurseries over a large area. These areas run through dark zones in the spiral arms. “We were also able to measure the signatures in great detail over a wide area that covers different zones with different conditions. Even at first glance, it becomes clear that the two molecules can visualize dense gas with approximately the same ability - but there are also interesting differences," says co-author Eva Schinnerer from MPIA.

While the intensity of the radiation from hydrogen cyanide and diazenylium rises and falls across the spiral arms in line with the gas density, the team found a clear deviation from this rule in the central region of the galaxy: compared to diazenylium, the brightness of the hydrogen cyanide emission increases there disproportionately strong there. According to this, something seems to cause the hydrogen cyanide to glow, but not the diazenylium. “We suspect that the active galactic nucleus in the Whirlpool Galaxy is responsible for this,” says Schinnerer. This is the turbulent zone around the galaxy's central black hole, which emits intense radiation. It could cause the additional emissions of hydrogen cyanide molecules. “But we still have to research what exactly makes the difference between the two gases,” emphasizes Schinnerer.

As the team summarizes, the results now open up the prospect of being able to explore early phases of star evolution outside of the Milky Way: The Whirlpool Galaxy can now serve as the first research object to explore star formation on a galactic scale. “We would like to use this approach to examine more galaxies in the future,” says Stuber. So far, however, the technical possibilities are not yet sufficient for this: the radiation sensitivity is too low. But the astronomers are pinning their hopes on further development of the possibilities: “The ngVLA (next-generation Very Large Array), which is currently being planned, could be correspondingly powerful,” says Schinnerer.

Source: Max Planck Institute for Astronomy in Heidelberg, specialist article: Astronomy & Astrophysics Letters, doi: 10.1051/0004-6361/202348205

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