Molecular clouds in the Whirlpool Galaxy

A detailed study of the molecular clouds in the Whirlpool Galaxy, led by PAWS project, revealed that they are immersed in a kind of fog of molecular hydrogen, much denser than expected. An aura that permeates the entire galactic disk, and the influence of star formation.



To make a star, you know, you thicken a soup at the right point of molecular hydrogen. But is this recipe always working? To this question has attempted to answer the group of international research project PAWS , led by Eva Schinnerer from the Max Planck Institute for Astronomy in Heidelberg, who has patiently surveyed the giant molecular clouds in the Whirlpool Galaxy , drawing the most comprehensive and detailed map of its kind ever built for a galaxy. A map that has forced researchers to revise their views on what they are actually molecular clouds, and under what conditions they can form stars.

The Whirlpool Galaxy, also known as M51, was chosen as the ideal subject for the study. It is a typical spiral galaxy similar to the Milky Way, it is not too far, around 23 million light-years, and it appears to be “flat” compared to us. Moreover, it is an object already widely observed in all wavelengths.

The project PAWS – PDBI arc second Whirlpool Survey – has mainly used the Plateau de Bure Interferometer telescope interferometer ( PDBI ), a set of six deployable antennas 15 meters in diameter each, located in the French Alps and managed by IRAM, the Institut of Millimetric Radioastronomy of Grenoble. With the data of PDBI radio telescope it has been possible to identify more than 1,500 distinct molecular clouds in M51 and reconstruct the overall distribution of molecular hydrogen in the galaxy. Distribution which was in turn correlated with already known, atomic hydrogen, the dust and the different types of stars.

This new “cloud atlas” has for researchers, at least a couple of surprises. The first is that a large amount of molecular hydrogen – about half – is not found in molecular clouds, but in a scattered disc and not delineated, a kind of fog that pervades the galaxy. According to the researchers , this unexpected distribution leads to the conclusion that the external pressure exerted by the fog on molecular clouds is completely irrelevant, as was previously thought , but instead it plays a key role in precipitating the collapse of the cloud and the resulting star formation.



The second surprise is for a specific region, both located in the spiral arms of M51, where no star formation is observed, while, according to current theories, the density of molecular clouds in those areas would necessarily lead to the ignition of new stars. According to the authors, the explanation still calls into question the external pressure, in this case influenced by the mutual motion between the arms of the spiral, of the one part, and the molecular clouds and fog in which they are contained, on the other. For effects of fluid dynamics, the motion reduces the inner pressure of the fog and, consequently, the external pressure on the molecular cloud, with the final result of inhibiting the star formation.

In essence, we could say that the results of the project PAWS add new instructions to the recipe galactic clouds of molecular hydrogen that must be under pressure to clot and start forming stars. Before rewriting the manuals, we really have to wait a while. “So far, the Whirlpool Galaxy is the only example that we have studied in depth. Now we have to make sure that what we found also applies to other galaxies. “” – said Eva Schinnerer

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