A completely isolated dwarf galaxy exposed to an exciting loss of gas – the Observatoire de Paris

Dwarf galaxies are affected by the same evolutionary processes that work in large galaxies such as the Milky Way or the Andromeda Galaxy. They are less luminous and less massive than their big sisters, and they are particularly sensitive to the environmental mechanisms caused by the presence of a giant galaxy in their vicinity. In this case, they may be exposed to the effects of gravitational tides or the dynamic pressure caused by the surrounding gaseous medium. To determine the significance of these effects, astronomers can compare the differences between the populations of dwarf galaxies: on the one hand those called “satellites” and, on the other hand, those called “isolated”.

WLM is considered the archetype of an isolated dwarf galaxy. It was discovered in 1909 by Max Wolf, then confirmed by Knut Lundmark and Philibert Melotte (hence the name “WLM” after the initials of its co-discoverers). It is located equally far between the Milky Way and the M31, almost 3 million light-years away. It would have formed in total isolation without any external disturbances. Its mass of dark matter is thus considered to be stable and fixed, up to 90 times greater than its baryonic matter, which consists of stars and gas.

But recent observations made at the MeerKAT radio telescope by an international team led by a CNRS engineer at the Paris Observatory – PSL revealed a strong interaction between the WLM and the intergalactic medium. The neutral hydrogen of the galaxy is repelled by a dynamic pressure effect caused by its own motion through the intergalactic medium.

A completely isolated dwarf galaxy exposed to an exciting loss of gas – the Observatoire de Paris

Composite image, in the optical and radio domain, of WLM, an isolated dwarf galaxy, just a short distance from the Milky Way and the Andromeda Galaxy.

Observations at the MeerKAT radio telescope reveal four gaseous traces of neutral hydrogen (blue transparent), in the wake of its displacement (represented by the orange arrow): a peculiarity of this object, which is not supposed to interact with the intergalactic medium as the surroundings.

Yang et al. / Paris Observatory – PSL / CNRS / ESO / NOAO / MeerKAT

From the deepest radio observations, scientists have actually identified in the WLM four gaseous clouds of neutral hydrogen extending in the opposite direction of its motion in the sky, determined by recent data from ESA’s Gaia satellite. .

The four clouds represent 10% of the galaxy’s total neutral hydrogen mass. We also found that there is a spatial delay between the gas and the stars in the WLM explains Roger Ianjamasimanana, a radio astronomer at Rhodes University in South Africa. With the discovery of these four gas streams, the research team concludes that WLM gas is being driven out of them due to the dynamic pressure exerted by the surrounding gaseous medium.

This discovery is a complete surprise to researchers: The intergalactic medium in which the WLM is located is thought to be almost empty, composed of a thin layer of gas. Until now, scientists have assumed that such a thin medium could not cause such an effect on a galaxy. », Explains Yanbin Yang, CNRS engineer at the Paris Observatory – PSL, and first author of the article.


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To study the effect in more detail, the team performed computer simulations. ” In our simulations, we also found that the observations can only be explained if the intergalactic medium turns out to be much denser than expected. It seems that we have found a very large, unexpected reservoir of matter in areas that are among the least dense in the universe. “, adds the scientist.

Another hypothesis would be that the mass of WLM would be much lower and could even be devoid of dark matter. This would explain how the intergalactic medium, with such low density, could be able to strip WLM at dynamic pressure. But it would also be a groundbreaking result, for so far it is believed that the mass of galaxies is dominated by this mysterious dark matter.

In summary, the team concludes with two scenarios: either these almost “empty” intergalactic regions are not really empty, or the WLM galaxy is deficient in dark matter.

In any case, this study revolutionizes the understanding of dwarf galaxies. In the future, for future work, they can never again be considered completely isolated.


Article ‘Evidence of ram-pressure stripping of WLM, a dwarf galaxy far

away from any large host galaxy ‘by Yanbin Yang et al. (2022) is published in a journal letter Astronomy and astrophysicsApril 27, 2022.

DOI: https://www.aanda.org/10.1051/0004-6361/202243307

International cooperation

The international team includes researchers from the Paris Observatory – PSL and CNRS, as well as the Instituto de Astrofisica de Andalucia (Spain), Rhodes University (South Africa), University of Cape Town (South Africa) South), Victoria University (Canada), University of Montreal (Canada) ), Ouaga University (Burkina Faso), Max-Planck-Institut für Radioastronomie (Germany) and NRC Herzberg Astronomy and Astrophysics (Canada).

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