A completely isolated dwarf galaxy subject to an exciting (…) –

Dwarf galaxies are affected by the same evolutionary processes that operate in large galaxies such as the Milky Way or the Andromeda Galaxy. They are less luminous and less massive than their older 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 subject 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”.

The 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 equidistant between the Milky Way and M31, almost 3 million light years away. It would have formed in total isolation without any external disturbance. Its dark matter mass is thus considered to be stable and fixed, up to 90 times larger than its baryonic matter, which consists of stars and gas.

However, recent observations performed 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 galaxy’s neutral hydrogen is repelled by a dynamic pressure effect caused by its own motion through the intergalactic medium.

Composite image, in the optical and radio domains, of the WLM, an isolated dwarf galaxy equidistant from the Milky Way and the Andromeda Galaxy.

Observations by the MeerKAT radio telescope reveal four gaseous tracks of neutral hydrogen (blue transparent), in the wake of its displacement (represented by the orange arrow): an oddity for this object, which is not supposed to interact with the intergalactic medium like its surroundings.

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

Indeed, from the deepest radio observations, scientists have identified in the WLM four gaseous clouds of neutral hydrogen extending in the opposite direction of its motion on the sky, as 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 tracks, the research team concludes that WLM gas is driven out of them due to the dynamic pressure exerted by the surrounding gaseous medium.

This discovery is a complete surprise to the researchers: The intergalactic medium in which the WLM resides is believed to be nearly empty, composed of a thin layer of gas. Until now, scientists assumed that this 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 appears that we have found a very large, unexpected reservoir of matter in regions that are among the least dense in the universe. “, adds the scientist.

Another hypothesis would be that the mass of the 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 remove the WLM by dynamical pressure. But it would also be a ground-breaking result, because 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 nearly “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.


The article ‘Evidence for ram pressure stripping of the WLM, a far dwarf galaxy

away from any large host galaxy’ by Yanbin Yang et al. (2022) is published in a journal letter Astronomy and astrophysics27 April 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) Sud), 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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