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The “Odd” Nature of Dark Matter Halos of Ultra-Diffuse Galaxies

Physicists at UC Riverside (UCR) and UC Irvine (UCI) co-led a study in which they discovered that dark matter halos of ultra-diffuse galaxies are very strange, raising queries concerning the researchers’ comprehension of the formation of the galaxy and the universe’s structure.

Hai-Bo Yu is a theoretical physicist at UC Riverside. Image Credit: Samantha Tieu.

Ultra-diffuse galaxies are named so based on their very low luminosity. The spread of baryons—stars and gas—is much more scattered in ultra-diffuse galaxies compared to “normal” galaxies with analogous masses.

Hai-Bo Yu, an associate professor of astronomy and physics at UCR, divulges his thoughts on the findings he and UCI’s Manoj Kaplinghat, a longstanding research partner of Yu’s, in the following Q&A. The Astrophysical Journal has reported details of recently learned ultra-diffuse galaxies and their dark matter halos.

Besides Yu and Kaplinghat, Demao Kong of Tufts University, and Filippo Fraternali and Pavel E. Mancera Piña of the University of Groningen in the Netherlands participated in the study. The first author of the study Kong will be joining UCR later this year.

The study received support via grants from the National Science Foundation, Department of Energy, John Templeton Foundation, the National Aeronautics and Space Administration, the Netherlands Research School for Astronomy, and ASTRON, the Netherlands Institute for Radio Astronomy.

What is a Dark Matter Halo?

A dark matter halo can be defined as the halo of invisible matter that infiltrates and encompasses a galaxy or a collection of galaxies. Although dark matter has never been identified in labs, physicists are certain dark matter, which composes nearly 85% of the universe’s matter, does exist.

What is Odd About Dark Matter Halos of the Ultra-Diffuse Galaxies?

The ultra-diffuse galaxies that were studied by the team are a lot less colossal compared to the Milky Way. They comprise plenty of gas, however, and they possess much higher gas mass than total stellar mass, contrary to what one observes in the Milky Way. The ultra-diffuse galaxies also have large-sized ones.

The scattering of dark matter in these galaxies can be deduced from the motion of gas particles. The presence of baryonic matter surprises the researchers. It is mainly in the form of gas, and is almost adequate to elucidate the measured velocity of gas particles and leaves just a tiny space for dark matter in the inner regions, where most of the gas and stars are found.

This is astonishing as in the case of regular galaxies, whose masses are akin to those of the ultra-diffuse galaxies, it is the reverse: dark matter rules over baryonic matter.

To accommodate this finding, dark matter halos must possess much lower “concentrations.” That is, they possess a lot less mass in their inner areas, compared to those of standard galaxies. In this aspect, the dark matter halos of the ultra-diffuse galaxies are “odd.”

At an initial glance, researchers would anticipate that such low-concentration halos are so uncommon that the ultra-diffuse galaxies would not be present. After studying the data from advanced numerical simulations of cosmic structure formation, however, the team learned that the population of low-concentration halos is greater than expected. 

What Was Involved in the Study?

This is a joint effort. Filippo Fraternali and Pavel E. Mancera Piña (his student) are specialists in the gas dynamics of galaxies. They found that ultra-diffuse galaxies revolve more sluggishly than standard galaxies with comparable masses.

The team worked together to understand measurement data of the gas motion of these galaxies and deduce the distribution of their dark matter. Furthermore, the team examined data from simulations of cosmic structure creation and detected dark matter halos that have analogous properties to those deduced from the ultra-diffuse galaxies.

Findings Raise Questions About Our Understanding of Galaxy Formation/Structure Formation of the Universe

Several questions concern the development and evolution of these recently discovered galaxies. For example, ultra-diffuse galaxies comprise plenty of gas, and no one knows how this gas is preserved during the creation of a galaxy. Furthermore, the study’s findings show that these galaxies could be younger than standard galaxies. The development of ultra-diffuse galaxies is not comprehended correctly, and more studies are required.

What Makes Ultra-Diffuse Galaxies Interesting?

These are incredible objects to explore because of their astonishing properties, as deliberated in the study. The recently discovered ultra-diffuse galaxies offer new perspectives on the additional exploration of galaxy formation, perhaps even the identity of dark matter.

Journal Reference:

Kong, D., et al. (2022) The Odd Dark Matter Halos of Isolated Gas-rich Ultradiffuse Galaxies. The Astrophysical Journal.


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