Dark matter halos of ultra-diffuse galaxies are very odd, raising questions about physicists' understanding of the structure of the universe.
The ultra-diffuse galaxies are very low in luminosity. Gas and stars are spread out more in ultra-diffuse galaxies than in normal ones.
In the following Q&A, Hai-Bo Yu, an associate professor of physics and astronomy at UCR, gives his thoughts on the findings he and UCI's Manoj Kaplinghat have published in The Astrophysical Journal.
Yu and Kaplinghat were joined in the research by other people. Kong is the first author of the book.
A dark matter halo is a question.
A dark matter halo is the halo of invisible matter around a galaxy. Dark matter, which makes up 85% of the universe's matter, has never been found in laboratories.
What do you think about the halos of the ultra-diffuse galaxies? Is it odd that you are comparing them to something?
We found that the ultra-diffuse galaxies are not as big as the Milky Way. They have more gas than the total stellar mass, which is different to what we see in the sky. Large sizes are also found in the ultra- diffuse galaxies.
The motion of gas particles can be used to estimate the distribution of dark matter. The presence of baryonic matter, mostly in the form of gas, is enough to explain the measured velocities of gas particles and leaves little room for dark matter in the inner regions, where most of the stars and gas are located.
In normal galaxies, dark matter dominates over baryonic matter, which is very surprising. We conclude that the halos must have lower concentrations. They have less mass in their inner regions. The halos of the ultra-diffuse galaxies are not normal.
One would think that the ultra-diffuse galaxies would not exist because of the low concentration halos. The population of low-concentration halos is higher than expected after looking into the data from the state-of-the-art simulations.
The study was done.
This is not a one man show. Filippo Fraternali is an expert on gas dynamics. They found that the ultra-diffuse galaxies were rotating at a slower rate than normal ones. We were able to infer the dark matter distribution from the measurement data we used. We found dark matter halos that have the same properties as those inferred from the ultra-diffuse galaxies by analyzing data from simulations of Cosmic structure formation.
Questions are raised about our understanding of the structure of the universe. What is the way?
There are a lot of questions regarding the formation and evolution of these new galaxies. We don't know how gas is retained during galaxy formation in the ultra- diffuse galaxies. Our results show that the galaxies are younger than normal ones. There is more work that needs to be done on the formation of the galaxies.
Why are ultra-diffuse galaxies so interesting?
The objects discussed in our work are amazing. Our understanding of galaxy formation is likely to be tested by the newly discovered ultra-diffuse galaxies.
More information: Demao Kong et al, The Odd Dark Matter Halos of Isolated Gas-rich Ultradiffuse Galaxies, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac8875 Journal information: Astrophysical Journal
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