Could supermassive black holes anchor the tiniest galaxies? | by Ethan Siegel | Starts With A Bang! | Jun, 2025

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posted on Jun. 23, 2025 at 2:44 pm

At the centers of galaxies, orbiting stars experience close interactions with the central supermassive black holes at a galaxy’s core. Stars that pass too close, particularly in multi-star systems, are at risk of receiving hypervelocity kicks and being ejected from the galaxy entirely. While all galaxies have a dark matter halo, many low-mass galaxies, long thought to not have supermassive black holes, may in fact possess them. (Credit: ESO)

The tiniest galaxies of all are the most severely dominated by dark matter. Could black holes be the cause of the extra gravity instead?

Here in our Universe, galaxies come in various sizes.

This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Over time, overdense clumps of matter grow richer and more massive, growing into galaxies, groups, and clusters of galaxies, while the less dense regions than average preferentially give up their matter to the denser surrounding areas. The “void” regions between the bound structures continue to expand, but the structures themselves do not. (Credit: Ralf Kaehler and Tom Abel (KIPAC)/Oliver Hahn)

On cosmological scales, all structures exhibit the same (5-to-1) dark matter/normal matter ratio.

This composite JWST (above left) and Hubble (lower right) image of nearby spiral galaxy NGC 628, located 32 million light-years away, showcases the gas and dust network that traces out the spiral arms and the current star-forming regions within them. For large, high-mass spiral galaxies, it’s very common to find a dark matter to normal matter ratio consistent with the cosmic average: 5-to-1. However, in extreme environments and for low mass galaxies, the ratios can be extreme, both in terms of producing galaxies overabundant and deficient in dark matter. (Credit: NASA, ESA, CSA, STScI, Janice Lee (STScI), Thomas Williams (Oxford), PHANGS Team)

But on smaller scales, tiny galaxies are more dark matter dominated.

Galaxies, when we examine their stars inside, range from ultra-diffuse to ultra-compact, depending on where their stars are located. While some ultra-diffuse galaxies are rich in dark matter and others appear to be dark matter-poor, the general trend is that, with lower mass, galaxies become more and more severely dominated by dark matter, with a lower stellar mass to total mass ratio. (Credit: Sloan Digital Sky Survey, Canada-France-Hawaii Telescope and the NGVS team)

After stars form, winds and radiation expel gas, leaving only dark matter, stars, and stellar corpses behind.

Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity. Although the winds and ejecta are copious, this galaxy’s high overall mass will allow it to retain most of its normal matter. Smaller mass galaxies aren’t so lucky. (Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation))

MarylandDigitalNews.comJune 23, 2025

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