Tiny Galaxy Has Unusually Massive Black Hole and Scientists Don't Know Why

Astronomers have discovered that a small satellite galaxy falling towards the Milky Way is home to an unusually massive black hole.

The black hole at the heart of the dwarf galaxy Leo I, a satellite galaxy of the Milky Way, is nearly as massive as the supermassive black hole at the heart of our galaxy. The findings were published in The Astrophysical Journal.

María José Bustamante, doctoral graduate from the University of Texas, Austin, and co-author of the study on the findings, said in a statement: "there is no explanation for this kind of black hole in dwarf spheroidal galaxies."

Bustamante told Newsweek: "There are several things that are remarkable about this discovery. It's the second-closest known supermassive black hole, the first one being the one at the center of our galaxy. And it lives inside a dwarf spheroidal galaxy, which are galaxies that shouldn't have supermassive black holes inside them.

"This begs the question, is this galaxy special or are other dwarf spheroidals likely to have supermassive black holes inside them?"

The research could not only shake up astrophysical models of how galaxies evolve, but could also help scientists better understand how the mysterious substance dark matter—which accounts for the majority of a galaxy's mass content—is distributed throughout galactic structures.

Researchers chose to study Leo I because it is a dwarf galaxy that seems to have very little dark matter, unlike other galaxies orbiting the Milky Way.

The team of astronomers reached their discovery by studying the dark matter profile of Leo I, which is how the density of dark matter changes from the dwarf galaxy's outer edge through to its center. This is achieved by measuring the gravitational influence of Leo I's dark matter content on its stars.

Dark matter differs from the everyday matter we see all around us because it interacts weakly with other matter and also with light. The only interaction dark matter displays that is measurable is with gravity. That means it can be inferred by its effect on stars and other cosmic bodies.

The faster the stars are moving, the more matter there is enclosed in their orbits. This allowed the team to measure if Leo I's dark matter density increases toward the dwarf galaxy's center.

This central region has been somewhat overlooked in previous studies of Leo I, which focused more on the velocities of individual stars. What the team found was in the measurements previously taken there was a bias towards low-velocity stars. This had the effect of reducing the amount of matter that seemed to be enclosed within the star's orbits.

By removing this bias, the astronomers found that the amount of mass contained at the center of Leo I ballooned tremendously.

This suggested the presence of a compact and truly massive object about 3.3 million times as heavy as the sun. For comparison, Sagittarius A*, the supermassive black hole at the heart of the Milky Way, is 4.4 million times as massive as our star.

The team believes that the presence of this massive black hole in such a small galaxy represents a new way for black holes to merge. The astronomers suggest that over time, as small galaxies like Leo I fall into larger galaxies, the smaller galaxy's black hole merges with that of the larger galaxy, increasing its mass.

"If the mass of Leo I's black hole is high, that may explain how black holes grow in massive galaxies," Gebhardt said.

This could give gravitational wave detectors like LIGO and Virgo an entirely new source of gravitational wave sources to investigate. And because astronomers use these dwarf galaxies to study dark matter, the findings made by the team could shake up the search for answers regarding one of physics' most pressing mysteries.

"There is a really big black hole in a galaxy not so far away. Which is cool in itself. And that there could be more, lurking in those fluffy old galaxies that orbit us," Bustamante concludes. "If this discovery is confirmed and I ever get a chance to name it I would call it 'Yacana,' the shadow of the llama.

"Yacana is one of the dark constellations of the Incas in my home country of Peru, which are formed from the dark patches of the Milky Way."

Article updated to include comments from researcher María José Bustamante.

Milky Way Leo I
(Main) An image of the Milky Way (inset) the small dwarf galaxy Leo I. Astronomers have discovered an unusually massive black hole at the heart of one of the Milky Way’s dwarf satellite galaxies, called Leo I. Gaia/DPAC; SDSS/ESA