Leo’s tiny galaxy orbiting the Milky Way has a huge secret.
According to a new analysis, the central region of Leo I has an absolutely monstrous supermassive black hole compared to the mass of the galaxy. The galaxy itself is about 20 million times the mass of the Sun. The black hole has about 3.3 million solar masses – about 16 percent of the total mass of the galaxy.
Despite the large margin of uncertainty, the result is still a big surprise. This 3.3 million solar masses is fairly close to the mass of Sagittarius A *, the supermassive black hole at the center of the Milky Way. Sgr A * has a mass of about 4 million solar masses, and recent calculations show that the mass of the Milky Way is about 1.3 trillion solar masses.
An absolutely staggering difference in mass ratio. The possible presence of such a large black hole in such a small galaxy is surprising, as we thought we had pretty reliable rules for the relationship of a central black hole to its galaxy. But if confirmed, it could tell us something new about how galaxies and supermassive black holes at their centers grow and develop.
“Such a large mass of a black hole in Leo I is of great importance in many ways,” the researchers wrote in their article.
“This is the first detection of a black hole in a dwarf spheroidal galaxy using spatial resolution kinematics, its mass is similar to the total stellar mass of the system, and it is comparable to the mass of a black hole in the center of the Milky Way.”
In the space around the Milky Way (and around other galaxies too) there is a swarm of dwarf galaxies, some of which are in the process of colliding with another galaxy and being absorbed by it. It is believed to be an important means by which galaxies gradually grow in size.
But not all dwarf galaxies are built the same, and studying them can help us understand galactic diversity and how these objects are formed.
Leo I, located about 820,000 light-years from Earth, is one such potential ejection. Unlike most satellites in the Milky Way, there is not much dark matter – the directly undetectable gravitational glue that binds the universe together.
While we cannot investigate dark matter directly, we can measure it based on the effect it has on what we can detect. For example, stars revolve around their galaxies faster than they would if they were only influenced by detectable matter.
Astronomers at the University of Texas at the MacDonald Observatory in Austin wanted to study the dark matter profile of Leo I, or how the density of dark matter changes from the center of the galaxy to its outskirts.
They made new observations of Leo I with the Harlan J. Smith Telescope at MacDonald Observatory, and then fed that data and complex galaxy models into a supercomputer for analysis. And their results showed that a supermassive black hole must be lurking in the center of the Galaxy.
“Models scream that you need a black hole in the center; you really don’t need a lot of dark matter, ”said astronomer Karl Gebhardt of UT Austin.
“You have a very small galaxy that is falling into the Milky Way, and its black hole is about as massive as the Milky Way. The ratio of the masses is absolutely enormous. The Milky Way dominates; Leo I’s black hole is almost comparable. “
The group’s work showed that previous measurements of the orbital velocities of stars in Leo I showed a strong bias towards the slower stars; therefore, previous calculations could have missed such a huge mass. Because the new study claims to have no such bias, it was able to find what others missed.
But the result is not entirely unprecedented.
In 2014, it was discovered that a dwarf galaxy with a total mass of 140 million solar masses has a supermassive block hole with a clock frequency of 21 million solar masses – about 15 percent of the total mass of the galaxy. Then, in 2017, two more dwarf galaxies with chunks of black holes were discovered with masses of 4.4 million and 5.8 million Suns – 13 and 18 percent of the masses of their host galaxies, respectively.
So, while we do not currently have “an explanation for this type of black hole in dwarf spheroidal galaxies,” according to UT Austin astronomer Maria Jose Bustamante, it may actually be a fairly common occurrence.
And it could help explain why supermassive black holes are getting so supermassive. When two galaxies merge, their supermassive black holes should eventually occur. This means that dwarf galaxies can feed huge black holes.
“If the mass of a Leo I black hole is large, that could explain how black holes grow in massive galaxies,” Gebhardt said.
Research published in Astrophysical Journal…