Astronomers determine the mass of the little black hole in the center of the neighboring galaxy

If astronomers want to know how the supermassive black holes are formed, they have to start little by little, really small.

In fact, a team including astronomer Elena Gallo of the University of Michigan discovered that a black hole in the center of a nearby dwarf galaxy, called NGC 4395, is about 40 times more small than we thought before. Their results are published in the journal Nature Astronomy.

Currently, astronomers believe that supermassive black holes are at the center of each galaxy as big as or larger than the Milky Way. But they are also curious about black holes in small galaxies such as NGC 4395. Knowing the mass of the black hole in the center of NGC 4395 – and being able to measure it accurately – can help astronomers apply these techniques to others. black holes.

"The question remains open for small or dwarf galaxies: do these galaxies have black holes, and if so, do they have a scale identical to that of supermassive black holes?" Gallo said. "Answering these questions could help us understand the very mechanism by which these monster black holes were assembled when the universe was in its infancy."

To determine the NGC's black hole mass, Gallo and his fellow researchers used reverb mapping. This technique measures the mass by monitoring the radiation emitted by what is called an accretion disk around the black hole. An accretion disk is a mass of matter collected by the gravitational attraction of black holes.

As the radiation moves outward from this accretion disk, it passes through another cloud of material farther away from the black hole that is more diffuse than the accretion disk. This area is called the wide line area.

When the radiation strikes the gas in the region of the main lines, it causes a transition of the atoms. This means that the radiation pushes an electron out of the shell of a hydrogen atom, causing it to occupy a more energetic level of the atom. Once the radiation has passed, the atom returns to its previous state. Astronomers can visualize this transition, which looks like a flash of brightness.

By measuring the time it takes for the accretion disk to reach the wide-line region and cause these lightning flashes, astronomers can estimate how far the region of the wide lines is from the black hole. . With the help of this information, they can then calculate the mass of the black hole.

"We think the distance depends on the mass of the black hole," said Gallo. "The larger the black hole, the greater the distance and the more the light emitted by the accretion disk will reach the wide zone."

Using the data from the MDM observatory, astronomers calculated that it took about 83 minutes, about 14 minutes, for the radiation to reach the mainline region from the accretion disk. To calculate the mass of the black hole, they also had to measure the intrinsic velocity of the wide-line region, ie the speed at which the cloud of the region moves under the influence of black hole gravity. To do this, they used a high quality spectrum with the GMOS spectrometer on the GEMINI North telescope.

By knowing this number, the speed of the broad line, the speed of light and what is called the gravitational constant, or a measure of the gravitational force, astronomers were able to determine that the mass of the black hole was about 10,000 times greater than the mass of the black hole. our sun, about 40 times lighter than expected. This is also the smallest black hole found via reverb mapping.

"This regime of dwarf galaxies is largely unexplored with respect to the properties of their nuclear black holes," said Gallo. "We do not even know if every galaxy has a black hole, it adds a new member to the family of black holes we have information on."

This information could also help astronomers understand how larger black holes shape the galaxies they occupy. A field called black hole feedback explores how black holes affect the properties of host galaxies at scales much larger than their gravitational attraction should reach.

"There is no reason for stars that live to orders of magnitude larger than the area dominated by the gravity of the black hole should even know that there is a black hole in their galaxy, but they do it one way or another, "Gallo said. "Black holes shape the galaxy in which they live on a very large scale, and since we do not know much about small galaxies with their smaller black holes, we do not know if this is true until the end. With this measure, can add more information to this relationship ".

This result is the result of a partnership between U-M Astronomy and the Department of Physics and Astronomy of the Seoul National University. Observations were made at the GEMINI North Observatory in Hawaii and at the MDM Observatory in Arizona. GEMINI is managed in partnership by the United States, Canada, Chile, Brazil, Argentina and Korea.