University of Michigan, Ann Arbor | 2019 Jun 10
If astronomers want to learn about how supermassive black holes form, they have to start small—really small, astronomically speaking.
In fact, a team including University of Michigan astronomer Elena Gallo has discovered that a black hole at the center of a nearby dwarf galaxy, called NGC 4395, is about 40 times smaller than previously thought. ...
Currently, astronomers believe that supermassive black holes sit at the center of every galaxy as massive as or larger than the Milky Way. But they’re curious about black holes in smaller galaxies such as NGC 4395 as well. Knowing the mass of the black hole at the center of NGC 4395—and being able to measure it accurately—can help astronomers apply these techniques to other black holes. ...
To determine the mass of NGC 4395’s black hole, Gallo and her fellow researchers employed reverberation mapping. This technique measures mass by monitoring radiation thrown off by what’s called an accretion disk around the black hole. An accretion disk is a mass of matter collected by the gravitational pull of black holes.
As radiation travels outward from this accretion disk, it passes through another cloud of material farther out from the black hole that’s more diffuse than the accretion disk. This area is called the broad-line region.
When the radiation hits gas in the broad-line region, it causes atoms in it to undergo a transition. This means that the radiation bumps an electron out of the shell of an atom of hydrogen, for example, causing the atom to occupy a more energetic level of the atom. After the radiation passes, the atom settles back into its previous state. Astronomers can image this transition, which looks like a flash of brightness.
By measuring how long it takes for the accretion disk radiation to hit the broad-line region and cause these flashes, the astronomers can estimate how far the broad-line region is from the black hole. Using this information, they can then calculate the black hole’s mass. ...
Gemini Focuses on a Mid-sized Galactic Black Hole
Gemini Observatory | 2019 Jun 10
A 10,000-Solar-Mass Black Hole in the Nucleus of a Bulgeless Dwarf Galaxy ~ Jong-Hak Woo et al