Researchers explain how black holes can fill galaxies

Light-based techniques used to study far away stars, big black holes



Visiting scientist Anil Seth, said black holes in some galaxies are 20 million times the size of our sun.

BAILEY CAMPBELL, Evergreen reporter

The twinkling of stars indicates movement, which can be used to determine their mass and ultimately the black hole they surround. Ultra-compact dwarf galaxies can contain black holes 20 million times the mass of Earth’s sun.

Anil Seth, a visiting assistant professor from the University of Utah Physics and Astronomy Department, explained his research team’s discoveries of black holes in ultra-compact dwarf galaxies to students and faculty Tuesday at a Distinguished Colloquium Series hosted by the WSU Department of Physics and Astronomy.

“Astronomy explains the origins of how our galaxy formed,” Seth said, “and black holes are a part of that formation.”

Seth spent five years as a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics. He and his team found black holes in four of the five galaxies they studied.

“We found that there really are black holes in low-mass galaxies,” Seth said.

Seth explained how they have found large black holes in small galaxies. Messier 60 is a nearby, largest known galaxy. It has a black hole four and a half times the mass of the sun.

Seth said there is a smaller galaxy nearby that is possibly falling into Messier 60.

In close proximity to Messier 60 is yet another small galaxy, which has a black hole 20 million times the mass of the sun. Small galaxies like this with massive black holes are called ultra-compact dwarf galaxies, or UCDs. This galaxy is less than one hundred light-years across, yet has a massive black hole that is 15 percent of the galaxy.

“It is one of the densest objects we know of in the universe,” Seth said.

Astrophysicists have noticed that these UCDs live near big galaxies.

“What we think is these UCD’s fall in and have their outer layers stripped away by the big galaxy,” said Seth.

Tidal forces of the bigger galaxy begin to strip away outer layers of the smaller galaxy when it passes close to the center of a neighboring galaxy. Through repeated orbits, the outer layers of the small galaxy get torn away and form structures that dissipate over time, Seth said. The stripped galaxy is thrown to the side, leaving behind a small galaxy with a massive black hole. This explains why a galaxy less than a hundred light-years across can have a black hole 20 million times the mass of the sun.

To measure the mass of a black hole, scientists look for stellar movements.

“We measure the motion of stars to tell us how much mass is there,” Seth said.

By looking at Hubble Space Telescope images of the galaxy, scientists can see how mass is distributed.

“If there is a mismatch between how mass is distributed and what we infer from the mass from the motions of the stars, then that can tell us there is a black hole,” Seth said.

If there is fast motion at the center of a galaxy and it doesn’t come from stars, then it must come from something else, he added.

However, Seth said light doesn’t always trace mass very nicely, making it hard to measure star mass profiles. Seth and his team at the University of Utah use adaptive optics observations to measure star mass.

“When you go out at night and see the stars twinkling,” Seth said, “that is because there is turbulence in our atmosphere which causes the light to bend and dance around.”

Astrophysicists don’t like twinkling stars because it makes them hard to measure, he said. His team uses a telescope to shine a laser at the star to remove the twinkle. When the laser hits, the star becomes distorted, and they undo the distortion by using a mirror. Every time the star moves, they move it back.

Seth explained that different stars at the center of a galaxy have different ages and each age has a different mass, making the mass-to-light ratio change. This is a problem when measuring low-mass galaxies.

“Luminosity can tie the formation of gas in the galaxy to the formation of the black hole,” Seth said. “The black hole can then regulate how many stars are formed within the galaxy.”