UAA astronomer puzzles over black holes' sway on galaxies
by Kathleen McCoy |
UAA astronomy professor Erin Hicks hopes to answer a classic chicken-or-egg question: Which came first, the galaxy or the black hole?
To set the stage, black holes are objects of extreme density out there in the cosmos, with a gravitational pull so strong that not even light can escape them. They have thrillingly scary terms, like "event horizon" and "the point of no return," a lip at the mouth of a black hole where the gravitational influence is so strong no escape is possible. It's the stuff of science fiction and nightmares.
Albert Einstein first predicted them in 1916 with his general theory of relativity. It showed that as a star dies, it leaves behind a small, dense remnant core. His equations showed that if the core's mass is more than three times the mass of the Sun, the force of gravity overwhelms all other forces and produces a black hole. There are two types, these stellar remnant black holes, and super massive black holes. Hicks focuses on the latter.
So, what's the connection to galaxies?
Astronomers now believe black holes reside at the center of all galaxies, including our Milky Way. They discovered this by measuring the motions of stars in the center of nearby galaxies and found they were moving much faster than expected.
After many theories failed to explain this, scientists were left with just one: Supermassive black holes were occupied the centers of these galaxies. In some galaxies, material is falling toward this black hole. Before the gas or stars caught in its gravitational pull actually splinter and fall inside, they heat up to very high temperatures and produce tremendous light.
Most galaxies, however, harbor black holes that are dormant, currently consuming very little or no stars and gas. These are relics of the past, from a time in which black holes rapidly consumed material and grew to the mass that we measure today.
Our very own black hole is four million times the size of our Sun. That might conjure up fears of planet Earth getting caught in a black hole's hungry spiral. But Hicks assures us that we are well beyond the gravitational influence of our galaxy's own black hole.
Hollywood doomsday scenarios don't animate Hicks's imagination. She's much more interested in understanding the role black holes play in the evolution of galaxies.
"These galaxies are on such a large scale that the black hole should have no idea what's going on on the outskirts of the galaxy, and the outskirts of the galaxy should have no idea that a black hole is even sitting there," Hicks said.
Yet, there is evidence that an evolutionary link between them exists. The mass of these black holes is tightly related to other properties of the galaxy on very large scales, such as how stars in a galaxy are distributed. They therefore influence each other. And here's that chicken-or-egg question: Is the galaxy influencing the black hole, or is the black hole influencing the galaxy?
Astronomers know that when material moves toward a black hole, some falls in, but some gets shot back out. Images often depict gas jets squirting out in opposite directions from the black hole. "This might be a mechanism for outflows from the region that could potentially reach scales that could influence the whole galaxy," Hicks says.
Hicks's role in all this is figuring out a more accurate way to measure black hole mass. With seed money from UAA's Innovate fund, she'll spend most of next summer modeling the motions of the gas and stars in the vicinity of a black hole to more accurately estimate its mass for a single object. She'll use this "proof of concept" to apply for a National Science Foundation grant next November, extending the method to a larger sample of galaxies.
That work will secure the foundation of a method allowing astronomers to make one simple measurement for a reasonably accurate black hole mass of thousands of galaxies, and to follow and study the evolutionary relationship between black hole mass and galaxies through time.
I was surprised that Hicks could do this work in Anchorage, with no massive telescope at her disposal. But Hicks has been successful in applying for access to government-run facilities in the U.S. and the European Southern Observatory in Chile. In addition, her previous doctoral work at UCLA and post-doctoral work in Germany helped solidify strong collaborations with astronomers with a wide range of expertise. She works by teleconference with these colleagues on a weekly basis.
That happy fact led Hicks and her husband, Nate, a professor of physics at UAA, and their two toddlers, to choose UAA and Anchorage as their new home last summer.
NOTE: A version of this story by Kathleen McCoy appeared in the Anchorage Daily News on Feb. 16, 2014.