Eight years and 130 observations: that’s all it takes to prove the winds coming from supermassive black holes have accelerated.
Led by University of Wisconsin-Madison Assistant Astronomy Professor Catherine Grier and recent graduate Robert Wheatly, a team of researchers compiled years of data to find that quasars, the cores of galaxies where supermassive black holes are messily feeding, are emitting winds that are speeding up over time. This research may mark the first step in understanding how black holes communicate with the galaxies they’re in, according to Grier.
The team presented their findings on June 11 at the 244th meeting of the American Astronomical Society in Madison. This research was part of the Sloan Digital Sky Survey Project, a spectral imaging collaboration of hundreds of astronomers across the globe, which, according to Grier, is no stranger to producing earth-shattering discoveries.
While this isn’t the first discovery of quasar winds’ acceleration, due to the density of UW-Madison’s data set — made up of about 130 observations spanning almost a decade — it is the first time astronomers have been able to prove this phenomenon with such certainty, according to Grier.
“I think the study with the most data before us had five observations, and we have 130 over eight years. We really just have a great data set to work with,” Grier told The Daily Cardinal.
What are quasars, and why are we observing them?
Supermassive black holes are the home of quasars and are found at the center of some galaxies. The dust and gas falling into supermassive black holes combined with the forces of gravity and friction are what powers quasars, according to NASA.
“The quasar is a supermassive black hole that has a lot of stuff falling into it,” Grier said. “And when stuff falls into a black hole, it swirls around and spirals in, making a disc around the black hole.”
Due to the friction between gas and the dust within the disk, quasars earn their trademark feature: extremely high temperatures and luminosity, according to Grier.
Quasars are ripe for observation because, in addition to their easily visible luminous appearance, looking at them gives us a peek into our universe’s past. Quasars are located up to 13 billion light years away from Earth, meaning the light that reaches telescopes is from tens of billions of years in the past, according to the University of Michigan.
“They made really good ways for us to look at the universe when it was younger,” said Grier.
“Because as we look at things that are farther away in the universe, we're actually looking backward in time because it takes time for the light to travel to us.”
How were the changes in wind speed discovered?
The UW-Madison Astronomy Department has observed around 850 quasars since 2014, according to Grier.
Grier wrote her first paper on quasar winds in 2015, reflecting on a year of observations. At the time, they found rapid changes in the strength of the winds coming from quasars.
After years of continued observations and data accumulation, Grier and her team thought it was time to revisit the quasars, and they were shocked when they found that the winds had gotten increasingly faster.
“We were expecting to see the same thing we saw back in 2015 when I wrote the first paper on it,” Grier said. “But when we looked at this data, we actually found that the wind was getting faster, and we weren't expecting to see that.”
They detected the changing strength and speed of the winds by examining the spectra of various quasars. An object’s spectrum displays the amount of light it emits of each color, allowing researchers to determine its composition by matching the object’s spectral lines to those of specific elements.
Observing the light coming from quasars gives astronomers information about their makeup and temperature. As showcased in the study, the amount of light observed showed how quasars wind speeds have changed over time.
If they have enough energy, scientists speculate these winds could travel far enough out in their galaxies to affect star formation in their host galaxies, according to Grier.
“That's kind of the motivation behind what we're doing, to try to figure out what's going on with these winds to see if they can be the way that the black hole talks to the galaxy around it,” Grier said.
