A team of astronomers, including Penn State researchers, from the Sloan Digital Sky Survey (SDSS) has used eight years of monitoring observations to discover unexpected changes in the winds surrounding a distant black hole.
Their work - presented at the American Astronomical Society (AAS) meeting in Madison, Wisconsin, and published today (June 11) in the Astrophysical Journal - revealed that the high-speed motions of gas surrounding the supermassive black hole at the center of the quasar known as SBS 1408+544 have been accelerating. The results were featured during a press conference that was livestreamed and is available to view at the AAS Press Office YouTube page.
"We would have never determined this was acceleration if we hadn't had eight years of data to work with," said Robert Wheatley, a recent graduate from the University of Wisconsin-Madison and the lead author of the paper. "Previous studies looking at quasar winds just didn't have the data to make such detailed measurements."
Supermassive black holes (SMBHs) reside in the centers of most - if not all - massive galaxies across the Universe; astronomers investigate these black holes by examining the behavior of the surrounding material as it becomes overwhelmed by their strong gravitational pulls. The material that wanders too close to a SMBH gets pulled in, forming an "accretion disk" of swirling gas and dust on its way to being devoured by the black hole. If the SMBH has a constant supply of new material to consume, the disk remains in a steady state, and the material within it is heated to extremely high temperatures, emitting light across the entire electromagnetic spectrum. These SMBHs are called "active," and the most luminous ones are referred to as quasars; the accretion disks in quasars are so luminous that we can observe them out to billions of light-years away.
Astronomers study quasars to learn about SMBHs and to understand the Universe when it was very young.
"The light observed from quasars is variable, changing in brightness over time," said Yasaman Homayouni, Eberly Research Fellow at Penn State and a collaborator on the investigation. "Careful study of these variations effectively probes the immediate physical environment of the black hole."
To investigate quasars, astronomers examine their spectra, which is a measure of how much light the quasar gives off at each wavelength, from ultraviolet through the full visible spectrum and into infrared. A spectrum can reveal far more about a quasar than a simple telescope image, so by repeatedly measuring spectra over many years, astronomers can watch quasar light fluctuations and learn about the motion of the gas in the accretion disk, which can be used to determine the mass of the SMBH.
To learn about SMBHs in great detail, the SDSS-V collaboration has repeatedly observed thousands of quasars as part of its Black Hole Mapper (BHM) program. SDSS observes roughly 1,800 quasars as a part of a high-cadence monitoring project, obtaining more than 100 spectra of each quasar over the course of several years. In addition to the current observations, almost 500 of these quasars have been observed as a part of a previous SDSS program called the Sloan Digital Sky Survey Reverberation Mapping Project; these quasars have been monitored by SDSS since 2014, and team members have already amassed more than 150 spectra of these quasars.
"This kind of dataset is unprecedented," said Catherine Grier, an assistant professor of astronomy at the University of Wisconsin-Madison and Wheatley's research supervisor while he worked on this investigation. "We have hundreds of spectral observations each for thousands of quasars, which allows us to study quasar variability on both long and short timescales in a very large quasar sample."
Many of the quasar spectra in the sample show evidence for what astronomers call "Broad Absorption Line (BAL) regions," which are produced by winds that launched off of the quasar accretion disk itself. These winds are carried away with enormous amounts of energy and can potentially have a strong effect on the galaxy that the quasar lives in by regulating star formation in its surroundings.
The team has been investigating many of these quasars observed with SDSS since 2014, a few of which have been the subject of previous investigations. In 2015, Grier and colleagues studied a quasar known as SBS 1408+544, located in the constellation Boötes, that showed signs of unusually fast-changing winds. However, that study included only one year of SDSS data - enough to recognize the quasar as changing, but not enough to learn how it was changing.
Years later, Grier and her team revisited this quasar, now with eight years of monitoring and more than 130 spectra in hand and were surprised to discover that not only was the BAL variable in strength, but that the gas outflow was accelerating.
"The wind was already moving incredibly fast - more than 10,000 miles per second, and we've found that it's only getting faster!" Wheatley said.
Quasar winds are thought to be produced by strong radiation from the accretion disk blasting gas off of it. If this theory is correct, BALs should display acceleration as the winds are launched. However, as Wheatley noted, observations of acceleration are extremely rare because it is incredibly difficult to disentangle acceleration signatures from more typical variability in strength and shape of the absorption line. The team's work implies that BALs may be accelerating far more often than we thought.
"There have been a few other reports of possible acceleration, but most other studies only had a handful of observations spread over several years, and it's really hard to tell for sure whether you are seeing real acceleration," Grier said. "We now have very solid evidence for acceleration, which we can use to refine our theories for how these winds are produced."
The researchers said this work could inform theorists who are working on theories of quasar wind production to better understand interactions between quasar winds and their home galaxies, as well as galaxy evolution. The SDSS-V Black Hole Mapper team will continue to monitor and investigate the hundreds of BAL quasars observed by the BHM program over the next few years.
"It has been a delight, over the past 15 years, to help push forward time-domain quasar spectral studies with the SDSS," said W. Niel Brandt, Eberly Family Chair Professor of Astronomy and Astrophysics and professor of physics at Penn State and a project collaborator. "The incredibly rich resulting data have spun off a feast of astronomical surprises, with the clear demonstration of quasar wind acceleration being a particularly delectable recent example."
In addition to Homayouni and Brandt, the team at Penn State includes Donald P. Schneider, Distinguished Professor of Astronomy and Astrophysics and the SDSS-V Publication Coordinator. Catherine Grier was formerly a postdoctoral researcher at Penn State, where she developed her ongoing interest in quasar wind variability.
