For decades, astronomers have tried to pin down why two of the most common types of growing supermassive black holes, known as Type I and Type II active galaxies nuclei (AGN), appear different when observed from Earth. Although both galaxy types host voracious supermassive black holes at their center that actively swallow matter and emit massive amounts of radiation, Type I galaxies appear brighter to astronomers’ telescopes.
New research from an international team of astronomers, with contributions from the University of Maryland, make a major modification to a popular theory called the unified model. According to this model the growing black holes in Type I and Type II AGN have the same fundamental structure and energetic profile, but appear different solely because the galaxies point toward Earth at different angles. Specifically, Type II galaxies are tilted such that they are obscured by their own rings of dust, making Type I galaxies appear brighter by comparison.
The new results, published September 28, 2017, in the journal Nature, suggest that Type I and Type II galaxies do not just appear different—they are, in fact, very different from each other, both structurally and energetically. The key factor that distinguishes Type I and Type II galaxies is the rate at which their central black holes consume matter and spit out energy, according to the researchers.
“The unified model has been the prevailing wisdom for years. However, this idea does not fully explain the differences we observe in galaxies’ spectral fingerprints, and many have searched for an additional parameter that fills in the gaps,” said Richard Mushotzky, a professor of astronomy at UMD and a co-author of the study. “Our new analysis of X-ray data from NASA’s Swift Burst Alert Telescope suggests that Type I galaxies are much more efficient at emitting energy.”
To conduct the study, Mushotzky and his colleagues re-examined data from 836 active galaxies detected by NASA’s Swift Burst Alert Telescope that strongly emit high-energy, or “hard,” X-rays—the same X-rays that medical technicians use to visualize the human skeleton. In order to measure the mass of the supermassive black holes and how fast they were growing they used data from 12 different ground based telescopes spread across the globe to carefully estimate the stellar velocity dispersion and broad emission lines in the spectra which are strongly related to the black hole mass. “This is a project that began in 2009 during Dr. Koss's thesis at the University of Maryland with Professor Mushotzky and Professor Veilleux and has radically grown with the help of over 40 researchers across the globe.
"When I started the project, I had a month of lonely nights observing by myself at the 2.1m telescope at Kitt Peak national observatory to study a few dozen galaxies. I could never have dreamed we would eventually such a large sample, where we can answer many amazing scientific questions for the first time”, said Dr. Koss.
By comparing differences in the X-ray spectra between Type I and Type II galaxies, the researchers concluded that, regardless of which way the galaxy faces Earth, the central black holes in Type I galaxies consume matter and emit energy much faster compared with the black holes at the center of Type II galaxies.
“Our results suggest this has a lot to do with the amount of dust that sits close to the central black hole,” Claudio Ricci, the lead author of this work, said. “Type II galaxies have a lot more dust close to the black hole, and this dust pushes against the gas as it enters the black hole.”
For decades, astronomers preferentially studied Type II galaxies, largely because the central black holes of Type I galaxies are too bright to see the stars and gas clouds that constitute the rest of the galaxy. Because the unified model suggested that all active galaxies were fundamentally the same, astronomers focused their efforts on Type II galaxies because they are easier to observe.
“Because our results suggest that the two types of galaxies are indeed fundamentally different, it is likely that a lot of researchers will re-evaluate their data and take another look at Type I galaxies now,” Mushotzky said. “By putting us on a path to better understand the differences between Type I and Type II galaxies, this work will help us better understand how supermassive black holes influence the evolution of their host galaxies.”
The research was led by Dr. Claudio Ricci (PUC Chile/CASSACA/KIAA Beijing) , and was based on data collected by a team led by Dr. Mike Koss (Eureka Scientific) a former University of Maryland graduate student.
The research paper, “The close environments of accreting massive black holes are shaped by radiative feedback,” Claudio Ricci et al., was published in the journal Nature on September 28, 2017. More information on the survey and team involved in this research can be found at www.bass-survey.com
* Provided by: University of Maryland and ETH Zurich