Supermassive black holes are associated with the vast majority of galaxies. They’re believed to evolve with their host galaxies and even to affect galaxy growth over time, owing to their ability to gobble up vast amounts of gas and dust and shoot high-energy radiation back out into their surroundings.
There are measurable correlations between the mass of a supermassive black hole and the properties of its host galaxy’s bulge, such as the luminosity of the bulge and the movements of stars within it. The reasons for these correlations are still unknown, but astronomers have long believed that supermassive black holes affect the star formation around them via some sort of feedback process.
In a letter printed today in Nature, a group of astronomers led by Michael Parker at the Institute of Astronomy in Cambridge, UK, report their observations of IRAS 13224−3809, a nearby Seyfert galaxy hosting an active galactic nucleus, or AGN. Seyfert galaxies shine intensely in infrared light due to the activity of their supermassive black holes, which are relatively low mass but are accreting at high rates. IRAS 13224−3809 hosts a central supermassive black hole weighing about 6,000,000 times the mass of our Sun.
Parker and his coauthors studied observations of IRAS 13224−3809 taken with the X-ray Multi-Mirror Mission over the course of 17 days and with the Nuclear Spectroscopic Telescope Array over the course of six days. They observed X-ray variability on scales of minutes to weeks. By looking at the X-ray spectrum of the source, they were able to determine that this object offers a relatively unhindered view right down into the inner portions of the accretion disk near the black hole itself.
When astronomers “look” at a supermassive black hole, they’re actually observing light from the accretion disk of matter around the black hole, which hasn’t yet fallen past the event horizon and become invisible. Supermassive black holes show variability over time in a variety of wavelengths, including optical light, infrared light, and X-rays. This variability is believed to arise from changes in the accretion disk, such as clumps of matter or outflows of gas and radiation.
IRAS 13224−3809’s black hole shows extraordinary X-ray variability — in fact, it’s the most variable AGN observed at X-ray wavelengths. Parker’s group was able to watch the effects of an ultrafast outflow, which is associated with areas of the accretion disk within a few hundred times the size of the event horizon. Ultrafast outflows, or UFOs, are outflows moving faster than about 6,000 miles per second (10,000 km/s). They’re believed to be triggered by X-ray radiation associated with accretion at the innermost portions of the disk, just a few times the size of the event horizon.
IRAS 13224−3809’s outflow was clocked at 44,000 miles per second (71,000 km/s), or about 0.236 times the speed of light. This puts it in the top 5 percent of UFOs ever observed. What’s more, the power it’s putting out is on par with quasars that are three orders of magnitude more massive.
Because of their immense power, IRAS 13224−3809’s outflows may be strong enough to drive feedback in its host galaxy, just as more massive quasars do in the much more distant universe.
While all black holes are variable, the timescale of variability typically scales with size. This makes sense when you think of variability relating to the accretion disk, which also scales with size. Thus, IRAS 13224−3809 shows much faster variability than the variability observed in quasars, which are similar but much more massive objects. Parker and his group were able to watch IRAS 13224−3809’s X-ray light undergo changes that took only hours, rather than months in a quasar.
Studying IRAS 13224−3809 could thus help astronomers finally start to answer questions about how UFOs and other outflows are created. It could also shed light on how black hole feedback affects the host galaxy. This object’s unique properties would allow studies to be performed more easily and with much shorter observing times than those focused on faraway, slower-acting quasars.