What happens when three galaxies collide with huge black holes at the center of each? A new study using NASA’s Chandra X-ray Observatory and several other telescopes reveals new information about how many black holes are growing violently after these galactic collisions.
Astronomers want to know more about galaxy collisions because subsequent mergers are a key pathway for galaxies and giant black holes in their cores to grow over cosmic time.
“There has been a lot of research into what happens to supermassive black holes when two galaxies merge. Our laboratory is one of the first to systematically study what happens to black holes when three galaxies merge.”
Adi Ford of Stanford University
Scientists have identified triple galaxy merger systems by matching archives containing data now publicly available from NASA’s WISE and Sloan Digital Sky Survey (SDSS) missions to the Chandra archive. Using this method, they discovered seven triple mergers of galaxies located 370 million to one billion light-years from Earth.
Using specialized software, the team examined Chandra data targeting these systems to find X-ray sources that mark the location of growing supermassive black holes. When material falls toward the black hole, it heats up to millions of degrees and produces X-rays.
Chandra, with its sharp X-ray vision, is ideal for detecting growing supermassive black holes in mergers. Associated X-ray sources are difficult to detect because they are usually located close to each other in images and are often faint. The software is designed specifically to find such sources. The data from other telescopes were then used to rule out other possible sources of X-rays not associated with supermassive black holes.
The results show that of the seven mergers of triple galaxies, there is one growing supermassive black hole, four with a double growing supermassive black hole, and one with a triple. The last triple merger they studied appears to have happened without X-rays from supermassive black holes. In systems with many black holes, the distance between them is approximately 10,000 to 30,000 light-years.
“Why do we care about the percentage of these black holes hitting? Because these statistics can tell us more about how black holes and the galaxies they inhabit grow.”
Jesse Runno of Vanderbilt University in Nashville, Tennessee
Once they found evidence of bright X-ray sources as candidates for supermassive black hole growth in the Chandra data, the researchers incorporated archived data from other telescopes.
The authors studied infrared data from the WISE mission, the infrared astronomical satellite, and the All Sky Telescope to see how quickly stars are forming in various galaxies in their survey. This allowed them to estimate how many detected X-rays were likely to come from X-ray emitting systems containing massive stars rather than from a growing supermassive black hole. Because such stellar systems are young, they are more common when stars form faster. Scientists used this technique to conclude that one of the X-ray sources they discovered probably belongs to a collection of X-ray emitting star systems.
Chandra and WISE data show that the system with growing supermassive black holes has the most dust and gas. This is in line with theoretical computer simulations of mergers, which suggest that higher levels of gas near black holes are more likely to cause black holes to grow rapidly.
Research on triple mergers could help scientists understand whether pairs of supermassive black holes could get so close to each other that they create ripples in spacetime called gravitational waves. The energy lost by these waves will inevitably lead to the merging of black holes.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo facility in Europe have shown astronomers that stellar-mass black holes create gravitational waves and merge. Still, it is not known if supermassive black holes do.