A Missing Link in the Family Tree of Cosmic Black Holes

Intermediate-mass black hole with a powerful jet

March 05, 2015

A black hole discovered wandering all by itself inside one of the spiral arms of the galaxy NGC 2276 may present an important clue that would fill the gap in the evolutionary story of black holes. This discovery has been reported recently by a research team which is led by Mar Mezcua from the Harvard Centre for Astrophysics in Boston and includes Andrei Lobanov from the Max-Planck-Institut für Radioastronomie (MPIfR) in Bonn. They identify this elusive black hole, called NGC 2276-3c. The astronomers had to look at it simultaneously at radio waves with the European Very Long Baseline Interferometry Network, or EVN, and in X-rays with NASA’s Chandra Space Observatory. The combination of X-ray and radio data enables the researchers to ‘weigh’ the black hole, which has turned out to be as heavy as about 50,000 Suns. With this mass, it fills a gap between stellar black holes found in our own Galaxy and supermassive black holes residing in centers of most of the massive galaxies. Such intermediate-mass black holes are probably the seeds from which supermassive black holes will form.

Left: Optical image of the galaxy NGC 2276, taken with the Hubble Space Telescope. Right: The image is a zoom into the radio structure of the newly found source NGC 2276-3c in one of the galaxy’s spiral arms at high resolution.The radio image, observed with the European VLBI Network shows details of only a few light years in extension. — Left: Optical image of the galaxy NGC 2276, taken with the Hubble Space Telescope. Right: The image is a zoom into the radio structure of the newly found source NGC 2276-3c in one of the galaxy’s spiral arms at high resolution. The radio image, observed with the European VLBI Network shows details of only a few light years in extension.

adapted from Mezcua et al. 2015, MNRAS 448, 1893

Left: Optical image of the galaxy NGC 2276, taken with the Hubble Space Telescope.
Right: The image is a zoom into the radio structure of the newly found source NGC 2276-3c in one of the galaxy’s spiral arms at high resolution.
The radio image, observed with the European VLBI Network shows details of only a few light years in extension.

adapted from Mezcua et al. 2015, MNRAS 448, 1893

The new source is located in a spiral arm of the galaxy NGC 2276, which is separated by about 100 million light years from Earth, in the direction of the constellation Cepheus not too far from the North pole in the sky. Its detection may provide answers to some long-standing questions about how black holes evolve and influence their surroundings.

“In paleontology, the discovery of certain fossils can help scientists fill in the gaps of the dinosaurs,” says Mar Mezcua of the Harvard-Smithsonian Center for Astrophysics, who led the study. “We do the same thing in astronomy, but we often have to ‘dig’ up our discoveries in galaxies that are trillions of miles away.”

The intriguing object appears to be what astronomers call an “intermediate-mass black hole” or IMBH. For many years, scientists have found conclusive evidence for smaller black holes that contain only a few times the mass of the sun. There is also a lot of information about so-called supermassive holes that reside at the center of galaxies and weigh millions or even billions the sun’s mass.

As their name suggests, IMBHs represent a class of black holes that fall in between these two well-established groups. One reason that IMBHs are important is that they could be the seeds from which supermassive black holes formed in the early universe. The discovery in NGC 2276 would represent just the second IMBH that is found outside the center of a galaxy.

“Astronomers have been looking very hard for these medium-sized black holes,” explains co-author Timothy Roberts of the University of Durham in the UK. “There have been hints that they exist, but the IMBHs have been acting like a long-lost relative that isn’t interested in being found.”

To learn about this object, the researchers observed NGC 2276 almost simultaneously in radio waves with the EVN (including MPIfR’s Effelsberg 100m radio telescope) and in X-rays with Chandra. Each telescope provided critical and complementary information about this source. Moreover, both radio and X-ray data were necessary to get an accurate determination of the black hole’s mass, estimated to be about 50,000 times that of the Sun.

“We found that this IMBH has traits similar to both stellar-mass black holes and supermassive black holes” says co-author Andrei Lobanov of MPIfR. “In other words, this object helps tie the whole black hole family together.”

In addition to its size and location, there are several intriguing properties to the IMBH in NGC 2276. The black hole appears to have a powerful jet that extends for up to 2,000 light years, demonstrating the reach this black hole has.

A region about 1,000 light years long next to the jet seems to be missing young stars. This might provide evidence that the jet has cleared out a cavity in the gas, suppressing the formation of new stars. This is an important hint in how IMBHs influence their environment. This jet detection might also show that supermassive black hole seeds in the early universe had a large impact on their surroundings.

“There is an interesting open question”, concludes Mar Mezcua. “We are trying to determine if the black hole in NGC 2276 was formed within the galaxy or if it was created during a merger with a dwarf galaxy in the past.”

The study on NGC 2276-3c was conducted by Mar Mezcua (after completing her PhD at MPIfR’s IMPRS research school in 2011 she was in the Instituto de Astrofisica de Canarias and is now at the Harvard-Smithsonian Center for Astrophysics), Tim Roberts (University of Durham, UK), Andrei Lobanov (MPIfR), and Andrew Sutton (University of Durham). The study appears in the Monthly Notices of the Royal Astronomical Society (MNRAS).