NASA has announced that it has observed a black hole at the center of dwarf galaxy Henize 2-10 that defies expectations. Thanks to detailed evidence from the Hubble Space Telescope’s imaging and spectroscopy instruments, scientists can see that the black hole is ‘creating stars rather than gobbling them up.’
The typical understanding of a black hole is that it’s a region within space from which no particles, including light, can escape. However, something unusual occurs in the dwarf galaxy Henize 2-10. The dwarf galaxy is what’s called a ‘starburst galaxy,’ which means that, as the name suggests, it’s a galaxy that experiences a relatively high rate of star formation. As NASA puts it, ‘Stars are born within the clouds of dust and scattered throughout most galaxies.’ Henize 2-10 includes at least two star-forming regions near its center.
Besides featuring unusually high amounts of star formation, Henize 2-10 is special because its conditions are analogous to those of the early Universe. In 2011, NASA wrote, ‘Stars are forming in Henize 2-10 at a prodigious rate, giving the star clusters in this galaxy their blue appearance. This combination of a burst of star formation and a massive black hole is analogous to conditions in the early Universe. Since Henize 2-10 does not contain a significant bulge of stars in its center, these results show that supermassive black hole growth may precede the growth of bulges in galaxies. This differs from the relatively nearby Universe where the growth of galaxy bulges and supermassive black holes appears to occur in parallel.’
Back in 2011, the observations of Henize 2-10 set off a debate among astronomers ‘as to whether dwarf galaxies were home to black holes proportional to the supermassive behemoths found in the hearts of larger galaxies.’ Amy Reines published the first research on Henize 2-10 in 2011 and Reines is the principal researcher for the latest paper.
‘Ten years ago, as a graduate student thinking I would spend my career on star formation, I looked at the data from Henize 2-10 and everything changed,’ said Reines. ‘From the beginning I knew something unusual and special was happening in Henize 2-10, and now Hubble has provided a very clear picture of the connection between the black hole and a neighboring star forming region located 230 light-years from the black hole.’
|Credits: Science: NASA, ESA, Zachary Schutte (XGI), Amy Reines (XGI)
Image processing: Alyssa Pagan (STScI)
The debate a decade ago centered around the idea of if a black hole at the center of a dwarf galaxy behaved in proportion to a supermassive black hole at the center of much larger galaxies. The latest research shows that Henize 2-10 isn’t producing results like a larger black hole on a smaller scale.
With a supermassive black hole, material needed for star formation is ‘whisked away by surrounding magnetic fields,’ which forms super-hot plasma moving at nearly the speed of light. Any gas clouds, which contain star-forming material, are heated so much that they can’t cool enough to form stars.
Henize 2-10 is doing something different. The outflow is moving much slower. And while that behavior may be like a supermassive black hole, just slower, the effect is dramatically different. NASA writes, ‘That connection is an outflow of gas stretching across space like an umbilical cord to a bright stellar nursery. The region was already home to a dense cocoon of gas when the low-velocity outflow arrived. Hubble spectroscopy shows the outflow was moving about 1 million miles per hour, slamming into the dense gas like a garden hose hitting a pile of dirt and spreading out. Newborn star clusters dot the path of the outflow’s spread, their ages also calculated by Hubble.’
‘At only 30 million light-years away, Henize 2-10 is close enough that Hubble was able to capture both images and spectroscopic evidence of a black hole outflow very clearly. The additional surprise was that, rather than suppressing star formation, the outflow was triggering the birth of new stars,’ said Zachary Schutte, Reines’ graduate student and lead author of the new study.
There remained some controversy over whether there even is a black hole at the center of Heinze 2-10. When Reines discovered radio and x-ray emissions in 2011, some astronomers believed that the radiation was due to a supernova remnant and not a black hole. However, ‘Hubble’s amazing resolution clearly shows a corkscrew-like pattern in the velocities of the gas, which we can fit to the model of a precessing, or wobbling, outflow from a black hole. A supernova remnant would not have that pattern, and so it is effectively our smoking-gun proof that this is a black hole,’ Reines said.
Reines believes that more research will be focused on dwarf galaxy black holes, as they may hold the key to solving the persistent puzzle of supermassive black hole formation in the early Universe. ‘The relationship between the mass of the galaxy and its black hole can provide clues. The black hole in Henize 2-10 is around 1 million solar masses. In larger galaxies, black holes can be more than 1 billion times our Sun’s mass. The more massive the host galaxy, the more massive the central black hole,’ writes NASA.
As of now, there are three primary theories on the origin of supermassive black holes. Supermassive black holes may have formed like smaller stellar-mass black holes. Perhaps instead there may have been special conditions in the early Universe that gave rise to supermassive stars that then collapsed into massive black holes. Or maybe the ‘seeds’ of supermassive black holes were formed in dense star clusters where the cluster’s mass was great enough to create supermassive black holes through gravitational collapse. None of these black hole ‘seeding’ theories has taken off. It’s hoped that dwarf galaxy black holes, like the one in Henize 2-10, will be able to serve as an analog for black holes in the early Universe.
‘The era of the first black holes is not something that we have been able to see, so it really has become the big question: where did they come from?’ asked Reines. ‘Dwarf galaxies may retain some memory of the black hole seeding scenario that has otherwise been lost to time and space.’