Black holes are often described as the monsters of the universe—tearing
apart stars, consuming anything that comes too close, and holding light
captive. Detailed evidence from NASA's Hubble Space Telescope, however,
shows a black hole in a new light: Fostering rather than suppressing star
formation. Hubble imaging and spectroscopy of the dwarf starburst galaxy
Henize 2-10 clearly show a gas outflow stretching from the black hole to a
bright star birth region like an umbilical cord, triggering the already
dense cloud into forming clusters of stars. Astronomers have previously
debated that a dwarf galaxy could have a black hole analogous to the
supermassive black holes in larger galaxies. Further study of dwarf
galaxies, which have remained small over cosmic time, may shed light on the
question of how the first seeds of supermassive black holes formed and
evolved over the history of the universe.
Often portrayed as destructive monsters that hold light captive, black holes
take on a less villainous role in the latest research from NASA's Hubble
Space Telescope. A black hole at the heart of the dwarf galaxy Henize 2-10
is creating stars rather than gobbling them up. The black hole is apparently
contributing to the firestorm of new star formation taking place in the
galaxy. The dwarf galaxy lies 30 million light-years away, in the southern
constellation Pyxis.
A decade ago this small galaxy set off 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. This new
discovery has little Henize 2-10, containing only one-tenth the number of
stars found in our Milky Way, poised to play a big part in solving the
mystery of where supermassive black holes came from in the first place.
"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 Amy Reines, who published the first evidence for a black hole
in the galaxy in 2011 and is the principal investigator on the new Hubble
observations, published in the January 19 issue of Nature.
"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," Reines said.
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.
This is the opposite effect of what's seen in larger galaxies, where
material falling toward the black hole is whisked away by surrounding
magnetic fields, forming blazing jets of plasma moving at close to the speed
of light. Gas clouds caught in the jets' path would be heated far beyond
their ability to cool back down and form stars. But with the less-massive
black hole in Henize 2-10, and its gentler outflow, gas was compressed just
enough to precipitate new star formation.
"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's graduate student and lead author of
the new study.
Ever since her first discovery of distinctive radio and X-ray emissions in
Henize 2-10, Reines has thought they likely came from a massive black hole,
but not as supermassive as those seen in larger galaxies. Other astronomers,
however, thought that the radiation was more likely being emitted by a
supernova remnant, which would be a familiar occurrence in a galaxy that is
rapidly pumping out massive stars that quickly explode.
"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 expects that even more research will be directed at dwarf galaxy
black holes in the future, with the aim of using them as clues to the
mystery of how supermassive black holes came to be in the early universe.
It's a persistent puzzle for astronomers. 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.
Current theories on the origin of supermassive black holes break down into
three categories: 1) they formed just like smaller stellar-mass black holes,
from the implosion of stars, and somehow gathered enough material to grow
supermassive, 2) special conditions in the early universe allowed for the
formation of supermassive stars, which collapsed to form massive black hole
"seeds" right off the bat, or 3) the seeds of future supermassive black
holes were born in dense star clusters, where the cluster's overall mass
would have been enough to somehow create them from gravitational collapse.
So far, none of these black hole seeding theories has taken the lead. Dwarf
galaxies like Henize 2-10 offer promising potential clues, because they have
remained small over cosmic time, rather than undergoing the growth and
mergers of large galaxies like the Milky Way. Astronomers think that dwarf
galaxy black holes could serve as an analog for black holes in the early
universe, when they were just beginning to form and grow.
"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?
Dwarf galaxies may retain some memory of the black hole seeding scenario
that has otherwise been lost to time and space," Reines said.
Reference:
Zachary Schutte et al, Black-hole-triggered star formation in the dwarf
galaxy Henize 2-10, Nature (2022).
DOI: 10.1038/s41586-021-04215-6
Tags:
Space & Astrophysics