In 2020, the X-ray telescope eRosita took images of two enormous bubbles
extending far above and below the center of our galaxy.
Since then, astronomers have debated their origin. Now, a study including
University of Michigan research suggests the bubbles are a result of a
powerful jet of activity from the supermassive black hole at the center of
the Milky Way. The study, published in Nature Astronomy, also shows the jet
began spewing out material about 2.6 million years ago, and lasted about
100,000 years.
The team's results suggest that Fermi bubbles, discovered in 2010, and
microwave haze—a fog of charged particles roughly at the center of the
galaxy—were formed by the same jet of energy from the supermassive black
hole. The study was led by the National Tsing Hua University in
collaboration with U-M and the University of Wisconsin.
"Our findings are important in the sense that we need to understand how
black holes interact with the galaxies that they are inside, because this
interaction allows these black holes to grow in a controlled fashion as
opposed to [growing] uncontrollably," says U-M astronomer Mateusz
Ruszkowski, a co-author of the study. "If you believe in the model of these
Fermi or eRosita bubbles as being driven by supermassive black holes, you
can start answering these profound questions."
There are two competing models that explain these bubbles, called Fermi and
eRosita bubbles after the telescopes that named them, says Ruszkowski. The
first suggests that the outflow is driven by a nuclear starburst, in which a
star explodes in a supernova and expels material. The second model, which
the team's findings support, suggests that these outflows are driven by
energy thrown out from a supermassive black hole at the center of our
galaxy.
These outflows from black holes occur when material travels toward the black
hole, but never crosses the black hole's event horizon, or the mathematical
surface below which nothing can escape. Because some of this material is
thrown back into space, black holes don't grow uncontrollably. But the
energy thrown from the black hole does displace material near the black
hole, creating these large bubbles.
The structures themselves are 11 kiloparsecs tall. One parsec is equivalent
to 3.26 light-years, or about three times the distance that light travels
over the course of a year. The structures, then, are nearly 36,000
light-years tall.
For comparison, the Milky Way galaxy is 30 kiloparsecs in diameter, and our
solar system resides about 8 kiloparsecs from the center of the galaxy. The
eRosita bubbles are about two times the size of the Fermi bubbles and are
expanded by the wave of energy, or a shockwave, pushed out by the Fermi
bubbles, according to the researchers.
Astronomers are interested in the observation of these eRosita bubbles in
particular because they occur in our own galactic backyard as opposed to
objects in a different galaxy or at extreme cosmological distance. Our
proximity to the outflows means astronomers can collect an enormous amount
of data, Ruszkowski says. This data can tell astronomers the amount of
energy in the jet from the black hole, how long this energy was injected and
what material comprises the bubbles.
"We not only can rule out the starburst model, but we can also fine tune the
parameters that are needed to produce the same images, or something very
similar to what's in the sky, within that supermassive black hole model,"
Ruszkowski says. "We can better constrain certain things, such as how much
energy was pumped in, what's inside these bubbles and how long was the
energy injected in order to produce these bubbles."
What's inside them? Cosmic rays, a form of high-energy radiation. The
eRosita bubbles enclose the Fermi bubbles, the contents of which are
unknown. But the researchers' models can predict the amount of cosmic rays
inside each of the structures. The energy injection from the black hole
inflated the bubbles, and the energy itself was in the form of kinetic,
thermal and cosmic ray energy. Of these forms of energy, the Fermi mission
could only detect the gamma ray signal of the cosmic rays.
Karen Yang, lead author of the study and an assistant professor at the
National Tsing Hua University in Taiwan, began working on an early version
of the code used in the modeling in this paper as a postdoctoral researcher
at U-M with Ruszkowski. To arrive at their conclusions, the researchers
performed numerical simulations of energy release that take into account
hydrodynamics, gravity and cosmic rays.
"Our simulation is unique in that it takes into account the interaction
between the cosmic rays and gas within the Milky Way. The cosmic rays,
injected with the jets of the black hole, expand and form the Fermi bubbles
that shine in gamma rays," Yang says.
"The same explosion pushes gas away from the galactic center and forms a
shock wave that is observed as the eRosita bubbles. The new observation of
the eRosita bubbles has allowed us to more accurately constrain the duration
of the black hole activity, and better understand the past history of our
own galaxy."
The researchers' model rules out the nuclear starburst theory because the
typical duration of a nuclear starburst, and therefore the length of time
into which a starburst would inject the energy that forms the bubbles, is
about 10 million years, according to study co-author Ellen Zweibel,
professor of astronomy and physics at University of Wisconsin.
"On the other hand, our active black hole model accurately predicts the
relative sizes of the eRosita X-ray bubbles and the Fermi gamma ray bubbles,
provided the energy injection time is about 1% of that, or one tenth of a
million years," Zweibel says.
"Injecting energy over 10 million years would produce bubbles with a
completely different appearance. It's the opportunity to compare the X-ray
and gamma ray bubbles which provides the crucial previously missing piece."
The researchers used data from the eRosita mission, NASA's Fermi Gamma-ray
Space Telescope, the Planck Observatory and the Wilkinson Microwave
Anisotropy Probe.
Reference:
H.-Y. Karen Yang et al, Fermi and eROSITA bubbles as relics of the past
activity of the Galaxy's central black hole, Nature Astronomy (2022).
DOI: 10.1038/s41550-022-01618-x
Tags:
Space & Astrophysics