An
artist's illustration of the longest black hole jet system ever observed.
Nicknamed Porphyrion after a mythological Greek giant, these jets span roughly
7 megaparsecs, or 23 million light-years. That is equivalent to lining up 140
Milky Way galaxies back-to-back. Porphyrion dates back to a time when our
universe was less than half its present age. During this early epoch, the wispy
filaments that connect and feed galaxies, known as the cosmic web, were closer
together than they are now. Consequently, this colossal jet pair extended
across a larger portion of the cosmic web compared to similar jets in our
nearby universe. Porphyrion's discovery thus implies that jets in the early
universe may have influenced the formation of galaxies to a greater extent than
previously believed. Credit: E. Wernquist / D. Nelson (IllustrisTNG
Collaboration) / M. Oei
Astronomers have spotted the biggest pair of black hole jets ever seen, spanning 23 million light-years in total length. That's equivalent to lining up 140 Milky Way galaxies back to back.
"This pair is not just the size of
a solar system, or a Milky Way; we are talking about 140 Milky Way diameters in
total," says Martijn Oei, a Caltech postdoctoral scholar and lead author
of a Nature paper reporting the
findings. "The Milky Way
would be a little dot in these two giant eruptions."
The jet megastructure, nicknamed
Porphyrion after a giant in Greek mythology, dates to a time when our universe
was 6.3 billion years old, or less than half its present age of 13.8 billion
years. These fierce outflows—with a total power output equivalent to trillions
of suns—shoot out from above and below a supermassive black hole at the heart
of a remote galaxy.
Prior to Porphyrion's discovery, the
largest confirmed jet system was Alcyoneus, also named after a giant in Greek mythology.
Alcyoneus, which was discovered in 2022 by the same team that found Porphyrion,
spans the equivalent of around 100 Milky Ways. For comparison, the well-known Centaurus A jets, the closest major jet system to Earth,
spans 10 Milky Ways.
The latest finding suggests that these
giant jet systems may have had a larger influence on the formation of galaxies
in the young universe than previously believed.
Porphyrion existed during an early epoch
when the wispy filaments that connect and feed galaxies, known as the cosmic web, were closer together than they are now. That means
enormous jets like Porphyrion reached across a greater portion of the cosmic
web compared to jets in the local universe.
Credit: S. Landis & K. Rappaport (Science
Communication Lab)
"Astronomers believe that
galaxies and their central black holes co-evolve, and one key aspect of this is
that jets can spread huge amounts of energy that affect the growth of their
host galaxies and other galaxies near them," says co-author George
Djorgovski, professor of astronomy and data science at Caltech. "This
discovery shows that their effects can extend much farther out than we
thought."
Unveiling a vast population
The Porphyrion jet system is the
biggest found so far during a sky survey that has revealed a shocking number of
the faint megastructures: more than 10,000. This massive population of
gargantuan jets was found using Europe's LOFAR (LOw Frequency ARray) radio telescope.
While hundreds of large jet systems
were known before the LOFAR observations, they were thought to be rare and on
average smaller in size than the thousands of systems uncovered by the radio
telescope.
"Giant jets were known before
we started the campaign, but we had no idea that there would turn out to be so
many," says Martin Hardcastle, second author of the study and a professor
of astrophysics at the University of Hertfordshire in England.
"Usually when we get a new
observational capability, such as LOFAR's combination of wide field of view and
very high sensitivity to extended structures, we find something new, but it was
still very exciting to see so many of these objects emerging."
Back in 2018, Oei and his
colleagues began using LOFAR to study not black hole jets but the cosmic web of
wispy filaments that crisscrosses the space between galaxies. As the team
inspected the radio images for the faint filaments, they began to notice several
strikingly long jet systems.
"When we first found the giant jets, we were quite surprised," says Oei, who is also affiliated with Leiden Observatory in the Netherlands. "We had no idea that there were this many."
The bipolar jets drill their way through the
surrounding intergalactic medium. Where the ends of the jets hit this medium, a
hotspot forms, surrounded by a shock wave. The core of the jet system is the
host galaxy containing the supermassive black hole. The backflow consists of
material from the hotspot that has decelerated and flowed back toward the host
galaxy. Credit: S. Landis & K. Rappaport (Science Communication Lab)
To systematically search for more
hidden jets, the team inspected the radio images by eye, used machine-learning
tools to scan the images for signs of the looming jets, and enlisted the help
of citizen scientists around the globe to eyeball the images further.
A paper describing their most
recent batch of giant outflows, containing more than 8,000 jet pairs, has been
accepted for publication in the journal Astronomy & Astrophysics.
Lurking in the past
To find the galaxy from which
Porphyrion originated, the team used the Giant Metrewave Radio Telescope(GMRT) in India along with ancillary data from a
project called Dark Energy Spectroscopic Instrument(DESI), which operates from Kitt Peak National
Observatory in Arizona. The observations pinpointed the home of the jets to a
hefty galaxy about 10 times more massive than our Milky Way.
The team then used the W. M. Keck
Observatory in Hawai'i to show that Porphyrion is 7.5 billion light-years from
Earth. "Up until now, these giant jet systems appeared to be a phenomenon
of the recent universe," Oei says. "If distant jets like these can
reach the scale of the cosmic web, then every place in the universe may have
been affected by black hole activity at some point in cosmic time," Oei
says.
The observations from Keck also
revealed that Porphyrion emerged from what is called a radiative-mode active
black hole, as opposed to one that is in a jet-mode state.
When supermassive black holes
become active—in other words, when their immense forces of gravity tug on and
heat up surrounding material—they are thought to either emit energy in the form
of radiation or jets. Radiative-mode black holes were more common in the young,
or distant, universe, while jet-mode ones are more common in the present-day
universe.
The fact that Porphyrion came from
a radiative-mode black hole came as a surprise because astronomers did not know
this mode could produce such huge and powerful jets. What is more, because
Porphyrion lies in the distant universe where radiative-mode black holes
abound, the finding implies there may be a lot more colossal jets left to be
found.
"We may be looking at the tip
of the iceberg," Oei says. "Our LOFAR survey only covered 15% of the
sky. And most of these giant jets are likely difficult to spot, so we believe
there are many more of these behemoths out there."
Ongoing mysteries
How the jets can extend so far
beyond their host galaxies without destabilizing is still unclear.
"Martijn's work has shown us that there isn't anything particularly
special about the environments of these giant sources that causes them to reach
those large sizes," says Hardcastle, who is an expert in the physics of
black hole jets.
"My interpretation is that we
need an unusually long-lived and stable accretion event around the
central, supermassive black hole to allow it to be active for so long—about a
billion years—and to ensure that the jets keep pointing in the same direction
over all of that time. What we're learning from the large number of giants is
that this must be a relatively common occurrence."
As a next step, Oei wants to better
understand how these megastructures influence their surroundings. The jets
spread cosmic rays, heat, heavy atoms, and magnetic fields throughout the space
between galaxies. Oei is specifically interested in finding out the extent to
which giant jets spread magnetism.
"The magnetism on our planet allows life to thrive, so we want to understand how it came to be," he says. "We know magnetism pervades the cosmic web, then makes its way into galaxies and stars, and eventually to planets, but the question is: Where does it start? Have these giant jets spread magnetism through the cosmos?"
No comments:
Post a Comment