A team using NASA’s Hubble Space Telescope has uncovered a new type of astronomical object — a starless, gas-rich, dark-matter cloud considered a “relic” or remnant of early galaxy formation. Nicknamed “Cloud-9,” this is the first confirmed detection of such an object in the universe — a finding that furthers the understanding of galaxy formation, the early universe, and the nature of dark matter itself.
“This is a tale of a failed galaxy,”
said the program’s principal investigator, Alejandro Benitez-Llambay of the
Milano-Bicocca University in Milan, Italy. “In science, we usually learn more
from the failures than from the successes. In this case, seeing no stars is
what proves the theory right. It tells us that we have found in the local
universe a primordial building block of a galaxy that hasn't formed.”
The results, published in The Astrophysical Journal Letters, were presented at a press conference Monday at the 247th meeting of the American Astronomical Society in Phoenix.
“This cloud is a window into the dark
universe,” said team member Andrew Fox of the Association of Universities for
Research in Astronomy/Space Telescope Science Institute (AURA/STScI) for the
European Space Agency. “We know from theory that most of the mass in the
universe is expected to be dark matter, but it’s difficult to detect this dark
material because it doesn’t emit light. Cloud-9 gives us a rare look at a
dark-matter-dominated cloud.”
This image shows the location of Cloud-9, which is 14
million light-years from Earth. The diffuse magenta is radio data from the
ground-based Very Large Array (VLA) showing the presence of the cloud. The
dashed circle marks the peak of radio emission, which is where researchers
focused their search for stars. Follow-up observations by the Hubble Space
Telescope’s Advanced Camera for Surveys found no stars within the cloud. The
few objects that appear within its boundaries are background galaxies. Before
the Hubble observations, scientists could argue that Cloud-9 is a faint dwarf
galaxy whose stars could not be seen with ground-based telescopes due to the
lack of sensitivity. Hubble’s Advanced Camera for Surveys shows that, in
reality, the failed galaxy contains no stars.
Science: NASA, ESA, VLA, Gagandeep Anand (STScI),
Alejandro Benitez-Llambay (University of Milano-Bicocca); Image Processing:
Joseph DePasquale (STScI)
The object is called a
Reionization-Limited H I Cloud, or "RELHIC.” The term “H I” refers to
neutral hydrogen, and “RELHIC” describes a natal hydrogen cloud from the
universe’s early days, a fossil leftover that has not formed stars. For years,
scientists have looked for evidence of such a theoretical phantom object. It
wasn’t until they turned Hubble toward the cloud, confirming that it is indeed
starless, that they found support for the theory.
“Before we used Hubble, you could argue
that this is a faint dwarf galaxy that we could not see with ground-based
telescopes. They just didn't go deep enough in sensitivity to uncover stars,”
said lead author Gagandeep Anand of STScI. “But with Hubble’s Advanced Camera for Surveys, we're able to nail down that there's nothing there.”
The discovery of this relic cloud was a
surprise. “Among our galactic neighbors, there might be a few abandoned houses
out there,” said STScI’s Rachael Beaton, who is also on the research team.
Astronomers think RELHICs are dark
matter clouds that couldn’t accumulate enough gas to form stars. They represent
a window into the early stages of galaxy formation. Cloud-9 suggests the
existence of many other small, dark matter-dominated structures in the universe
— other failed galaxies. This discovery provides new insights into the dark
components of the universe that are difficult to study through traditional
observations, which focus on bright objects like stars and galaxies.
Scientists have studied hydrogen clouds
near the Milky Way for many years, but these clouds tend to be much bigger and
more irregular than Cloud-9. Compared with other observed hydrogen clouds,
Cloud-9 is smaller, more compact, and highly spherical, making it look very
different from the others.
The core of this object is composed of
neutral hydrogen and is about 4,900 light-years in diameter. Researchers
measured the hydrogen gas in Cloud-9 by the radio waves it emits, measuring it
to be approximately one million times the mass of the Sun. Assuming that the
gas pressure is balancing the dark matter cloud’s gravity, which appears to be
the case, researchers calculated Cloud-9’s dark matter must be about five
billion solar masses.
Cloud-9 is an example of structures and
mysteries that don't involve stars. Just looking at stars doesn’t give the full
picture. Studying the gas and dark matter helps provide a more complete
understanding of what's going on in these systems that would otherwise be
unknown.
Observationally, identifying these
failed galaxies is challenging because nearby objects outshine them. Such
systems are also vulnerable to environmental effects like ram-pressure
stripping, which can remove gas as the cloud moves through intergalactic space.
These factors further reduce their expected numbers.
The starless relic was discovered three
years ago as part of a radio survey by the Five-hundred-meter Aperture
Spherical Telescope (FAST) in Guizhou, China, a finding later confirmed by the
Green Bank Telescope and the Very Large Array facilities in the United States.
But only with Hubble could researchers definitively determine that the failed
galaxy contains no stars.
Cloud-9 was simply named sequentially,
having been the ninth gas cloud identified on the outskirts of a nearby spiral
galaxy, Messier 94 (M94). The cloud is close to M94 and appears to have a
physical association with the galaxy. High-resolution radio data shows slight
gas distortions, possibly indicating interaction between the cloud and galaxy.
The cloud may eventually form a galaxy
in the future, provided it grows more massive — although how that would occur
is under speculation. If it were much bigger, say, more than 5 billion times
the mass of our Sun, it would have collapsed, formed stars, and become a galaxy
that would be no different than any other galaxy we see. If it were much
smaller than that, the gas could have been dispersed and ionized and there
wouldn't be much left. But it’s in a sweet spot where it could remain as a
RELHIC.
The lack of stars in this object
provides a unique window into the intrinsic properties of dark matter clouds.
The rarity of such objects and the potential for future surveys is expected to
enhance the discovery of more of these “failed galaxies” or “relics,” resulting
in insights into the early universe and the physics of dark matter.
The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
Source: NASA's
Hubble Examines Cloud-9, First of New Type of Object - NASA Science
