Looking back
13-billion years into our universe's past, a group of scientists first
discovered the monster COSMOS-AzTEC-1 galaxy using the James Clerk MaxwellTelescope in Hawai'i and later followed up with observations using the Large
Millimeter Telescope (LMT) in Mexico and the Atacama LargeMillimeter/submillimeter Array (ALMA) in Chile.
What they
found was astonishing: not only was this galaxy forming stars at an
unexpectedly high rate, but the clouds that fuel the birth of stars were
unusual as well. Galaxies that form stars at higher rates are called starburst
galaxies, and COSMOS-AzTEC-1 seemed to fit that archetype. However, in
starburst galaxies, the star-formation predominantly occurs at the center of
the galaxy.
Surprisingly,
astronomers detected two massive clouds of gas several thousand light-years
away from where they thought the star-formation should be concentrated. When
they investigated more closely, they found that the gas in COSMOS-AzTEC-1 is
extremely unstable, fueling runaway star-formation, and turning it into a
monster galaxy. And yet, when scientists attempted to model this galaxy, their
computer simulations could not explain all of its characteristics. Simply put,
COSMOS-AzTEC-1 shouldn't exist.
Monster
galaxy COSMOS-AzTEC-1 observed with ALMA. ALMA revealed the distribution of
molecular gas (left) and dust particles (right). In addition to the dense cloud
in the center, the research team found two dense clouds several thousand
light-years away from the center. These dense clouds are dynamically unstable
and thought to be the sites of intense star formation. CREDIT: ALMA
(ESO/NAOJ/NRAO), TADAKI ET AL.
The
international team of scientists, led by Dr. Ken-ichi Tadaki from the National
Astronomical Observatory of Japan, that discovered COSMOS-AzTEC-1 published
their findings in Nature on Wednesday, reporting that they achieved
unprecedented angular resolution by observing the galaxy with ALMA. Poor
resolution has impeded studies of submillimeter-bright galaxies in the early
universe and has been an obstacle to understanding their physical and dynamical
properties.
Studying
galaxies like COSMOS-AzTEC-1 is important, as they are thought to be the
ancient predecessors of massive galaxies in the present universe, and the
improved resolution has revealed several unexpected findings. In addition to
the mysterious off-center clouds, COSMOS-AzTEC-1 has a regularly rotating,
ordered gas disk instead of a more chaotic distribution, which is what
scientists expected. Not only does this complicate the common model of early
galaxy evolution, but it makes future high-resolution observations of similar
galaxies an imperative.
Radio
telescope antennas of the ALMA ( Atacama Large Millimeter/submillimeter Array)
project are seen in the Chajnantor plateau, Atacama desert, some 1500 km north
of Santiago, on March 12,2013. The ALMA, an international partnership project
between Europe, North America and East Asia, with the cooperation of Chile, is
presently the largest astronomical project in the world. On March 13, 66 high
precision antennas will be opened, located at an altitude of 5000 meters above
sea level in the extremely arid Atacama desert. Credit: AFP PHOTO/Martin
BERNETTI/Getty Images
Thankfully,
the ALMA observations didn't just raise more puzzles for scientists to solve;
they also answered many long-standing questions about the nature of galaxies in
the early universe. Before ALMA, astronomers knew that massive, young galaxies
existed just 1 billion years after the Big Bang, but they were unable to answer
how these galaxies formed or how they were able to stockpile massive reserves
of gas so quickly.
One of the
clues that this research team uncovered was hidden in the nature of
COSMOS-AzTEC-1's gas clouds, a feature that was previously unable to be
observed. In normal galaxies, the gas clouds are in equilibrium; the gravity pulling
the cloud inward is balanced by the gaseous pressure pushing outwards. Given
the right environmental conditions, the gravity eventually becomes stronger
than the pressure, causing the cloud to collapse, become extremely dense, and
begin to form stars.
These stars
and their eventual supernovae increase the outward pressure within the gas,
stabilizing it, and regulating the rate of star formation. In COSMOS-AzTEC-1,
the gas clouds are extremely unstable, and the outward pressure is
significantly weaker than the gravity, which prevents any of the aforementioned
stabilization. Instead, COSMOS-AzTEC-1 has a runaway star-formation rate over
1000 times that of our Milky Way galaxy, and Tadaki's team estimates it will
run out of gas in just 100 million years.
The
mechanism that causes COSMOS-AzTEC-1's gas instability remains unclear. Tadaki
and his colleagues hypothesize that it could be caused by a galaxy merger: a
collision between two or more galaxies that can transport gas efficiently
throughout the galaxy and can initiate a burst of star-formation. However,
their current observations didn't detect any of the signs of such an event, and
a merger wouldn't entirely explain how COSMOS-AzTEC-1 was able to keep its
enormous surplus of gas from becoming stars before it began its starburst
period.
Monster
galaxies, also known as starburst galaxies, are thought to be ancestors of
massive galaxies like the Milky Way in today's universe. This image is an
artist's impression of ZF-COSMOS-20115, a monster galaxy discovered in 2017
Despite the
lingering questions though, Tadaki and his team's research is a perfect example
of how technological improvements will find the solutions to long-standing
scientific mysteries and reveal unexpected secrets hidden throughout our
universe.
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