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.
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.
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. 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.