When Galaxies Collide: How the First Super-Massive Black Holes Were Born
SOURAV KUMAR SAHOO
Astronomers believe they have discovered the origin of our universe's
first super-massive black holes, which formed some 13 billion years
ago...........
The discovery fills in a missing chapter of our universe's early
history, and could help write the next chapter -- in which scientists
better understand how gravity and dark matter formed the universe as
we know it.
In the journal Nature, Ohio State University astronomer Stelios
Kazantzidis and colleagues describe computer simulations in which they
modeled the evolution of galaxies and black holes during the first few
billion years after the Big Bang.
Our universe is thought to be 14 billion years old. Other astronomers
recently determined that big galaxies formed much earlier in the
universe's history than previously thought -- within the first 1
billion years, Kazantzidis explained.
These new computer simulations show that the first-ever super-massive
black holes were likely born when those early galaxies collided and
merged together.
"Our results add a new milestone to the important realization of how
structure forms in the universe," he said.
For more than two decades, the prevailing wisdom among astronomers has
been that galaxies evolved hierarchically -- that is, gravity drew
small bits of matter together first, and those small bits gradually
came together to form larger structures.
Kazantzidis and his team turn that notion on its head.
"Together with these other discoveries, our result shows that big
structures -- both galaxies and massive black holes -- build up
quickly in the history of the universe," he said. "Amazingly, this is
contrary to hierarchical structure formation."
The paradox is resolved once one realizes that dark matter grows
hierarchically, but ordinary matter doesn't," he continued. "The
normal matter that makes up visible galaxies and super-massive black
holes collapses more efficiently, and this was true also when the
universe was very young, giving rise to anti-hierarchical formation of
galaxies and black holes."
For Kazantzidis and other astronomers, our Milky Way galaxy is small
compared to others.
So when it comes to normal matter, big bits like giant galaxies and
super-massive black holes come together quickly, and smaller bits like
our own Milky Way galaxy -- and the comparatively small black hole at
its center -- form more slowly. The galaxies that formed those first
super-massive black holes are still around, Kazantzidis added.
"One of them is likely our neighbor in the Virgo Cluster, the
elliptical galaxy M87," he said. "The galaxies we saw in our
simulation would be the biggest galaxies known today, about 100 times
the size of the Milky Way. M87 fits that description."
They started their simulations with two giant primordial galaxies --
ones made of the kinds of stars that were around at the beginning of
the universe. Astronomers believe that back then, all stars would have
been much more massive than present-day stars -- up to 300 times the
mass of our sun.
Then the astronomers simulated the galaxies colliding and merging together.
The astronomers were able to make their discovery because they used
supercomputers to provide a high-resolution view of what happened
next.
Previous simulations showed details of the merged galaxy down to only
about 300 light-years across. A light-year is the distance that light
travels in year, about six trillion miles.
These new simulations contained features that were 100 times smaller,
and revealed details in the heart of the merged galaxies on a scale of
less than a light year.
The astronomers saw two things happen. First, gas and dust in the
center of the galaxies condensed to form a tight nuclear disk. Then
the disk became unstable, and the gas and dust contracted again, to
form an even denser cloud that eventually spawned a super-massive
black hole.
The implications for cosmology are far-reaching, Kazantzidis said.
"For example, the standard idea -- that a galaxy's properties and the
mass of its central black hole are related because the two grow in
parallel -- will have to be revised. In our model, the black hole
grows much faster than the galaxy. So it could be that the black hole
is not regulated at all by the growth of the galaxy. It could be that
the galaxy is regulated by the growth of the black hole."
He and his cohorts also hope that their work will aid astronomers who
are searching the skies for direct evidence of Einstein's theory of
general relativity: gravitational waves.
According to general relativity, any ancient galaxy mergers would have
created massive gravitational waves -- ripples in the space-time
continuum -- the remnants of which should still be visible today.
New gravitational wave detectors, such as NASA's Laser Interferometer
Space Antenna, were designed to detect these waves directly, and open
a new window into astrophysical and physical phenomena that cannot be
studied in other ways.
Scientists will need to know how super-massive black holes formed in
the early universe and how they are distributed in space today in
order interpret the results of those experiments. The new computer
simulations should provide a clue.
Coauthors on the Nature paper include Lucio Mayer and Simone Callegari
of the Institute for Theoretical Physics at the University of Zürich,
and Andres Escala, formerly of Stanford University and now at the
Universidad de Chile.
This work was funded by the Swiss National Science Foundation, the
Center for Cosmology and Astro-Particle Physics at Ohio State, and the
Kavli Institute for Particle Astrophysics at Stanford University.
Simulations were performed on the Zbox3 supercomputer at the
University of Zürich and on the Brutus cluster at ETH Zürich.
Images Attached:
1)100825131439-large(early universe)- The panel above illustrate the
complexity of dynamical evolution in a typical collision between two
equal-mass disk galaxies. The simulation follows dark matter, stars,
gas, and supermassive black holes, but only the gas component is
visualized. Brighter colors indicate regions of higher gas density and
the time corresponding to each snapshot is given by the labels. The
first 10 panel images measure 100 kpc on a side, roughly five times
the diameter of the visible part of the Milky Way galaxy. The next
five panels represent successive zooms on the central region. The
final frame shows the inner 300 pc of the nuclear region at the end of
the simulation.
--
Sourav Kumar Sahoo
Int BS-MS,
Convener,
IISER-K Astronomy and Model Making Club,
Indian Institute of Science Education and Research-Kolkata(IISER-K)
Ph:+919614734100
EMAIL:sahoo...@iiserkol.ac.in
sahoo...@gmail.com
