This is a Hubble image of the most massive cluster
of galaxies ever seen to exist when the universe was just half its current age
of 13.8 billion years. The cluster contains several hundred
galaxies.
Image Credit:
NASA, ESA, and J. Jee (University of California,
Davis)
NASA's Hubble Space Telescope has weighed the largest known galaxy cluster in
the distant universe, catalogued as ACT-CL J0102-4915, and found it definitely
lives up to its nickname -- El Gordo (Spanish for "the fat one").
By measuring how much the cluster's gravity warps images of galaxies in the
distant background, a team of astronomers has calculated the cluster's mass to
be as much as 3 million billion times the mass of our sun. Hubble data show the
galaxy cluster, which is 9.7 billion light-years away from Earth, is roughly 43
percent more massive than earlier estimates.
The team used Hubble to measure how strongly the mass of the cluster warped
space. Hubble's high resolution allowed measurements of so-called "weak
lensing," where the cluster's immense gravity subtly distorts space like a
funhouse mirror and warps images of background galaxies. The greater the
warping, the more mass is locked up in the cluster.
"What I did is basically look at the shapes of the background galaxies that
are farther away than the cluster itself," explained lead author James Jee of
the University of California at Davis. "It's given us an even stronger
probability that this is really an amazing system very early in the
universe."
A fraction of this mass is locked up in several hundred galaxies that inhabit
the cluster and a larger fraction is in hot gas that fills the entire volume of
the cluster. The rest is tied up in dark matter, an invisible form of matter
that makes up the bulk of the mass of the universe.
Though equally massive galaxy clusters are found in the nearby part of the
universe, such as the Bullet cluster, nothing like this has ever been discovered
to exist so far back in time, when the universe was roughly half its current
estimated age of 13.8 billion years. The team suspects such monster galaxy
clusters are rare in the early universe, based on current cosmological
models.
The immense size of El Gordo was first reported in January 2012. Astronomers
estimated its mass based on observations made by NASA's Chandra X-ray
Observatory, and galaxy velocities measured by the European Southern
Observatory's Very Large Telescope array in Paranal, Chile. They were able to
put together estimates of the cluster's mass based on the motions of the
galaxies moving inside the cluster and the temperatures of the hot gas between
those galaxies.
The challenge was that El Gordo looked as if it might have been the result of
a titanic collision between a pair of galaxy clusters -- an event researchers
describe as two cosmic cannonballs hitting each other.
"We wondered what happens when you catch a cluster in the midst of a major
merger and how the merger process influences both the X-ray gas and the motion
of the galaxies," explained John Hughes of Rutgers University. "So, the bottom
line is because of the complicated merger state, it left some questions about
the reliability of the mass estimates we were making."
That is where the Hubble data came in, according to Felipe Menanteau of the
University of Illinois at Urbana-Champaign.
"We were in dire need for an independent and more robust mass estimate given
how extreme this cluster is and how rare its existence is in the current
cosmological model. There was all this kinematic energy that was unaccounted for
and could potentially suggest that we were actually underestimating the mass,"
Menanteau said.
The expectation of "unaccounted energy" comes from the fact the merger of
galaxy clusters is occurring tangentially to the observers' line-of-sight. This
means they are potentially missing a good fraction of the kinetic energy of the
merger because their spectroscopic measurements only track the radial speeds of
the galaxies.
The team's next step with Hubble will be to compile an image of the cluster.
Because El Gordo does not fit into Hubble's field of view, the team will capture
images of sections of the galaxy cluster and piece them together into a
mosaic.
Researchers say it is like observing a giant from the side.
"We can tell it's a pretty big El Gordo, but we don't know what kind of legs
he has, so we need to have a larger field of view to get the complete picture of
the giant," said Menanteau.
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages the telescope. The Space Telescope Science Institute
(STScI) in Baltimore conducts Hubble science operations. STScI is operated for
NASA by the Association of Universities for Research in Astronomy, Inc., in
Washington.
For images and more information about Hubble, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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