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: http://www.nasa.gov/hubble
J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Ray Villard
Space Science Telescope Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
villard@stsci.edu
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Ray Villard
Space Science Telescope Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
villard@stsci.edu
Source: NASA's Hubble Space Telescope
