April 4, 2014
Hubble Data Shows True Size Of El Gordo Monster Galaxy
El Gordo is Spanish for "the fat one," and the nickname of the largest galaxy known galaxy cluster in the distant universe, ACT-CLJ0102-4915. A new study from NASA's Hubble Space Telescope reveals that the galaxy cluster, which is 9.7 billion light-years from Earth, definitely lives up to its nickname.
According to a NASA report, a team of astronomers measured how much the cluster's gravity warps images of galaxies in the distant background. This allowed the scientists to calculate the cluster's mass to be at least 3 million billion times the mass of our sun—roughly 43 percent more massive than was previously thought.
To measure how strongly the mass of the cluster warped space, the research team used Hubble's high resolution, which allowed for measurements of "weak lensing." Much like a funhouse mirror, weak lensing is seen when the cluster's immense gravity subtly distorts space, warping the images of background galaxies. The more the images are warped, the greater the mass contained 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 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."
El Gordo's mass is divided three ways. First, a small fraction of the mass is represented in the several hundred galaxies that make up the cluster. Second, a larger fraction of the mass is made up of the hot gas filling the entire volume of the cluster. The third and largest portion of the mass is found in dark matter, an invisible form of matter that makes up the bulk of the mass of the universe.
In the closer, and younger, parts of the Universe, there are equally massive galaxy clusters — for example, the Bullet cluster. In the far distant reaches of the Universe, however, nothing like this has been discovered to exist so far back in time — approximately half the current estimated age of the Universe, 13.8 billion years. Based on the current cosmological models, the researchers suggest that such massive cluster galaxies are very rare in the early Universe.
El Gordo's impressive size was first reported in January 2012. Using observations from NASA's Chandra X-Ray Observatory, and galaxy velocities measured by the European Southern Observatory's (ESO) Very Large Telescope (VLT) array in Chile, astronomers estimated the mass of the cluster galaxy using the motions of the galaxies moving inside the cluster and the temperatures of the hot gas between those galaxies.
El Gordo presented a challenge, however. The cluster looks as if it might have been formed as a result of a titanic collision between a pair of cluster galaxies. Astronomers describe such an event 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."
This is where the team decided to include the data from Hubble.
"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," Felipe Menanteau of the University of Illinois at Urbana-Champaign said.
The scientists expected to find the "unaccounted energy" because the merger of the galaxy clusters is tangential to the observers' line-of-sight, meaning that there is a possibility that the observers are 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 intends to continue their observations, using Hubble data to compile an image of the cluster. This image will be a stitched together mosaic rather than a single image because El Gordo is too large to fit into Hubble's field of view.
"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.