Astronomy Now Home
Home Magazine Resources Store

On Sale Now!

The October 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). Astronomy Now is the only astronomy magazine specially designed to be read on tablets and phones. Download the app from Google Play Store or the Apple App Store.

Top Stories

Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...

Solid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...

Steamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...

A fat galaxy cluster
found faraway

Posted: 11 January 2012

Bookmark and Share

A titanic collision between two galaxy clusters is creating the most massive collection of galaxies ever observed in the distance Universe. The observation of this new super-cluster, nicknamed ‘El Gordo’ after the Spanish for ‘fat one’, fits in neatly with models of dark matter and dark energy in the Universe.

Galaxies tend to arrange themselves in clusters at the nodes of the ‘cosmic web’ – the filaments of matter distributed across the Universe. Our Milky Way exists in a small cluster, the Local Group, which also incorporates the Triangulum and Andromeda spiral galaxies as well as a handful of diminutive dwarf galaxies. The Local Group, however, exists close to the edge of the Virgo Supercluster, a gargantuan agglomeration of at least 1,300 galaxies. Large clusters are common in the local Universe, but become more scarce the farther back in time we look. We see El Gordo (official designation: ACT-CL-J0102-4915) as it was seven billion years ago at a redshift of 0.87. It’s not the most distant galaxy cluster ever seen, given that some have been identified at almost ten billion light years (see our news story here) but it is the most massive, measuring in at 2 x 1015 (2,000 trillion) times the mass of the Sun. To put that into context, the mass of the Milky Way is on the order of a trillion solar masses. El Gordo is composed of two smaller clusters that are merging at velocities of several million kilometres per hour.

A composite image of the ‘El Gordo’ galaxy supercluster, featuring observations from the Very Large Telescope, the Southern Astrophysics Research Telescope (SOAR) Telescope and the Chandra X-ray Observatory (the latter indicated by the X-ray emitting, false-colour blue gas). Image: ESO/SOAR/NASA.

El Gordo was discovered by the six-metre Atacama Cosmology Telescope at the European Southern Observatory in Chile after a disturbance in the cosmic microwave background (CMB) radiation was detected. The CMB is the now faint radiation emitted in the first light after the big bang and its now long-cooled microwave photons permeate the Universe. When these photons interact with electrons in hot gas the CMB becomes distorted. The more hot the gas – like in the environment of a galaxy cluster – the greater the distortion, which is known as the Sunyaev-Zel’dovich effect. One of these distortions tipped off a team led by Felipe Menanteau of Rutgers University in New Jersey, USA and, using the Atacama telescope, they identified El Gordo.

Subsequent observations by NASA’s orbiting Chandra X-ray Observatory measured the position and abundance of hot X-ray emitting ‘intra-cluster’ gas and found the distribution to be almost identical to previous studies of other colliding galaxy clusters such as the famed Bullet Cluster, which has a similar mass (about 3 x 1015 solar masses) to El Gordo (see our previous news story here). In the Bullet Cluster the galaxies themselves had become separated from the intra-cluster gas, but there was something else. Gravitational lensing, whereby more distant objects in the background are magnified by the mass of the cluster warping space-time, revealed that there was unseen, dark matter present in the cluster, that had decoupled from both the gas – the hot gas had slowed during the collision, but the dark matter had drifted right through. This is strong evidence for the existence of dark matter.

“For El Gordo we have made the educated assumption that galaxies are a good tracer of dark matter,” Menanteau told Astronomy Now. “So we have used the galaxies in the cluster in lieu of dark matter maps from galaxy lensing and we see the gas and galaxies (and dark matter) are decoupled, just as in the Bullet Cluster.”

To be sure, Menanteau’s team plan on making Hubble Space Telescope observations of the cluster in order to identify any weak gravitational lensing that may hint at where the dark matter is present.

The detection of the cluster also tells us about the strength of dark energy at the time the cluster was forming. Dark energy is the mysterious force that is causing the expansion of the Universe to accelerate. As it does so, it makes it more difficult for galaxy clusters to grow as it is continually working against gravity, pulling small galaxy groups away from one another. By measuring the mass and growth rate of clusters through cosmic history, it is possible to constrain the strength of dark energy.

“The mass of El Gordo, although rare, is still consistent with the predictions of the current cosmological model and it validates it independently via the growth of structure,” says Menanteau. “However, a merger like El Gordo is not expected, so we hope that its discovery will shed light in the merging process of clusters in the early Universe.”

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


© 2014 Pole Star Publications Ltd.