Hubble sees ‘ghost light’ from dead galaxies


Even galaxies die. Now, Nasa has published an image showing the ‘ghost light’ from a quartet of dead galaxies, ripped apart by gravitational forces over the course of six billion years and now bleeding star matter and planetary debris into the cosmos.
The crime scene is Abell 2744, otherwise known as “Pandora’s Cluster”; a mind-bogglingly vast collection of some 500 galaxies that’s located 4 billion light years away and was formed when four smaller galactic clusters collided. Estimates suggest the cluster’s mass is roughly 400 trillion times that of our own Sun – or 1,000 times that of our galaxy.
The doomed galaxies in the picture currently being “pulled apart like taffy” (according to Nasa) would have been around the size of our own Milky Way Galaxy, and presumably got too close to the center of the cluster where the gravitational tidal forces are strongest.
The vague haze is from the newly-orphaned stars with the image (which was analysed in a study from the Astrofísica de Canarias (IAC) in Spain and published in the October issue of The Astrophysical Journal) and confirms theories about how ‘disassembled’ galaxies.
“This is an original piece of work which marks a “before and after” in our understanding of the formation and evolution of galaxy clusters,” said Mireia Montes of the IAC. “Previous studies of the intra-cluster light had suffered from serious limitation in the depth and the spectral range of the observations.”
The images is thanks to Hubble’s Frontier Fields program, an ambitious three-year project which actually uses galactic clusters such as Abel 2744 as gravitational lenses to see further than any unassisted space telescope.
These clusters are so massive that they warp the light from stars and galaxies behind them, distorting and magnifying it – an effect that was first predicted by Albert Einstein’s general theory of relativity.
By exploiting this natural phenomenon, astronomers manning the Hubble telescope can peer even further into the origins of the universe than before, glimpsing light from stars formed in the first billion years after the Big Bang.
It’s perhaps no surprise then that while looking back into the Universe’s violent past they found this grisly scene – and the ghost light of dead galaxies.

Colossal black holes

Accelerating past the speed of light, the outward flow of space deposits you in a white hole, where space is falling outward faster than the speed of light.

The corpses of three “dead” galaxies – which to the surprise of astronomers stopped forming stars long ago – have been identified by the Spitzer Space Telescope during a survey of the distant, early universe. The find bolsters a theory that colossal black holes can starve galaxies of the gas needed to create new stars.
An infrared telescope on Earth first found the galaxies two years ago. They appeared red – a sign that most of their stars were old. But our planet’s own heat clouded the observations, making it impossible to rule out whether dust was obscuring the light from younger stars.
Now, using NASA’s Spitzer telescope, which trails behind the Earth in the coldness of space, astronomers have determined the galaxies are red because they are dead – no stars appear to have formed for 1.5 billion years. That arrested development happened early in the history of the universe – their distance means Spitzer is viewing them just 2 billion to 3 billion years after the big bang.
“We think galaxies form over tens of billions of years,” says lead researcher Ivo Labbé, an astronomer at the Carnegie Observatories in Pasadena, California, US. For example, he notes the 13 billion-year-old Milky Way is still forming stars today. “Surprisingly, we found galaxies that are fully formed and dead when the universe was only one-fifth its present age.”

No freak monsters
The researchers estimate a significant fraction of galaxies in the early universe were dead – just 10 times as many exist today. “It means these are not freak monsters,” Labbé told New Scientist.
David Hogg, a cosmologist at New York University, who is not part of the team, agrees. “What is most impressive to me is that a significant fraction of the galaxies look ‘long dead’, which does suggest that some kinds of galaxies could be fully in place at very early times,” he says.
Labbé believes that black holes lurking inside the galaxies were their undoing. The galaxies appear to be as massive as the Milky Way and like our galaxy, may harbour “supermassive” black holes containing the mass of millions of Suns. Gas falling into these black holes may have spawned powerful outpourings of energy, creating what astronomers call “active” galaxies.
This enormous energy is thought to heat the gas remaining throughout the galaxy to tens of millions of degrees.

Turned off
Lars Hernquist, an astronomer at the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, recently published computer simulations of this heating phenomenon.
“Eventually the gas is so hot, it is no longer bound to the galaxy and simply starts to flow out to space,” Hernquist told New Scientist, a process that takes about 10 million years. When it is over “star formation is turned off”, says Hernquist. “The galaxy will just sit there and the stars will become older. I think this is a very likely explanation of what’s being seen in these galaxies.”
Another possibility is that supernovae exploding within the galaxies blow gas out to space, but both Labbé and Hernquist say this is a less efficient mechanism.

Source: Weekly Holiday