Astronomers are confident that the volume of space-time within range of our telescopes—‘the universe’—is only a tiny fraction of the aftermath of the big bang. “We’d expect far more galaxies located beyond the horizon, unobservable,” says renowned astrophysicist Martin Rees, “each of which (along with any intelligences it hosts) will evolve rather like our own.”
One of the great known unknowns in astronomy is just how many galaxies the universe contains. Astronomers have developed a new technique to discover the faintest galaxies yet seen in the early universe —10 times fainter than any previously seen, which will help astronomers probe the time a billion years after the Big Bang, when the early, dark universe was flooded with light from the first galaxies.
It has taken researchers at the Instituto de Astrofísica de Canarias almost three years to produce the deepest image of the Universe ever taken from space, shown above by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field. In dark grey you can see the new light that has been found around the galaxies in this field. That light corresponds to the brightness of more than one hundred billion suns.
“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what we will find when we observe these galaxies with the next generation of telescopes,” says astronomer Christopher Conselice, at the University of Nottingham, who led a team that discovered that there are ten times more galaxies in the universe than previously thought, and an even wider space to search for extraterrestrial life.
The Hubble Deep Field images, captured in the mid 1990s, revealed untold numbers of faint galaxies. It was estimated that the observable Universe contains between 100 to 200 billion galaxies.
Because gravitational attraction is overwhelmed by a mysterious force, dark energy, latent in empty space that pushes galaxies apart from each other, all that the human species will be able to view after a hundred billion years, will be the dead and dying stars of our Local Group.
But these, say astronomer Martin Rees in On the Future, “could continue for trillions of years—time enough, perhaps, for the long-term trend for living systems to gain complexity and ‘negative entropy’ to reach a culmination. All the atoms that were once in stars and gas could be transformed into structures as intricate as a living organism or a silicon chip—but on a cosmic scale. Against the darkening background, protons may decay, dark matter particles annihilate, occasional flashes when black holes evaporate—and then silence.”
In 2016, astronomers using data from the NASA/ESA Hubble Space Telescopes and other telescopes performed an accurate census of the number of galaxies, and came to the surprising conclusion that there are at least 10 times as many galaxies in the observable universe as previously thought. The image itself was produced by the Frontier Fields Collaboration (a joint effort between NASA’s Hubble, Spitzer, and Chandra space telescopes) allowing scientists to detect galaxies that are as much as 100 times fainter than those independently captured before.
Rachael Livermore and Steven Finkelstein of the University of Texas, along with Jennifer Lotz of the Space Telescope Science Institute, went looking for these faint galaxies in images from Hubble Space Telescope’s Frontier Fields survey. “These galaxies are actually extremely common,” Livermore said. “It’s very satisfying being able to find them.”
These faint, early galaxies gave rise to the Epoch of Reionization, when the energetic radiation they gave off bombarded the gas between all galaxies in the universe. This caused the atoms in this diffuse gas to lose their electrons (that is, become ionized).
Finkelstein explained why finding these faint galaxies is so important. “We knew ahead of time that for our idea of galaxy-powered reionization to work, there had to be galaxies a hundred times fainter than we could see with Hubble,” he said, “and they had to be really, really common.” This was why the Hubble Frontier Fields program was created, he said.
The Hubble Space Telescope view of the galaxy cluster MACS 0416 shown above is annotated in cyan and magenta to show how it acts as a ‘gravitational lens,’ magnifying more distant background galaxies. Cyan highlights the distribution of mass in the cluster, mostly in the form of dark matter. Magenta highlights the degree to which the background galaxies are magnified, which is related to the mass distribution.
(STScI/NASA/CATS Team/R. Livermore)
Lotz leads the Hubble Frontier Fields project, one of the telescope’s largest to date. In it, Hubble photographed several large galaxy clusters. These were selected to take advantage of their enormous mass which causes a useful optical effect, predicted by Albert Einstein.
A galaxy cluster’s immense gravity bends space, which magnifies light from more-distant galaxies behind it as that light travels toward the telescope. Thus the galaxy cluster acts as a magnifying glass, or a “gravitational lens,” allowing astronomers to see those more-distant galaxies — ones they would not normally be able to detect, even with Hubble. Even then, though, the lensed galaxies were still just at the cusp of what Hubble could detect.
“The main motivation for the Frontier Fields project was to search for these extremely faint galaxies during this critical period in the universe’s history,” Lotz said. “However, the primary difficulty with using the Frontier Field clusters as an extra magnifying glass is how to correct for the contamination from the light of the cluster galaxies.”
Livermore elaborates: “The problem is, you’re trying to find these really faint things, but you’re looking behind these really bright things. The brightest galaxies in the universe are in clusters, and those cluster galaxies are blocking the background galaxies we’re trying to observe. So what I did was come up with a method of removing the cluster galaxies” from the images.
Her method uses modeling to identify and separate light from the foreground galaxies (the cluster galaxies) from the light coming from the background galaxies (the more-distant, lensed galaxies).
According to Lotz, “This work is unique in its approach to removing this light. This has allowed us to detect more and fainter galaxies than seen in previous studies, and to achieve the primary goal for the Frontier Fields survey.”
Livermore and Finkelstein have used the new method on two of the galaxy clusters in the Frontier Fields project: Abell 2744 and MACS 0416. It enabled them to identify faint galaxies seen when the universe was about a billion years old, less than 10 percent of its current age — galaxies 100 times fainter than those found in the Hubble Ultra Deep Field, for instance, which is the deepest image of the night sky yet obtained.
Their observations showed that these faint galaxies are extremely numerous, consistent with the idea that large numbers of extremely faint galaxies were the main power source behind reionization.
There are four Frontier Fields clusters left, and the team plans to study them all with Livermore’s method. In future, she said, they would like to use the James Webb Space Telescope to study even fainter galaxies.
The Daily Galaxy, Sam Cabot via The University of Texas at Austin