Cosmologists believe they have discovered the source of the vast cluster of galaxies we see today, after combining observational data of the distant Universe from ESA’s Herschel and Planck space observatories. A galaxy cluster (cluster of galaxies) is a structure consisting of anywhere from hundreds to thousands of galaxies bound together by gravity.
Galaxies such as ours, the Milky Way with its 100 billion stars, are not generally found in isolation. Today in our Universe, 13.8 billion years after the Big Bang, many exist in dense clusters of many hundreds of galaxies.
One of the major unanswered questions in space science is how these massive clusters assembled in the early Universe – they have not always been there.
The Planck all-sky map at submillimeter wavelengths (545 GHz). The band running through the middle is the dust in our Milky Way galaxy. The black dots indicate the location of the proto-cluster candidates identified by Planck and subsequently observed by Herschel. (Image: University of Arizona)
Learning about galaxy cluster evolution and dark matter
Finding out when and how they formed could give cosmologists deeper insight into the process of galaxy cluster evolution, including what role dark matter has played in shaping these cosmic megalopolises.
Now, using the combined strengths of the Planck Satellite and the Herschel Space Observatory, astronomers have been able to find objects in the distant Universe, seen at a time when it was just three billion years old (10.8 billion light years from Earth). These objects could be forerunners of the clusters observed around us today.
Brenda L. Frye, an assistant astronomer at the University of Arizona’s Steward Observatory, who was involved in the research, said:
“Because we are looking so far back in time, and because the universe is assumed to be homogenous in all directions, we think it’s very similar to looking at the equivalent of what a baby cluster might look like.”
“In contrast to previous observations, for which the odd one or two baby clusters was found which one would put in a zoo, we now have found a real sample of 200 baby clusters.”
The European Space Agency’s Planck observatory’s main goal was to provide the most accurate map of the relic radiation of the Big Bang, the cosmic microwave background.
To be able to do this, the observatory surveyed the entire sky in nine different wavelengths from radio to far-infrared, in order to factor out foreground emissions from our Milky Way and others in the Universe.
234 bright sources located
However, those foreground sources offer important data for astronomers, and it was in Planck’s short wavelength data that astrophysicists were able to identify 234 bright sources with features that suggested they were located in the early (ultra-distant) Universe.
The Herschel space observatory then observed these objects across the far-infrared to submillimeter wavelength range (marginally shorter than microwaves), but with much greater sensitivity and angular resolution.
Herschel showed that most of the Planck-detected sources are consistent with dense galaxy concentrations in the early Universe, vigorously creating new stars.
Each of these very young galaxies is seen to be converting dust and gas into stars at a rate of a few hundred to about 1,500 times the mass of our Sun annually. This is a much greater creation rate compared to our Milky Way, which currently produces stars at about one solar mass per year.
Frye said:
“It was not known whether young galaxies form stars gradually, like marathon runners pacing themselves or in bursts.”
“It turns out these young galaxies were not forming slowly, but in a dramatic way. Lighting up with star formation, they appear like fireworks going off in the sky. It’s like sprinting the first mile of a 26-mile marathon, and then walking the rest of the way.”
These ‘fireworks’ cannot be seen by astronomers because of their extreme distance. However, as they occur in clusters, they appear bright. Even so, now and again they detected single galaxies that appeared much brighter than they should.
The natural telescope in space
Frye said:
“In a small number of cases, we are finding only one object instead of a cluster, which we shouldn’t be able to see, so there is some other mystery there.”
“The answer is that these objects are brightened up by what you could call a natural telescope in space. In a few cases, we can see individual far-away galaxies lighting up with star formation because they are situated along just at the right line of sight where their light passes through a massive galaxy clusters close to Earth.”
As forecast by the Theory of General Relativity established by Albert Einstein, the foreground cluster distorts light from the background galaxy because of its colossal gravity, resulting in an image from a super-distant galaxy that is enhanced in brightness.
Frye said:
“Why we find these enigmatic examples of single ones when they should be in clusters, is what we’re studying here at the UA. These might be fainter examples in general, or they, too, may have friends, which haven’t turned on their fireworks just yet.”
Frye took advantage of the Hectospec at the MMT Observatory, a spectrograph, a partnership between the University of Arizona and the Smithsonian Institution of Astrophysics.
Frye said:
“Hectospec acts like a prism, breaking the emissions coming from those protogalaxies into a rainbow of light, so we can very quickly study these really interesting objects, which may have been very much like what the Milky Way may have looked like earlier in cosmic history.”
“A key feature that sets our instrument apart from the rest of the world is that we can get the spectra of 300 objects at the same time.”
While the cosmologists haven’t yet conclusively established how old these newly-discovered distant galaxy clusters are, they are the best contenders yet found for ‘proto-clusters’ – precursors of the massive, mature galaxy clusters we now see in the Universe.
Planck added many more candidates to be studied
Lead scientist of the analysis published in the journal Astronomy & Astrophysics, Hervé Dole, of the Institut d’Astrophysique Spatiale in Orsay, France, said:
“Hints of these kinds of objects had been found earlier in data from Herschel and other telescopes, but the all-sky capability of Planck revealed many more candidates for us to study.”
“We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation.”
Ludovic Montier, of the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France, added:
“We are now preparing an extended catalogue of possible proto-clusters detected by Planck, which should help us identify even more of these objects.”
Citation: “The influence of wavelength, flux, and lensing selection effects on the redshift distribution of dusty, star-forming galaxies,” Matthieu Béthermin, Carlos De Breuck, Mark Sargent and Emanuele Daddi. Astronomy & Astrophysics. Published on 2 April, 2015. DOI: 10.1051/0004-6361/201525718.
Video – Clusters of Galaxies
Galaxy clusters are the biggest organized structures in the Universe that appear gravitationally bound. They contain thousands of galaxies all confined to a volume of space only tens of millions of light years across. Carolin Crawford, Gresham Professor of Astronomy, explains.