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First galaxies older than expected

Abell 383 and a distorted background galaxy

The gravity of this giant cluster of galaxies, including the huge one in the middle, acts as a sort of magnifying glass, distorting and concentrating the light of a distant background galaxy. Visible as two tiny dots (labelled), the galaxy is seen as it was less than a billion years after the Big Bang.

ASTRONOMERS HAVE DISCOVERED a distant galaxy whose stars were born unexpectedly early in cosmic history.

“We have discovered a distant galaxy that began forming stars just 200 million years after the Big Bang,” says Johan Richard, the lead author of a new study. “This challenges theories of how soon galaxies formed and evolved in the first years of the Universe.”

Richard’s team spotted the galaxy in recent observations from the NASA/ESA Hubble Space Telescope, verified it with observations from the NASA Spitzer Space Telescope and measured its distance using W.M. Keck Observatory in Hawaii.

The distant galaxy is far beyond a cluster of galaxies called Abell 383, whose powerful gravity bends passing rays of light almost like a magnifying glass.

The chance alignment of the galaxy, the cluster and the Earth amplifies the light reaching us from the distant galaxy, enabling the astronomers to make detailed observations.

Without this gravitational lens, the galaxy would have been too faint to be seen even with today’s largest telescopes.

A young galaxy of old stars

After spotting the galaxy in Hubble and Spitzer images, the team carried out spectroscopic observations with the Keck-II telescope in Hawaii. Spectroscopy is the technique of breaking up light into its component colours.

By analysing the spectra, the team was able to make detailed measurements of the galaxy’s redshift and infer information about the properties of its component stars.

The redshift is 6.027, which means we’re seeing the galaxy as it was when the Universe was around 950 million years old.

Diagram explaining gravitational lensing

The gravity of huge galaxy clusters acts as a magnifying glass, amplifying the light of galaxies in the distant background and making them easier to see.

This doesn’t make it the most remote galaxy ever detected—several have been confirmed at redshifts of more than 8, and one has an estimated redshift of around 10, placing it 400 million years earlier.

However the newly discovered galaxy is very different to other distant ones, which generally shine brightly with only young stars.

“When we looked at the spectra, two things were clear,” explains co-author Eiichi Egami. “The redshift placed it very early in cosmic history, as we expected. But the Spitzer infrared detection also indicated that the galaxy was made up of surprisingly old and relatively faint stars.”

“This told us that the galaxy was made up of stars already nearly 750 million years old—pushing back the epoch of its formation to about 200 million years after the Big Bang, much further than we had expected,” adds Egami.

“This suggests that the first galaxies have been around for a lot longer than previously thought,” says Dan Stark, another co-author of the study.

Artist's impression of the James Webb Space Telescope

When operational, the James Webb Space Telescope will be able to see even further back in time.

Unseen army may solve the mystery

The discovery has implications beyond the question of when galaxies first formed. It might also help explain how the Universe became transparent to ultraviolet light in the first billion years after the Big Bang.

In the early years of the cosmos, a diffuse fog of neutral hydrogen gas blocked ultraviolet light in the Universe. Some source of radiation must have progressively ionised the diffuse gas, clearing the fog and making it transparent to ultraviolet rays as it is today—a process known as reionisation.

Astronomers believe that the radiation that powered this reionisation must have come from galaxies. But, so far, nowhere near enough of them have been found to provide the necessary radiation.

The new discovery may help solve this enigma.

“It seems probable that there are in fact far more galaxies out there in the early Universe than we previously estimated—it’s just that many galaxies are older and fainter, like the one we have just discovered,” says co-author Jean-Paul Kneib.

“If this unseen army of faint, elderly galaxies is indeed out there, they could provide the missing radiation that made the Universe transparent to ultraviolet light.”

As of today, astronomers can discover these galaxies only by seeing them through gravitational lenses. But the NASA/ESA/CSA James Webb Space Telescope, scheduled for launch later this decade, will be able to make high-resolution observations of the distant, highly redshifted bodies.

Astronomers hope then to be in a position to solve this mystery once and for all.

Adapted from information issued by the ESA Hubble Information Centre. Abell 383 image courtesy of NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM) (acknowledgement: Marc Postman (STScI)). Gravitational lensing diagram courtesy NASA, ESA & L. Calçada.

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Huge galaxy group reveals dark matter

Image of a cluster of galaxies

This wide-field, deep image reveals thousands of galaxies crowding an area on the sky roughly as large as the full Moon. Some of the galaxies are relatively close. The more distant look just like faint blobs — their light has travelled through the Universe for eight billion years or more before reaching Earth.

A new wide-field image released by the European Southern Observatory shows many thousands of distant galaxies, especially a large group belonging to the massive galaxy cluster known as Abell 315.

As crowded as it may appear, this assembly of galaxies is only the proverbial “tip of the iceberg”, as Abell 315 — like most galaxy clusters — is dominated by dark matter.

And the huge mass of this cluster bends the light coming from more-distant background galaxies, distorting their observed shapes slightly.

When looking at the sky with the unaided eye, we mostly only see stars within our Milky Way galaxy and some of its closest neighbours. More distant galaxies are just too faint to be perceived by the human eye, but if we could see them, they would literally cover the sky.

The new image is both a wide-field and long-exposure one, and reveals thousands of galaxies crowding an area on the sky roughly as large as the full Moon. The full-size image shows the full extent of the galaxy cluster (click here).

These galaxies span a vast range of distances from us. Some are relatively close, as it is possible to distinguish their spiral arms or elliptical halos, especially in the upper part of the image. The more distant appear just like the faintest of blobs — their light has travelled through the Universe for eight billion years or more before reaching Earth.

This diagram shows how gravitational lensing works.

This diagram shows how gravitational lensing works. A distant galaxy (right) is actually directly behind the cluster's core. The orange arrow show how the galaxy's real position and shape get distorted by the cluster's gravity. The white arrow shows the true path of the galaxy's light if the cluster didn't get in the way.

Dominated by dark matter

Galaxy clusters are some of the largest structures in the Universe held together by gravity.

But there is more in these structures than the many galaxies we can see.

Galaxies in these giants contribute to only ten percent of the mass, with hot gas in between galaxies accounting for another ten percent. The remaining 80 percent is made of an invisible and unknown ingredient called dark matter that lies in between the galaxies.

The presence of dark matter is revealed through its gravitational effect: the enormous mass of a galaxy cluster acts on the light from galaxies behind the cluster like a cosmic magnifying glass, bending the trajectory of the light and thus making the galaxies appear slightly distorted.

By observing and analysing the twisted shapes of these background galaxies, astronomers can infer the total mass of the cluster responsible for the distortion, even when this mass is mostly invisible.

However, this effect is usually tiny, and it is necessary to measure it over a huge number of galaxies to obtain significant results: in the case of Abell 315, the shapes of almost 10,000 faint galaxies in this image were studied in order to estimate the total mass of the cluster, which amounts to over a 100,000 billion times the mass of our Sun.

Adapted from information issued by ESO / J. Dietrich / NASA.

Hubble confirms the universe is expanding faster

A map showing the expected location of dark matter withing a region of deep space

A map showing the expected location of dark matter withing a region of deep space

A new study led by European scientists presents the most comprehensive analysis of data from the most ambitious survey ever undertaken by the NASA/ESA Hubble Space Telescope.

The researchers have, for the first time ever, used Hubble data to probe the effects of the natural gravitational “weak lenses” in space and characterise the expansion of the Universe.

A group of astronomers, led by Tim Schrabback of the Leiden Observatory, conducted an intensive study of over 446,000 galaxies within the COSMOS field, the result of the largest survey ever conducted with Hubble. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the same part of the Universe using the Advanced Camera for Surveys (ACS) onboard Hubble. It took nearly 1,000 hours of observations.

In addition to the Hubble data, researchers used redshift data from ground-based telescopes to assign distances to 194,000 of the galaxies surveyed (out to a redshift of 5).

“The sheer number of galaxies included in this type of analysis is unprecedented, but more important is the wealth of information we could obtain about the invisible structures in the Universe from this exceptional dataset,” says Patrick Simon from Edinburgh University.

An illustration showing how Hubble looks back in time to "map" evolving dark matter

Hubble looks back in time to "map" evolving dark matter by splitting the background galaxy population into discrete epochs of time (like cutting through rock strata). By measuring the redshift of the "lensing" galaxies used to map the dark matter distribution, scientists can put them into different time/distance "slices".

In particular, the astronomers could “weigh” the large-scale matter distribution in space over large distances. To do this, they made use of the fact that this information is encoded in the distorted shapes of distant galaxies, a phenomenon referred to as weak gravitational lensing.

Using complex algorithms, the team led by Schrabback has improved the standard method and obtained galaxy shape measurements to an unprecedented precision. The results of the study will be published in an upcoming issue of Astronomy and Astrophysics.

The meticulousness and scale of this study enables an independent confirmation that the expansion of the Universe is accelerated by an additional, mysterious component named dark energy. A handful of other such independent confirmations exist.

Astronomers compared real observations with two predictions – one for a dark matter-dominated universe, the other one dominated by dark energy.

COSMOS Project Astronomers compared real observations with two simulations – one for a dark matter-dominated universe, the other one dominated by dark energy. The dark energy one is the closest match.

Scientists need to know how the formation of clumps of matter evolved in the history of the Universe to determine how the gravitational force, which holds matter together, and dark energy, which pulls it apart by accelerating the expansion of the Universe, have affected them.

“Dark energy affects our measurements for two reasons. First, when it is present, galaxy clusters grow more slowly, and secondly, it changes the way the Universe expands, leading to more distant — and more efficiently lensed — galaxies. Our analysis is sensitive to both effects,” says co-author Benjamin Joachimi from the University of Bonn.

“Our study also provides an additional confirmation for Einstein’s theory of general relativity, which predicts how the lensing signal depends on redshift,” adds co-investigator Martin Kilbinger from the Institut d’Astrophysique de Paris and the Excellence Cluster Universe.

The large number of galaxies included in this study, along with information on their redshifts is leading to a clearer map of how, exactly, part of the Universe is laid out; it helps us see its galactic inhabitants and how they are distributed.

“With more accurate information about the distances to the galaxies, we can measure the distribution of the matter between them and us more accurately,” notes co-investigator Jan Hartlap from the University of Bonn.

“Before, most of the studies were done in 2D, like taking a chest X-ray. Our study is more like a 3D reconstruction of the skeleton from a CT scan. On top of that, we are able to watch the skeleton of dark matter mature from the Universe’s youth to the present,” comments William High from Harvard University, another co-author.

Image credits: NASA, ESA, J. Hartlap (University of Bonn), P. Simon (University of Bonn) and T. Schrabback (Leiden Observatory)