Galactic Magnifying Lens to Probe Elusive Dark Energy

Galactic Magnifying Lens to Probe Elusive Dark Energy

  10:26:00 AM    Science Lover

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An international team of astronomers using gravitational lensing observations from the NASA/ESA Hubble Space Telescope has taken an important step forward in the quest to solve the riddle of dark energy, a phenomenon which mysteriously appears to power the Universe's accelerating expansion.

This image shows the galaxy cluster Abell 1689, with the 
mass distribution of the gravitational lens overlaid 
(in purple). The mass in this lens is made up partly of normal 
(baryonic) matter and partly of dark matter. Distorted 
galaxies are clearly visible around the edges of the 
gravitational lens. The appearance of these distorted galaxies 
depends on the distribution of matter in the lens and on the 
relative geometry of the lens and the distant galaxies, as well
as on the effect of dark energy on the geometry of the universe. 
(Credit: NASA, ESA, E. Jullo (JPL/LAM), P. 
Natarajan (Yale) and J-P. Kneib (LAM).)

Their results appear in the 20 August 2010 issue of the journal Science.

Normal matter like that found in stars, planets and dust clouds only makes up a tiny fraction of the mass-energy content of the Universe. It is dwarfed by the amount of dark matter -- which is invisible, but can be detected by its gravitational pull. In turn, the amount of dark matter in the Universe is itself overwhelmed by the diffuse dark energy that permeates the entire Universe. Scientists believe that the pressure exerted by this dark energy is what pushes the Universe to expand at an ever-increasing rate.

Probing the nature of dark energy is, therefore, one of the key challenges in modern cosmology. Since its discovery in 1998, the quest has been to characterise and understand it better. This work presents an entirely new way to do so.

Eric Jullo, lead author of a new paper in the journal Science explains: "Dark energy is characterised by the relationship between its pressure and its density: this is known as its equation of state. Our goal was to try to quantify this relationship. It teaches us about the properties of dark energy and how it has affected the development of the Universe."

The team measured the properties of the gravitational lensing in the galaxy cluster Abell 1689. Gravitational lensing is a phenomenon predicted by Einstein's theory of general relativity, and was here used by the team to probe how the cosmological distances (and thus the shape of space-time) are modified by dark energy. At cosmic distances, a huge cluster of galaxies in the foreground has so much mass that its gravitational pull bends beams of light from very distant galaxies, producing distorted images of the faraway objects. The distortion induced by the lens depends in part on the distances to the objects, which have been precisely measured with large ground-based telescopes such as ESO's Very Large Telescope and the Keck Telescopes.

"The precise effects of lensing depend on the mass of the lens, the structure of space-time, and the relative distance between us, the lens and the distant object behind it," explains Priyamvada Natarajan, a co-author of the paper. "It's like a magnifying glass, where the image you get depends on the shape of the lens and how far you hold it from the object you're looking at. If you know the shape of the lens and the image you get, you can work out the path that light followed between the object and your eye."

Looking at the distorted images allows astronomers to reconstruct the path that light from distant galaxies takes to make its long journey to Earth. It also lets them study the effect of dark energy on the geometry of space in the light path from the distant objects to the lensing cluster and then from the cluster to us. As dark energy pushes the Universe to expand ever faster, the precise path that the light beams follow as they travel through space and are bent by the lens is subtly altered. This means that the distorted images from the lens encapsulate information about the underlying cosmology, as well as about the lens itself.

So why is the geometry of the Universe such a big issue?

"The geometry, the content and the fate of the Universe are all intricately linked," says Natarajan. "If you know two, you can deduce the third. We already have a pretty good knowledge of the Universe's mass-energy content, so if we can get a handle on its geometry then we will be able to work out exactly what the fate of the Universe will be."

The real strength of this new result is that it devises a totally new way to extract information about the elusive dark energy. It is a unique and powerful one, and offers great promise for future applications.

According to the scientists, their method required multiple, meticulous steps to develop. They spent several years developing specialised mathematical models and precise maps of the matter -- both dark and "normal" -- that together constitute the Abell 1689 cluster.

Co-author Jean-Paul Kneib explains: "Using our unique method in conjunction with others, we were able to come up with results that were far more precise than any achieved before."

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The international team of astronomers in this study consists of Eric Jullo (Jet Propulsion Laboratory/ Cal Tech, USA and Laboratoire d'Astrophysique de Marseille, France), Priyamvada Natarajan (Yale University, USA), Jean-Paul Kneib (Laboratoire d'Astrophysique de Marseille, France), Anson D'Aloisio (Yale University, USA), Marceau Limousin (Laboratoire d'Astrophysique de Marseille, France and University of Copenhagen, Denmark), Johan Richard (Durham University, UK) and Carlo Schimd (Laboratoire d'Astrophysique de Marseille, France)

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Bizarre Galaxy Is Result Of Pair Of Spiral Galaxies Smashing Together

Bizarre Galaxy Is Result Of Pair Of Spiral Galaxies Smashing Together

  9:42:00 PM    Science Lover

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A recent NASA/ESA Hubble Space Telescope image captures what appears to be one very bright and bizarre galaxy, but is actually the result of a pair of spiral galaxies that resemble our own Milky Way smashing together at breakneck speeds. The product of this dramatic collision, called NGC 2623, or Arp 243, is about 250 million light-years away in the constellation of Cancer (the Crab).

Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. NGC 2623 is in the late stages of the merging process, with the centres of the original galaxy pair now merged into one nucleus, but stretching out from the centre are two tidal tails of young stars, a strong indicator that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails. (Credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA))

  
Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. The object in the image, NGC 2623, is in the late stages of the merging process with the centres of the original galaxy pair now merged into one nucleus. However, stretching out from the centre are two tidal tails of young stars showing that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails.

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The history of the universe – in 3D!

The history of the universe – in 3D!

  12:54:00 AM    Science Lover

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In an international project, astronomers have obtained exceptional 3D images of distant galaxies, seen when the Universe was half its current age. And by looking at this unique “history book” of the universe, at an epoch when the Sun and the Earth did not even exist, scientists hope to solve the puzzle of how galaxies formed in the remote past.

The team – consisting of scientists from the US’ NASA, the European Space Agency and the European Southern Observatory – obtained the images by combining the Hubble Space Telescope’s acute eye with the capacity of the Very Large Telescope (VLT) to probe the motions of gas in tiny objects.

“Hubble and VLT are real ‘time machines’ for probing the universe’s history,” said Sébastien Peirani, lead author of one of the papers reporting on this study.

DOUBLING UP

For decades, distant galaxies that emitted their light six to eight billion years ago – over half the age of the universe – were no more than small specks of light on the sky.

With the launch of the Hubble Space Telescope in the early 1990s, astronomers were able to scrutinise the structure of these galaxies in some detail for the first time.

Now, researchers are using the Hubble’s capabilities in conjunction with those of the VLT – an array of four separate optical telescopes, situated at the Paranal Observatory in northern Chile. Its Fibre Large Array Multi Element Spectrograph (FLAMES) can observe up to 130 targets at a time, which enabled the team to measure the velocity of the gas in the distant galaxies.

“By seeing how the gas is moving, it provides us with a 3D view of galaxies halfway across the universe,” said François Hammer, who led the project.

The team is now reconstituting the history of about 100 remote galaxies. The project has already provided useful insights for three such galaxies.

UNRAVELLING GALACTIC SECRETS

In one galaxy, FLAMES revealed a region full of ionised gas – hot gas composed of atoms that have been stripped of one or several electrons. This is normally due to the presence of very hot, young stars.

However, even after staring at the region for more than 11 days, Hubble did not detect any stars! “Clearly, this unusual galaxy has some hidden secrets,” researcher Mathieu Puech said.

Computer simulations suggested that the explanation lies in the collision of two very gas-rich spiral galaxies. The heat produced by the collision would ionise the gas, making it too hot for stars to form.

Another galaxy that the scientists studied showed the opposite effect. There, they discovered a bluish central region enshrouded in a reddish disc, almost completely hidden by dust.

“The models indicate that gas and stars could be spiralling inwards rapidly,” said Hammer. “This might be the first example of a disc rebuilt after a major merger.”

Finally, in a third galaxy, the team saw a rare sight: an extremely blue, elongated structure – a bar, in fact – composed of young, massive stars. Comparisons with computer simulations showed that the properties of this object are well reproduced by a collision between two galaxies of unequal mass.

“The combination of Hubble and the VLT, along with modern computer simulations, allows us to model distant galaxies almost as nicely as the close ones,” Hammer said.

The team is now extending its analyses to the whole sample of galaxies observed.

“The next step will then be to compare this with closer galaxies, and so, piece together a picture of the evolution of galaxies over half the age of the universe,” he concluded.

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