VISTA finds new globular star clusters and sees right through the heart of the Milky Way

ScienceDaily (Oct. 20, 2011) — Two newly discovered globular clusters have been added to the total of just 158 known globular clusters in our Milky Way. They were found in new images from ESO's VISTA survey telescope as part of the Via Lactea (VVV) survey. This survey has also turned up the first star cluster that is far beyond the centre of the Milky Way and whose light has had to travel right through the dust and gas in the heart of our galaxy to get to us.

The dazzling globular cluster called UKS 1 dominates the right-hand side of the first of the new infrared images from ESO's VISTA survey telescope at the Paranal Observatory in Chile. But if you can drag your gaze away, there is a surprise lurking in this very rich star field -- a fainter globular cluster that was discovered in the data from one of VISTA's surveys. You will have to look closely to see the other star cluster, which is called VVV CL001: it is a small collection of stars in the left half of the image.

But VVV CL001 is just the first of VISTA's globular discoveries. The same team has found a second object, dubbed VVV CL002, which appears in image b [1]. This small and faint grouping may also be the globular cluster that is the closest known to the centre of the Milky Way. The discovery of a new globular cluster in our Milky Way is very rare. The last one was discovered in 2010, and only 158 globular clusters were known in our galaxy before the new discoveries.

These new clusters are early discoveries from the VISTA Variables in the Via Lactea (VVV) survey that is systematically studying the central parts of the Milky Way in infrared light. The VVV team is led by Dante Minniti (Pontificia Universidad Católica de Chile) and Philip Lucas (Centre for Astrophysics Research, University of Hertfordshire, UK).

As well as globular clusters, VISTA is finding many open, or galactic clusters, which generally contain fewer, younger, stars than globular clusters and are far more common (eso1128). Another newly announced cluster, VVV CL003, seems to be an open cluster that lies in the direction of the heart of the Milky Way, but much further away, about 15 000 light-years beyond the centre. This is the first such cluster to be discovered on the far side of the Milky Way.

Given the faintness of the newly found clusters, it is no wonder that they have remained hidden for so long; up until a few years ago, UKS 1 (seen in image a), which easily outshines the newcomers, was actually the dimmest known globular cluster in the Milky Way. Because of the absorption and reddening of starlight by interstellar dust, these objects can only be seen in infrared light and VISTA, the world's largest survey telescope, is ideally suited to searching for new clusters hidden behind dust in the central parts of the Milky Way [2].

One intriguing possibility is that VVV CL001 is gravitationally bound to UKS 1 -- making these two stellar groups the Milky Way's first binary globular cluster pair. But this could just be a line-of-sight effect with the clusters actually separated by a vast distance.

These VISTA pictures were created from images taken though near-infrared filters J (shown in blue), H (shown in green), and Ks (shown in red). The size of the images show only a small fraction of the full VISTA field of view.

Notes

[1] The discovery of the additional new clusters was just announced in San Juan, Argentina, during the first bi-national meeting of the Argentinian and Chilean astronomical associations.

[2] The tiny dust grains that form huge clouds within galaxies scatter blue light much more strongly than red and infrared light. As a result astronomers can see through the dust much more effectively if they study infrared light rather than the usual visible radiation that our eyes are sensitive to.

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How the Milky Way killed off nearby galaxies

ScienceDaily (Oct. 18, 2011) — Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of the first stars in our galaxy, the Milky Way. More than 12 billion years ago, the intense ultraviolet light from these stars dispersed the gas of our Galaxy's nearest companions, virtually putting a halt to their ability to form stars and consigning them to a dim future. Now Pierre Ocvirk and Dominique Aubert, members of the Light in the Dark Ages of the Universe (LIDAU) collaboration, have explained why some galaxies were killed off, while stars continued to form in more distant objects.

The two scientists publish their results in the October issue of the letters of the journal Monthly Notices of the Royal Astronomical Society.

The first stars of the Universe appeared about 150 million years after the Big Bang. Back then, the hydrogen and helium gas filling the universe was cold enough for its atoms to be electrically neutral. As the ultraviolet (UV) light of the first stars propagated through this gas, it broke apart the proton-electron pairs that make up hydrogen atoms, returning them to the so-called plasma state they experienced in the first moments of the Universe. This process, known as reionisation, also resulted in significant heating, which had dramatic consequences: the gas became so hot that it escaped the weak gravity of the lowest mass galaxies, thereby depriving them of the material needed to form stars.

It is now widely accepted that this process can explain the small number and large ages of the stars seen in the faintest dwarf galaxy satellites of the Milky Way. It also helps scientists understand why galaxies like the Milky Way have so few satellites around them -- the 'missing satellites' problem. The stripping out of gas from these galaxies makes them sensitive probes of the UV radiation in the reionisation epoch.

The satellite galaxies are also relatively close, from 30000 to 900000 light-years away, which allows us to study them in great detail, something that will be enhanced by the coming generation of larger telescopes. Comparing the population of their stars in each galaxy with its position could give us a unique insight into the structure of the UV radiation emitted from the earliest stars in the Milky Way.

Until now, models for this process assumed that the radiation leading to the removal of gas from galaxy satellites was produced collectively by all the large galaxies nearby, resulting in a uniform background of UV light. The new model put together by the two French researchers proves this assumption wrong.

Ocvirk and Aubert looked at the way the invisible 'dark matter' that makes up about 23% of the Universe structured itself with the stars in our Galaxy and its environs from shortly after the Big Bang to the present day. They used the high resolution numerical simulation Via Lactea II to model the formation of stars in gas trapped in the dark matter haloes that envelop galaxies, and then to describe how this gas reacted to UV radiation.

Pierre Ocvirk comments, "This is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the centre of the Milky Way on its satellite galaxies.

'In contrast to previous models, the radiation field produced is not uniform, but decreases in intensity as one moves away from the centre of the Milky Way.

'The satellite galaxies close to the galactic centre see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. At the same time, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous."

The new model appears to be a close match to observations of our Galaxy and its neighbourhood and suggests that the first stars of our galaxy played a major role in the photo-evaporation of the satellite galaxies' gas, adds Dr Ocvirk. "It is not large nearby galaxies but our own that caused the demise of its tiny neighbours, asphyxiating them through its intense radiation."

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Journal Reference:

Pierre Ocvirk, Dominique Aubert. A signature of the internal reionisation of the Milky Way? Monthly Notices of the Royal Astronomical Society, 2011 [link]

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VISTA finds new globular star clusters and sees right through the heart of the Milky Way

ScienceDaily (Oct. 20, 2011) — Two newly discovered globular clusters have been added to the total of just 158 known globular clusters in our Milky Way. They were found in new images from ESO's VISTA survey telescope as part of the Via Lactea (VVV) survey. This survey has also turned up the first star cluster that is far beyond the centre of the Milky Way and whose light has had to travel right through the dust and gas in the heart of our galaxy to get to us.

The dazzling globular cluster called UKS 1 dominates the right-hand side of the first of the new infrared images from ESO's VISTA survey telescope at the Paranal Observatory in Chile. But if you can drag your gaze away, there is a surprise lurking in this very rich star field -- a fainter globular cluster that was discovered in the data from one of VISTA's surveys. You will have to look closely to see the other star cluster, which is called VVV CL001: it is a small collection of stars in the left half of the image.

But VVV CL001 is just the first of VISTA's globular discoveries. The same team has found a second object, dubbed VVV CL002, which appears in image b [1]. This small and faint grouping may also be the globular cluster that is the closest known to the centre of the Milky Way. The discovery of a new globular cluster in our Milky Way is very rare. The last one was discovered in 2010, and only 158 globular clusters were known in our galaxy before the new discoveries.

These new clusters are early discoveries from the VISTA Variables in the Via Lactea (VVV) survey that is systematically studying the central parts of the Milky Way in infrared light. The VVV team is led by Dante Minniti (Pontificia Universidad Católica de Chile) and Philip Lucas (Centre for Astrophysics Research, University of Hertfordshire, UK).

As well as globular clusters, VISTA is finding many open, or galactic clusters, which generally contain fewer, younger, stars than globular clusters and are far more common (eso1128). Another newly announced cluster, VVV CL003, seems to be an open cluster that lies in the direction of the heart of the Milky Way, but much further away, about 15 000 light-years beyond the centre. This is the first such cluster to be discovered on the far side of the Milky Way.

Given the faintness of the newly found clusters, it is no wonder that they have remained hidden for so long; up until a few years ago, UKS 1 (seen in image a), which easily outshines the newcomers, was actually the dimmest known globular cluster in the Milky Way. Because of the absorption and reddening of starlight by interstellar dust, these objects can only be seen in infrared light and VISTA, the world's largest survey telescope, is ideally suited to searching for new clusters hidden behind dust in the central parts of the Milky Way [2].

One intriguing possibility is that VVV CL001 is gravitationally bound to UKS 1 -- making these two stellar groups the Milky Way's first binary globular cluster pair. But this could just be a line-of-sight effect with the clusters actually separated by a vast distance.

These VISTA pictures were created from images taken though near-infrared filters J (shown in blue), H (shown in green), and Ks (shown in red). The size of the images show only a small fraction of the full VISTA field of view.

Notes

[1] The discovery of the additional new clusters was just announced in San Juan, Argentina, during the first bi-national meeting of the Argentinian and Chilean astronomical associations.

[2] The tiny dust grains that form huge clouds within galaxies scatter blue light much more strongly than red and infrared light. As a result astronomers can see through the dust much more effectively if they study infrared light rather than the usual visible radiation that our eyes are sensitive to.

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Note: ScienceDaily reserves the right to edit materials for content and length. For further information, please contact the source cited above.

Note: If no author is given, the source is cited instead.

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Just four percent of galaxies have neighbors like the Milky Way

ScienceDaily (May 23, 2011) — How unique is the Milky Way?

To find out, a group of researchers led by Stanford University astrophysicist Risa Wechsler compared the Milky Way to similar galaxies and found that just four percent are like the galaxy Earth calls home.

"We are interested in how the Milky Way fits into the broader context of the universe," said Wechsler. "This research helps us understand whether our galaxy is typical or not, and may provide clues to its formation history."

The research team compared the Milky Way to similar galaxies in terms of luminosity--a measure of how much light is emitted--and distance to other bright galaxies. They found galaxies that have two satellites that are as bright and close by as the Milky Way's two closest satellites, the Large and Small Magellanic Clouds, are rare.

Published in the May 20 issue of the Astrophysical Journal, the findings are based on analyses of data collected from the Sloan Digital Sky Survey (SDSS). The work is the first of three papers that study the properties of the Milky Way's two most massive satellites.

Supported in part by the National Science Foundation (NSF), the SDSS is the most extensive survey of the optical sky performed to date.

In more than eight years of operations, SDSS has obtained images covering more than a quarter of the sky, and created 3-dimensional maps containing more than 930,000 galaxies and 120,000 quasars. For this analysis, Wechsler's group studied more than 20,000 galaxies with properties similar to the Milky Way and investigated the galaxies surrounding these Milky Way "twins," to create a "census" of galaxies similar to the Milky Way in the universe.

The work represents one of the most extensive studies of this kind ever performed.

Scientists can also compare the SDSS data to galaxies simulated by a computer model. Since they are currently unable to see all the way back to the Big Bang, this is one way researchers are trying to understand how the universe as we see it today began.

In order to learn more about possible conditions in the early universe, the group performed computer simulations to recreate the universe from specific sets of starting conditions. Then they compared their simulations to the SDSS data set. In this way, the group was able to test different theories of galaxy formation to determine whether or not each would result in a universe that matches what we see today. The results of their simulation matched the result found in the SDSS data set: just four percent of the simulated galaxies had two satellites like the Magellanic Clouds.

"This is an excellent example of data-enabled science," said Nigel Sharp, of NSF's Division of Astronomical Sciences. "Comparing the 'fake' and 'real' Universes is how we discriminate between successful and unsuccessful theories. This work interconnects three of the four legs of science: theory, observation and simulation, for a powerful scientific result."

Their results also lend support to a leading theory of galaxy formation called the Cold Dark Matter (CDM) theory. This theory provides what many consider to be the simplest explanation for the arrangement of galaxies throughout the universe following the Big Bang. It assumes that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation (dark) and whose constituent particles move slowly (cold). Dark matter, an invisible and exotic material of unknown composition, is believed to influence the distribution of galaxies in space and the overall expansion of the universe. The rareness of this aspect of the Milky Way may provide clues to its formation history.

"Because the presence of two galaxies like the Magellanic Clouds is unusual, we can use them to learn more about our own galaxy," said Wechsler. Using their simulation, the team identified a sample of simulated galaxies that had satellites matching the Milky Way's in terms of their locations and speeds.

"The combination of large surveys of the sky like the SDSS and large samples of simulated galaxies provides a new opportunity to learn about the place of our galaxy in the Universe," said Wechsler. "Future surveys will allow us to extend this study to even dimmer satellite galaxies, to build a full picture of the formation of our galaxy."

The theoretical and numerical work that produced the simulations used as a comparison for the SDSS data were supported by an award funded under the American Recovery and Reinvestment Act of 2009.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by National Science Foundation.

Journal Reference:

Lulu Liu, Brian F. Gerke, Risa H. Wechsler, Peter S. Behroozi, Michael T. Busha. How Common Are the Magellanic Clouds? The Astrophysical Journal, 2011; 733 (1): 62 DOI: 10.1088/0004-637X/733/1/62

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