Universe Today

Image credit: NRAO
A team of astronomers using the National Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT) has made the first conclusive detection of what appear to be the leftover building blocks of galaxy formation — neutral hydrogen clouds — swarming around the Andromeda Galaxy, located in the Andromeda constellation, the nearest large spiral galaxy to the Milky Way.

This discovery may help scientists understand the structure and evolution of the Milky Way and all spiral galaxies. It also may help explain why certain young stars in mature galaxies are surprisingly bereft of the heavy elements that their contemporaries contain.

“Giant galaxies, like Andromeda and our own Milky Way, are thought to form through repeated mergers with smaller galaxies and through the accretion of vast numbers of even lower mass ‘clouds’ — dark objects that lack stars and even are too small to call galaxies,” said David A. Thilker of the Johns Hopkins University in Baltimore, Maryland. “Theoretical studies predict that this process of galactic growth continues today, but astronomers have been unable to detect the expected low mass ‘building blocks’ falling into nearby galaxies, until now.”

Thilker’s research is published in the Astrophysical Journal Letters. Other contributors include: Robert Braun of the Netherlands Foundation for Research in Astronomy; Rene A.M. Walterbos of New Mexico State University; Edvige Corbelli of the Osservatorio Astrofisico di Arcetri in Italy; Felix J. Lockman and Ronald Maddalena of the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia; and Edward Murphy of the University of Virginia.

The Milky Way and Andromeda were formed many billions of years ago in a cosmic neighborhood brimming with galactic raw materials — among which hydrogen, helium, and cold dark matter were primary constituents. By now, most of this raw material has probably been gobbled up by the two galaxies, but astronomers suspect that some primitive clouds are still floating free.

Previous studies have revealed a number of clouds of neutral atomic hydrogen that are near the Milky Way but not part of its disk. These were initially referred to as high-velocity clouds (HVCs) when they were first discovered because they appeared to move at velocities difficult to reconcile with Galactic rotation.

Scientists were uncertain if HVCs comprised building blocks of the Milky Way that had so far escaped capture, or if they traced gas accelerated to unexpected velocities by energetic processes (multiple supernovae) within the Milky Way. The discovery of similar clouds bound to the Andromeda Galaxy strengthens the case that at least some of these HVCs are indeed galactic building blocks.

Astronomers are able to use radio telescopes to detect the characteristic 21-centimeter radiation emitted naturally by neutral atomic hydrogen. The great difficulty in analyzing these low-mass galactic building blocks has been that their natural radio emission is extremely faint. Even those nearest to us, clouds orbiting our Galaxy, are hard to study because of serious distance uncertainties. “We know the Milky Way HVCs are relatively nearby, but precisely how close is maddeningly tough to determine,” said Thilker.

Past attempts to find missing satellites around external galaxies at well-known distances have been unsuccessful because of the need for a very sensitive instrument capable of producing high-fidelity images, even in the vicinity of a bright source such as the Andromeda Galaxy.

One might consider this task similar to visually distinguishing a candle placed adjacent to a spotlight. The novel design of the recently commissioned GBT met these challenges brilliantly, and gave astronomers their first look at the cluttered neighborhood around Andromeda.

The Andromeda Galaxy was targeted because it is the nearest massive spiral galaxy. “In some sense, the rich get richer, even in space,” said Thilker. “All else being equal, one would expect to find more primordial clouds in the vicinity of a large spiral galaxy than near a small dwarf galaxy, for instance. This makes Andromeda a good place to look, especially considering its relative proximity — a mere 2.5 million light-years from Earth.”

What the GBT was able to pin down was a population of 20 discrete neutral hydrogen clouds, together with an extended filamentary component, which, the astronomers believe, are both associated with Andromeda. These objects, seemingly under the gravitational influence of Andromeda’s halo, are thought to be the gaseous clouds of the “missing” (perhaps dark-matter dominated) satellites and their merger remnants. They were found within 163,000 light-years of Andromeda.

Favored cosmological models have predicted the existence of these satellites, and their discovery could account for some of the missing “cold dark matter” in the Universe. Also, confirmation that these low-mass objects are ubiquitous around larger galaxies could help solve the mystery of why certain young stars, known as G-dwarf stars, are chemically similar to ones that evolved billions of years ago.

As galaxies age, they develop greater concentrations of heavy elements formed by the nuclear reactions in the cores of stars and in the cataclysmic explosions of supernovae. These explosions spew heavy elements out into the galaxy, which then become planets and get taken up in the next generation of stars.

Spectral and photometric analysis of young stars in the Milky Way and other galaxies, however, show that there are a certain number of young stars that are surprisingly bereft of heavy elements, making them resemble stars that should have formed in the early stages of galactic evolution.

“One way to account for this strange anomaly is to have a fresh source of raw galactic material from which to form new stars,” said Murphy. “Since high-velocity clouds may be the leftover building blocks of galaxy formation, they contain nearly pristine concentrations of hydrogen, mostly free from the heavy metals that seed older galaxies.” Their merger into large galaxies, therefore, could explain how fresh material is available for the formation of G-dwarf stars.

The Andromeda Galaxy, also known as M31, is one of only a few galaxies that are visible from Earth with the unaided eye, and is seen as a faint smudge in the constellation Andromeda. When viewed through a modest telescope, Andromeda also reveals that it has two prominent satellite dwarf galaxies, known as M32 and M110. These dwarfs, along with the clouds studied by Thilker and collaborators, are doomed to eventually merge with Andromeda. The Milky Way, M33, and the Andromeda Galaxy plus about 40 dwarf companions, comprise what is known as the “Local Group.”

Today, Andromeda is perhaps the most studied galaxy other than the Milky Way. In fact, many of the things we know about the nature of galaxies like the Milky Way were learned by studying Andromeda, since the overall features of our own galaxy are disguised by our internal vantage point. “In this case, Andromeda is a good analogue for the Milky Way,” said Murphy. “It clarifies the picture. Living inside the Milky Way is like trying to determine what your house looks like from the inside, without stepping outdoors. However, if you look at neighbors’ houses, you can get a feeling for what your own home might look like.”

The GBT is the world’s largest fully steerable radio telescope.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Image credit: RAS
In recent years, astronomers have obtained detailed measurements of the cosmic microwave background radiation – the ‘echo’ from the birth of the Universe during the Big Bang.

These results appear to indicate with remarkable precision that our Universe is dominated by mysterious ‘cold dark matter’ and ‘dark energy’. But now a group of UK astronomers has found evidence that the primordial microwave echoes may have been modified or ‘corrupted’ on their 13 billion year journey to the Earth.

The results from a team at the University of Durham, led by Professor Tom Shanks, are based on a new analysis of data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) satellite.

The team has found that nearby galaxy clusters appear to lie in regions of sky where the microwave temperature is lower than average. This behaviour could be accounted for if the hot gas in the galaxy clusters has interacted with the Big Bang photons as they passed by and corrupted the information contained in this echo of the primordial fireball. Russian physicists R. A. Sunyaev and Ya. B. Zeldovich predicted such an effect in the early 1970’s, shortly after the discovery of the cosmic microwave background radiation.

This Sunyaev-Zeldovich effect has previously been seen in the cases of detailed observations of the microwave background in the vicinity of a few rich galaxy clusters and the WMAP team themselves have reported seeing the effect in their own data, close to cluster centres.

Now the Durham team has found evidence that hot gas in the clusters may influence the microwave background maps out to a radius of nearly 1 degree from the galaxy cluster centres, a much larger area than previously detected. This suggests that the positions of “clusters of clusters” or “superclusters” may also coincide with cooler spots in the pattern of microwave background fluctuations.

“The photons in the microwave background radiation are scattered by electrons in nearby clusters,” said Professor Shanks. “This causes important changes to the radiation by the time it reaches us.”

“If the galaxy clusters located several billion light years from Earth also have the same effect, then we must consider whether it is necessary to modify our interpretation of the satellite maps of the microwave background radiation.”

If the Durham result is confirmed, then the consequences for cosmology could be highly significant. The signature for dark energy and dark matter lies in the detailed structure of the ripples detected in the microwave background, tiny temperature variations that were created at a time when the radius of the Universe was a thousand times smaller than it is today.

If this primordial pattern has been corrupted by processes taking place in the recent past, long after galaxies and galaxy clusters formed, then it will, at best, complicate the interpretation of the microwave echo and, at worst, begin to undermine the previous evidence for both dark energy and cold dark matter.

“The power of this wonderful WMAP data is that it indicates that interpreting the microwave background ‘echo’ may be less straightforward than previously thought,” said team member Sir Arnold Wolfendale (previously Astronomer Royal).

The WMAP team has already reported that their measurements of the Big Bang’s microwave echo may have been compromised by the process of galaxy formation at an intermediate stage in the Universe’s history. They presented evidence that gas heated by first-born stars, galaxies and quasars may have also corrupted the microwave signal when the Universe was 10 or 20 times smaller than at the present day. Thus both the WMAP and Durham results suggest that the microwave echo of the Big Bang may have had to come through many more obstacles on its journey to the Earth than had previously been thought, with consequent possible distortion of the primordial signal.

“Our results may ultimately undermine the belief that the Universe is dominated by an elusive cold dark matter particle and the even more enigmatic dark energy,” said Professor Shanks.

Although the observational evidence for the standard model of cosmology remains strong, the model does contain very uncomfortable aspects. These arise first because it is based on two pieces of “undiscovered physics” – cold dark matter and dark energy – neither of which has been detected in the laboratory. Indeed, the introduction of these two new components greatly increases the complication of the standard Big Bang inflationary model.

The problems of dark energy run particularly deep: for example, its observed density is so small that it may be quantum mechanically unstable. It also creates problems for the theories of quantum gravity, which suggest that we may live in a Universe with 10 or 11 dimensions, all of them shrunk, with the exceptions of three in space and one in time.

Many theorists would therefore like an escape route from today’s standard model of cosmology and it remains to be seen how far these observations discussed by the Durham group will go in this direction. But if correct, they suggest that the rumours that we are living in a “New Era of Precision Cosmology” may prove to be premature!

Original Source: RAS News Release