* Astronomy

"The distribution of dark matter bears no relationship to anything you will have read in the literature up to now. It comes in a 'magic volume' which happens to correspond to an amount which is 30 million times the mass of the Sun. It looks like you cannot ever pack it smaller than about 300 parsecs - 1,000 light-years; this stuff will not let you. That tells you a speed actually - about 9km/s - at which the dark matter particles are moving because they are moving too fast to be compressed into a smaller scale. These are the first properties other than existence that we've been able determine" - Professor Gerry Gilmore, Institute of Astronomy, Cambridge.

Different regions of space have different amounts of dark matter. The concentrations can be measured in terms of the equivalent weight of hydrogen, the lightest atom in the universe, per cubic centimetre. Around the Sun the concentration of dark matter is equivalent in weight to a third of an atom of hydrogen per cubic centimetre. According to the new results, the maximum density that dark matter can be packed into is much greater: the weight of four hydrogen atoms per cubic centimetre. While diffuse, it permeates the entire universe and adds up to more than five times the mass of all the stars and galaxies in existence.

The results, which are yet to be published, were obtained by analysing measurements made at the Very Large Telescope, an array of four 8m telescopes on the Paranal mountain in Chile, part of the European Southern Observatory.
The observations took 23 nights of work, the biggest British experiment carried out at Paranal.

Source

Gravitational Theory, Galaxy Rotation Curves and Cosmology without Dark Matter
Authors: J. W. Moffat

Details are available in the January 10th, 2006, edition of The Astrophysical Journal 636, pp. 721-741(subscription) The paper entitled "Galaxy Rotation Curves Without Nonbaryonic Dark Matter" is co-authored by John Moffat, Perimeter Institute for Theoretical Physics, and Joel Brownstein, Perimeter Institute for Theoretical Physics.


Abstract.
Einstein gravity coupled to a massive skew symmetric field Fμνλ leads to an acceleration law that modifies the Newtonian law of attraction between particles. The researchers use a framework of non-perturbative renormalisation group equations as well as observational input to characterise special renormalisation group trajectories to allow for the running of the effective gravitational coupling G and the coupling of the skew field to matter. Strong renormalisation effects occur at large and small momentum scales. The latter lead to an increase of Newton's constant at large galactic and cosmological distances. For weak fields a fit to the flat rotation curves of galaxies is obtained in terms of the mass (mass-to-light ratio M/L) of galaxies. The fits assume that the galaxies are not dominated by exotic dark matter and that the effective gravitational constant G runs with the distance scale. The equations of motion for test particles yield predictions for the solar system and the binary pulsar PSR 1913+16 that agree with the observations. The gravitational lensing of clusters of galaxies can be explained without exotic dark matter. A Friedmann–Lemaitre–Robertson–Walker cosmological model with an effective G = G(t) running with time can lead to consistent fits to cosmological data without assuming the existence of exotic cold dark matter.

Read the full article

The most prominent of the Milky Way's satellite galaxies - a pair of galaxies called the Magellanic Clouds - appears to be interacting with the Milky Way's ghostly dark matter to create a mysterious warp in the galactic disk that has puzzled astronomers for half a century.

The warp, seen most clearly in the thin disk of hydrogen gas permeating the galaxy, extends across the entire 200,000-light year diameter of the Milky Way, with the sun and earth sitting somewhere near the crease. Leo Blitz, professor of astronomy at the University of California, Berkeley, and his colleagues, Evan Levine and Carl Heiles, have charted this warp and analysed it in detail for the first time, based on a new galactic map of hydrogen gas (HI) emissions.
They found that the atomic gas layer is vibrating like a drum, and that the vibration consists almost entirely of three notes, or modes.

Astronomers previously dismissed the Magellanic Clouds - comprised of the Large and Small Magellanic Clouds - as a probable cause of the galactic warp because the galaxies' combined masses are only 2 percent that of the disk. This mass was thought too small to influence a massive disk equivalent to about 200 billion suns during the clouds' 1.5 billion-year orbit of the galaxy.

Nevertheless, theorist Martin D. Weinberg, a professor of astronomy at the University of Massachusetts, Amherst, teamed up with Blitz to create a computer model that takes into account the Milky Way's dark matter, which, though invisible, is 20 times more massive than all visible matter in the galaxy combined. The motion of the clouds through the dark matter creates a wake that enhances their gravitational influence on the disk. When this dark matter is included, the Magellanic Clouds, in their orbit around the Milky Way, very closely reproduce the type of warp observed in the galaxy.

"The model not only produces this warp in the Milky Way, but during the rotation cycle of the Magellanic Clouds around the galaxy, it looks like the Milky Way is flapping in the breeze" - Leo Blitz, director of UC Berkeley's Radio Astronomy Laboratory.

"People have been trying to look at what creates this warp for a very long time. Our simulation is still not a perfect fit, but it has a lot of the character of the actual data" - Martin D. Weinberg.

Levine, a graduate student, will present the results of the work in Washington, D.C., on January 9 2006, during a 10 a.m. session on galactic structure at the American Astronomical Society meeting. Blitz will summarize the work later that day during a 12:30 p.m. press briefing in the Wilson C Room of the Marriott Wardman Park Hotel.

The interaction of the Magellanic Clouds with the dark matter in the galaxy to produce an enigmatic warp in the hydrogen gas layer is reminiscent of the paradox that led to the discovery of dark matter some 35 years ago. As astronomers built better and better telescopes able to measure the velocities of stars and gas in the outer regions of our galaxy, they discovered these stars moving far faster than would be expected from the observed number and mass of stars in the entire Milky Way. Only by invoking a then-heretical notion, that 80 percent of the galaxy's mass was too dark to see, could astronomers reconcile the velocities with known theories of physics.
Though no one knows the true identity of this dark matter - the current consensus is that it is exotic matter rather than normal stars too dim to see - astronomers are now taking it into account in their simulations of cosmic dynamics, whether to explain the lensing effect galaxies and galaxy clusters have on the light from background galaxies, or to describe the evolution of galaxy clusters in the early universe.

Some physicists, however, have come up an alternative theory of gravity called Modified Newtonian Dynamics, or MOND, that seeks to explain these observations without resorting to belief in a large amount of undetected mass in the universe, like an invisible elephant in the room. Though MOND can explain some things, Weinberg thinks the theory will have a hard time explaining the Milky Way's warp.

"Without a dark matter halo, the only thing the gas disk can feel is direct gravity from the Magellanic Clouds themselves, which was shown in the 1970s not to work. It looks bad for MOND, in this case" - Martin D. Weinberg.

Because many galaxies have warped disks, similar dynamics might explain them as well. Either way, the researchers say their work suggests that warps provide a way to verify the existence of the dark matter.

The starting point for this research was new spectral data released this past summer about hydrogen's 21-centimeter emissions in the Milky Way. The survey, the Leiden-Argentina-Bonn or LAB Survey of Galactic HI, merged a northern sky survey conducted by astronomers in the Netherlands (the Leiden/Dwingeloo Survey) with a southern sky survey from the Instituto Argentino de Radioastronomía. The data were corrected by scientists at the Institute for Radioastronomy of the University of Bonn, Germany.
Blitz, Levine and Heiles, UC Berkeley professor of astronomy, took these data and produced a new, detailed map of the neutral atomic hydrogen in the galaxy. This hydrogen, distributed in a plane with dimensions like those of a compact disk, eventually condenses into molecular clouds that become stellar nurseries.
With map in hand, they were able to mathematically describe the warp as a combination of three different types of vibration: a flapping of the disk's edge up and down, a sinusoidal vibration like that seen on a drumhead, and a saddle-shaped oscillation. These three "notes" are about 3 million octaves below middle C.

"We found something very surprising, that we could describe the warp by three modes of vibration, or three notes, and only three" - Leo Blitz.

This rather simple mathematical description of the warp had escaped the notice of astronomers since the warp's discovery in 1957.

"We were actually trying to analyse a more complex 'scalloping' structure of the disk, and this simple, elegant vibrational structure just popped out" - Evan Levine.

The current warp in the gas disk is a combination of these three vibrational modes, leaving one-half of the galactic disk sticking up above the plane of stars and gas, while the other half dips below the disk before rising upward again farther outward from the centre of the galaxy. The results of this analysis will be published in an upcoming issue of the Astrophysical Journal.
Weinberg thought he could explain the observed warp dynamically, and used computers to calculate the effect of the Magellanic Clouds orbiting the Milky Way, ploughing through the dark matter halo that extends far out into the orbit of the clouds.
What he and Blitz found is that the clouds' wake through the dark matter excites a vibration or resonance at the centre of the dark matter halo, which in turn makes the disk embedded in the halo oscillate strongly in three distinct modes. The combined motion during a 1.5-billion-year orbit of the Magellanic Clouds is reminiscent of the edges of a tablecloth flapping in the wind, since the centre of the disk is pinned down.

"We often think of the warp as being static, but this simulation shows that it is very dynamic" - Leo Blitz .

Blitz, Levine and Heiles are continuing their search for anomalies in the structure of the Milky Way's disk. Weinberg hopes to use the UC Berkeley group's data and analysis to determine the shape of the dark matter halo of the Milky Way.

Astronomers have found clear indications that clumps of dark matter are the nursing grounds for newborn galaxies about twelve billion light years away. A single nest of dark matter can nurture several young galaxies. These results from researchers at the Space Telescope Science Institute, the National Astronomical Observatory of Japan, and the University of Tokyo confirm predictions of the currently dominant theory of cosmology known as the cold dark matter model.

Recent studies suggest that dark matter out weighs ordinary matter by a factor of seven. Although dark matter cannot be seen directly through a telescope, it reveals itself to astronomers by its strong gravitational pull on nearby stars and gas, and even galaxies.

Galaxies are often clustered together and how they cluster is determined mostly by gravity.


A scientifically accurate artistic image of galaxies twelve billion light years away. The blue nebulosity is dark matter. Denser regions are white. The blue-white regions correspond to the dark matter clumps or dark matter halos where young galaxies are forming.
Image credit Naomi Ishikawa and Takaaki Takeda, National Astronomical Observatory of Japan

By studying how galaxies cluster, it is possible to determine how dark matter is distributed and how it affects the birth and growth of galaxies. In the past, it was extremely difficult to study the clustering of young galaxies. Young galaxies appear faint due to their great distances, and finding enough of them to study how they cluster was an observational challenge.



Masami Ouchi from the Space Telescope Science Institute and colleagues used the Subaru telescope and its Suprime-Cam camera to study a piece of the sky in the constellation Cetus (the Whale) called the Subaru/XMM-Newton Deep Survey Field (SXDS). This piece of sky covers an area five times the size of the full moon. By taking deep and sensitive images of the field in three colours of visible light, the SXDS team was able to find about seventeen thousand (17,000) young galaxies twelve billion light years away. This number is ten times larger than previous studies of such young galaxies.



Based on these data, the team found that:

1) There are many pairs of galaxies with separations less than eight hundred thousand (800,000) light years.
2) Even at large distances, galaxies are strongly clustered.

Both of these results are expected if the galaxies are nestled within clumps of dark matter.
The SXDS team compared the observational results in detail to theoretical predictions based on a Cold Dark Matter model by team member Takashi Hamana and found that the average clump of dark matter nests weighs as much as six hundred billion (600,000,000,000) Suns, and that a single clump of dark matter harbours multiple young galaxies.

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Independently, Nobunari Kashikawa from the National Astronomical Observatory of Japan and colleagues also used Subaru's Suprime-Cam camera to study an area of sky in the constellation Coma Berenices (Berenice's Hair) called the Subaru Deep Field (SDF). This field is only the size of one full moon but the data available are twice as sensitive as the SXDS field data. The SDF team found about five thousand (5,000) young galaxies at a distance of twelve billion light years, and eight hundred (800)even younger galaxies at a distance of twelve billion five hundred million light years. The SDF team was also able to double-check the identities of the young galaxies by taking spectral data of the galaxies with the Subaru and Keck telescopes. The SDF team independently obtained the results 1) + 2) described above, and concluded that some single clumps of dark matter harbours multiple young galaxies. In the SDF images, it is possible to see several newborn galaxies huddled together in a small area.
By comparing the SDF data in detail to high precision computer simulations of the growth of clumps in Cold Dark Matter by team member Masahiro Nagashima of Kyoto University, the SDF team concludes that heavier clumps of dark matter have more bright galaxies, and that this preference produces the correlations found in real observation.

The two teams together have found the first concrete evidence that young galaxies in the early universe are nestled within clumps of dark matter, and that a single clump of dark matter nurses several young galaxies. Both teams took advantage of the Subaru telescope's unique ability to take deep sensitive images over a large area of sky.


-- Edited by Blobrana at 23:51, 2005-12-22