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TOPIC: Dark matter
Blobrana


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The way in which stars, galaxies and other cosmic structures originally formed depends strongly on the nature of dark matter, claim physicists in the UK and Belgium. By simulating the early effects of warm dark matter, which some researchers consider the most likely form of dark matter, the researchers found that the first stars would have been produced in massive filaments of dense gas -- thus going against physicists conventional view of star formation

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Blobrana


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Dark matter may be made of fast, lightweight particles contrary to the most widely accepted theory, according to a new computer simulation. That could explain the peculiarly pure chemical makeup of some stars in the Milky Way, and the enormous mass of black holes that live at the hearts of large galaxies.
Because dark matter reveals itself only by its gravity, astronomers have few clues to its nature. The most popular model is cold dark matter: heavy subatomic particles that tend to move very slowly.
Another possibility is warm dark matter: lighter particles that move faster. The rapid motion of these particles smoothes out the small dense knots of matter that would otherwise form in the cores of galaxies, and there are hints that such dense knots are indeed missing.
Liang Gao and Tom Theuns of Durham University in the UK have built a computer simulation to compare the behaviour of cold and warm dark matter in the early universe. At first the two varieties behave alike, collapsing under gravity into a network of filaments that crisscross the universe.
But cold dark matter then coalesces into blobs, or haloes (see image bottom right), while warm dark matter does not (see image below right). The random motion of its particles smoothes out these blobs, so warm dark matter filaments just keep collapsing and getting denser until there is a narrow tube of matter typically 10,000 light years long with the mass of 10 million Suns.
Ordinary gas is dragged in by the dark matter, and eventually the first stars form. They are made almost entirely of hydrogen and helium, the two main elements created in the big bang.

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Blobrana


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Using the giant Keck II telescope on Mauna Kea, astronomers have proved that at least eight nearly invisible clumps of stars around our Milky Way galaxy are dwarf galaxies acting like satellites of the Milky Way.
The groups of stars had been seen only in recent decades, and no one knew if they were bound together by gravity or are just random collections of stars, said Marla Geha, an astronomer with the Herzberg Institute of Astrophysics in Victoria, British Columbia, Canada.
Geha's team discovered that gravity keeps the stars buzzing around each other like bees, she said. That's different from the Milky Way, where most of the stars are spinning around the galactic centre in an orderly way.
Geha's discovery can best be understood by anyone who has heard of two large dwarfs called the Large and Small Magellanic Clouds, visible from the Southern Hemisphere.
The dwarfs Geha studied are as much as 1,000 times smaller -- "tiny," she said.
They're also much darker. In fact, they are composed of 99 percent mysterious dark matter. It's invisible in any wavelength of light astronomers look at, yet it must be there because the effects of its gravity can be seen.

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Blobrana


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Sharpless 269
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Title: Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve
Authors: Mareki Honma, Takeshi Bushimata, Yoon Kyung Choi, Tomoya Hirota, Hiroshi Imai, Kenzaburo Iwadate, Takaaki Jike, Osamu Kameya, Ryuichi Kamohara, Yukitoshi Kan-ya, Noriyuki Kawaguchi, Masachika Kijima, Hideyuki Kobayashi, Seisuke Kuji, Tomoharu Kurayama, Seiji Manabe, Takeshi Miyaji, Takumi Nagayama, Akiharu Nakagawa, Chung Sik Oh, Toshihiro Omodaka, Tomoaki Oyama, Satoshi Sakai, Katsuhisa Sato, Tetsuo Sasao, Katsunori M. Shibata, Motonobu Shintani, Hiroshi Suda, Yoshiaki Tamura, Miyuki Tsushima, Kazuyoshi Yama****a (the VERA project)

We have performed high-precision astrometry of H2O maser sources in Galactic star forming region Sharpless 269 (S269) with VERA. We have successfully detected a trigonometric parallax of 189 ±8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as proper motions are used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy's centre) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve and provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.


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Sharpless 269.kmz
Google Sky file

-- Edited by Blobrana at 16:34, 2008-01-12

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Blobrana


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Title: How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
Authors: M. Viel, G.D. Becker, J.S. Bolton, M.G. Haehnelt, M. Rauch, W.L.W. Sargent

We present new constraints on the mass of warm dark matter (WDM) particles derived from the lya flux power spectrum of 55 high- resolution lya forest spectra at 2.0 < z < 6.4 obtained with the HIRES spectrograph at the Keck telescope. From the HIRES spectra alone, we obtain a lower limit of mwdm > 1.2 keV (2 sigma) if the WDM consists of early decoupled thermal relics and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. This result improves the previous constraints from high-resolution spectra at lower redshift by a factor two. Adding the Sloan Digital Sky Survey lya flux power spectrum at 2.2<z<4.2 from a large sample of low resolution spectra we get mwdm > 4 keV and mwdm > 28 keV (2 sigma) for thermal relics and sterile neutrinos, respectively. This is also a factor two improvement compared to previous combined analysis of high and low-resolution data. The small scale matter power spectrum probed by the high-resolution high-redshift HIRES data is instrumental for this improvement.

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Blobrana


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From subterranean caverns in Europe to the plains of Illinois, Rice physicists travelled the Earth this summer in very different quests to answer the same fundamental question: What's the unseen cosmic glue that keeps galaxies from flying apart?
Their search is taking place in far-flung labs across the world, including the world's most powerful atom smasher in Batavia, Ill.; its heir-apparent in the Swiss Alps; and an Italian national laboratory that's buried under 9,000 feet of granite.

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Blobrana


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Title: Investigating the Relationship Between the Hot Gas and the Dark Matter Components of Galaxy Clusters
Authors: Leila C. Powell, Scott T. Kay, Arif Babul, Andisheh Mahdavi

Various differences in galaxy cluster properties derived from X-ray and weak lensing observations have been highlighted in the literature. One such difference is the observation of mass concentrations in lensing maps which have no X-ray counterparts (e.g. Jee, White, Ford et al. 2005). We investigate this issue by identifying substructures in maps of projected total mass (analogous to weak lensing mass reconstructions) and maps of projected X-ray surface brightness for three simulated clusters. We then compare the 2D mass substructures with both 3D subhalo data and the 2D X-ray substructures. Here we present preliminary results from the first comparison, where we have assessed the impact of projecting the data on subhalo identification.

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Blobrana


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Title: Mapping the distribution of luminous and dark matter in strong lensing galaxies
Authors: I. Ferreras (King's College London), P. Saha (Zurich), L. L. R. Williams (Minnesota), S. Burles (MIT)

We present the distribution of luminous and dark matter in a set of strong lensing (early-type) galaxies. By combining two independent techniques - stellar population synthesis and gravitational lensing - we can compare the baryonic and dark matter content in these galaxies within the regions that can be probed using the images of the lensed background source. Two samples were studied, extracted from the CASTLES and SLACS surveys. The former probes a wider range of redshifts and allows us to explore the mass distribution out to ~5Re. The high resolution optical images of the latter (using HST/ACS) are used to show a pixellated map of the ratio between total and baryonic matter. We find dark matter to be absent in the cores of these galaxies, with an increasing contribution at projected radii R>Re. The slopes are roughly compatible with an isothermal slope (better interpreted as an adiabatically contracted NFW profile), but a large scatter in the slope exists among galaxies. There is a trend suggesting most massive galaxies have a higher content of dark matter in the regions probed by this analysis.

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Blobrana


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Abell 520:
Dark Matter Mystery Deepens in Cosmic "Train Wreck"

Abell 520
Credit: X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.
JPEG (564.3 kb) Tiff (12.4 MB) PS (2.8 MB)

This multi-wavelength image of Abell 520 shows the aftermath of a complicated collision of galaxy clusters, some of the most massive objects in the Universe. In this image, the hot gas as detected by Chandra is coloured red. Optical data from the Canada-France-Hawaii and Subaru telescopes shows the starlight from the individual galaxies (yellow and orange). The location of most of the matter in the cluster (blue) was also found using these telescopes, by tracing the subtle light-bending effects on distant galaxies. This material is dominated by dark matter.

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Blobrana


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In deep underground laboratories around the globe, a high-tech race is on to spot dark matter, the invisible cosmic glue that's believed to keep galaxies from spinning apart.
Whoever discovers the nature of dark matter would solve one of modern science's greatest mysteries and be a shoo-in for the Nobel Prize. Yet it's more than just a brainy exercise. Deciphering dark matter -- along with a better understanding of another mysterious force called dark energy -- could help reveal the fate of the universe.

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