* Astronomy

Title: Detecting dark matter in electromagnetic field penetration experiments
Authors: Saibal Mitra

Dark matter in the form of particles from a hidden mirror sector has recently been proposed as an explanation for the DAMA annual modulation signal. Here one assumes that there exists a small mixing between photons and mirror photons. We show that dark matter with this property can also be detected in electromagnetic field penetration experiments. Such experiments can be used to measure the speed and direction of the dark matter halo wind, the local density, the temperature, and the strength of the photon-mirror photon mixing interaction. An additional result would be a significant improvement of the upper limit on the photon mass.

Read more (PDF)

Galaxies are born inside dark matter clumps.

Try mixing caramel into vanilla ice cream -- you will always end up with globs and swirls of caramel. Scientists are finding that galaxies may distribute themselves in similar ways throughout the universe and in places where there is lots of so-called dark matter.

"Our findings suggest that unseen dark matter -- which emits no light but has mass -- has had a major effect on the formation and evolution of galaxies, and that bright active galaxies are only born within dark matter clumps of a certain size in the young universe" - Duncan Farrah, Cornell University research associate, the lead author of a paper on spatial clustering that appeared in the April 10 issue of Astrophysical Journal Letters.

To investigate the spatial distribution of galaxies, Farrah used data that recently became available from the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey, one of the largest such surveys performed by the Spitzer Space Telescope, which was launched in 2003.
A galaxy is typically made up of hundreds of billions of stars grouped tightly together. But galaxies themselves often group together into what astronomers call "large-scale structures." And, just as galaxies themselves can take on such shapes as ellipticals and spirals, so, too, can the large-scale structures, ranging from galaxy clusters to long filaments of galaxies to large, empty voids.

"You might think that galaxies are just distributed randomly across the sky, like throwing a handful of sand onto the floor. But the problem is they are not, and this has been a great puzzle"- Duncan Farrah.

Farrah is interested in how large-scale structures form. To measure the amount of clustering in the early universe, he looked at light that had travelled for several billion years from extremely distant galaxies. From this he was able to calculate the amount of bunching in candidate galaxy clusters in the early universe.

"We wanted to find the beacons of the first stages of the formation of a galaxy cluster because, at that time, the clusters themselves had not formed yet" - Duncan Farrah.

In particular, he was interested in objects that emit strongly in the infrared and are surrounded by dense gas and dust. These objects, known as ultraluminous infrared galaxies (ULIRGs), were thought to be precursors of galaxy clusters. Farrah confirmed this by showing that ULIRGs do, indeed, tend to cluster in their early phases. The ability to pinpoint the locations of nascent galaxy clusters will enable researchers to investigate early cluster formations and when they occurred.
Farrah's finding that distant ULIRGs are linked with large clumps of dark matter was surprising for another reason. As its name suggests, dark matter doesn't emit light so no conventional telescope can see it. However, because dark matter has mass, its existence can be inferred by the way stars are drawn to regions where this mysterious mass is concentrated.
Unexpectedly, Farrah found that ULIRGs at different points in the history of the universe coincide with clumps of dark matter haloes of very similar masses. This observation suggests that a minimum amount of dark matter is necessary for galaxies to form and to coalesce into clusters. Farrah believes his study also provides valuable insights into understanding how dark matter helped mould the evolution of the universe.

Carol Lonsdale of NASA's Jet Propulsion Laboratory, which manages the Spitzer Space Telescope, is the principal investigator for the SWIRE project.

Read more

Using Gemini observations of globular clusters in NGC 3379 (M105), a team led by PhD student Michael Pierce and Prof. Duncan Forbes of Swinburne University in Australia, have found evidence for normal quantities of dark matter in the galaxy’s dark halo. This is contrary to previous observations of planetary nebulae that indicated a paucity of dark matter in the galaxy.

The observations of 22 globular clusters in the Leo Group elliptical galaxy were made using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North in early 2003. The data were obtained in the GMOS multi-slit mode with exposures of 10 hours on-source at a spectral resolution of FWHM ~4Å over an effective wavelength range of 3800Å-6660Å. The final spectra have a signal-to-noise ratio of 18-58/Å at 5000 Å. The spectroscopic data allowed the team to derive ages, metallicities and α-element abundance ratios for the sample of globular clusters. All of the globular clusters were found to be >~ 10 Gyr, with a wide range of metallicities. A trend of decreasing α-element abundance ratio with increasing metallicity is also identified.


Normalised globular cluster spectra that have been offset by one unit. These spectra have not been de-redshifted. These sample spectra show the wavelength range that the majority of our spectra cover and display the range of S/N and metallicity present. The spectrum of g1426 shows emission features around 5000 Å and 5890 Å of unknown origin. The planetary nebulae hosting globular cluster, g1420, is also plotted and the emission due to 4959 and 5007 Å [OIII] lines can be seen redshifted to 4971 and 5019 Å.

Most significantly, including 14 extra globular clusters from Puzia, et al. (2004), the projected velocity dispersion of the globular cluster system was found to be constant with radius from the galaxy centre, indicating significant dark matter at large radii in its halo. This result is in stark contrast to the “No/Low Dark Matter” interpretation by Romanowsky, et al. (2003) in the journal Science using observations of planetary nebula that indicated a decrease in the velocity dispersion profile with radius.

Reconciling the two velocity dispersion profiles is possible. Dekel, et al. (2005) recently showed that stellar orbits in the outer regions of merger-remnant elliptical galaxies are elongated and that declining planetary nebula velocity dispersions do not necessarily imply a dearth of dark matter.
Another possibility the authors suggest is that NGC 3379 could be a face-on S0 galaxy (as originally suggested by Capaccioli, et al. 1991). If a significant fraction of the planetary nebulae belong to the disk, this could suppress the line-of-sight velocity dispersion of the planetary nebulae relative to that of the globular clusters that lie in a more spherical halo.

These results are currently in press for publication in the Monthly Notices of the Royal Astronomical Society.

---

Gemini/GMOS Spectra of Globular Clusters in the Leo Group Elliptical NGC 3379
Authors: Michael Pierce (1), Michael A. Beasley (2), Duncan A. Forbes (1), Terry Bridges (3), Karl Gebhardt (4), Favio Raul Faifer (5,6), Juan Carlos Forte (5), Stephen E. Zepf (7), Ray Sharples (8), David A. Hanes (3), Robert Proctor (1) ((1) Swinburne (2) Lick Observatory, UCSC (3) Queen's (4) Texas (5) UNLP (6) IALP - CONICET (7) Michigan State (8) Durham)

We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and alpha-element abundance ratios from simple stellar population models using the multi-index chi^2 minimisation method of Proctor & Sansom (2002). All of these GCs are found to be consistent with old ages, i.e. >10 Gyr, with a wide range of metallicities. A trend of decreasing alpha-element abundance ratio with increasing metallicity is indicated.
The projected velocity dispersion of the GC system is consistent with being constant with radius. Non-parametric, isotropic models require a significant increase in the mass-to-light ratio at large radii. This result is in contrast to that of Romanowsky et al. (2003) who find a decrease in the velocity dispersion profile as determined from planetary nebulae. Our constant dispersion requires a normal sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass.


Spatial positions of Globular Clusters. The circles are GMOS data presented here, crosses are VLT data of Puzia et al. (2004). The two large circles show the effective radius of the galaxies NGC 3379 (lower right) and NGC 3384 (upper left) (de Vaucouleurset al. 1991, Capaccioli et al. 1990). The effective radius for NGC 3379 is 55'' (2.8 kpc). The four Globular Clusters approximately midway between NGC 3379 and NGC 3384, most likely belong to NGC 3379, however they cannot be decisively assigned to either galaxy and are therefore shown as open circles.

A two-sided chi^2 test over all radii, gives a 2 sigma difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. (2003). However, if we restrict our analysis to radii beyond one effective radius and test if the GC velocity dispersion is consistently higher, we determine a >3 sigma difference between the mass models, and hence favour the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo.

Read more (PDF)

-- Edited by Blobrana at 15:43, 2006-02-20

A Chinese astronomer from the University of St Andrews, Scotland, has creating a 'simple' theory which could solve the darkmatter mystery.

Dr Hong Sheng Zhao and his Belgian collaborator Dr Benoit Famaey of the Free University of Brussels (ULB), has developed a new law for gravity by modifying Einstein's theory of gravity.
Their formula suggests that gravity drops less sharply with distance as in Einstein, and changes subtly from solar systems to galaxies and to the universe.

It is similar to the solution proposed by Moti Milgrom in 1983 and backed up by Jacob Bekenstein in 2004. By boosting the gravity of ordinary matter , darkmatter would not be needed.
Dr Zhao and Dr Famaey have reworked a new formulation of that work that overcomes many of the problems previous versions have faced.
They have created a formula that allows gravity to change continuously over various distance scales and, most importantly, fits the data for observations of galaxies. To fit galaxy data equally well in the rival Dark Matter paradigm would be as challenging as balancing a ball on a needle, which motivated the two astronomers to look at an alternative gravity idea.

"It is not obvious how an apple would fall in a galaxy. Mr Newton's theory would be off by a large margin - his apple would fly out of the Milky Way. Efforts to restore the apple on a nice orbit around the galaxy have over the years led to two schools of thoughts: Dark Matter versus non-Newtonian gravity. Dark Matter particles come naturally from physics, with beautiful symmetries and explain cosmology beautifully; they tend to be everywhere. The real mystery is how to keep them away from some corners of the universe. Also Dark Matter comes hand- in-hand with Dark Energy. It would be more beautiful if there were one simple answer to all these mysteries. There has always been a fair chance that astronomers might rewrite the law of gravity. We have created a new formula for gravity which we call 'the simple formula', and which is actually a refinement of Milgrom's and Bekenstein's. It is consistent with galaxy data so far, and if its predictions are further verified for solar system and cosmology, it could solve the Dark Matter mystery. We may be able to answer common questions such as whether Einstein's theory of gravity is right and whether the so-called Dark Matter actually exists. A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function. It is ready for fellow scientists to falsify. It is time to keep an open mind for new fields predicted in our formula while we continue our search for Dark Matter particles" - Dr Hong Sheng Zhao, PPARC Advanced Fellow at University of St Andrews, School of Physics and Astronomy, and member of the Scottish Universities Physics Alliance (SUPA).

Dr Zhao and Dr Famaey will present their new theory to an international workshop at Edinburgh's Royal Observatory in April .

"It is possible that neither the modified gravity theory, nor the Dark Matter theory, as they are formulated today, will solve all the problems of galactic dynamics or cosmology. The truth could in principle lie in between, but it is very plausible that we are missing something fundamental about gravity, and that a radically new theoretical approach will be needed to solve all these problems. Nevertheless, our formula is so attractively simple that it is tempting to see it as part of a yet unknown fundamental theory. All galaxy data seem to be explained effortlessly" - Dr Benoit Famaey.

Their research was published on February 10th in the US-based Astrophysical Journal Letters.