When we talk about the diameter of the Milky Way, it’s usual to cite a figure of about 100,000 light years. But the much more diffuse halo of stars surrounding the galaxy actually extends almost twice as far. You would expect to find more or less the same situation in other galaxies, but new observations of the giant galaxy M87 have turned up a surprising fact: Its halo of stars is much smaller than expected. It’s true that the halo is three times the size of that around the Milky Way, but its diameter of a million light years is still much smaller than anticipated given the size of the parent galaxy.
Mysteries like this seem just the thing for the weekend, so consider the possibility, raised in the paper on this work, that the truncated halo is the result of a collapse of dark matter in the Virgo Cluster, where M87 resides. The Virgo Cluster is approximately 50 million light years from us and contains hundreds of galaxies of all descriptions, including spirals like the Milky Way. Another possibility: A close pass by the galaxy M84, which could have perturbed M87 about a billion years ago.
Image: The Virgo cluster of galaxies taken with the Palomar Observatory 48-inch Schmidt telescope as part of the Digitized Sky Survey 2. The giant elliptical galaxy Messier 87 is seen in the centre, while Messier 84 and 86 are the two bright galaxies forming part of the small group on the centre right of the image. New observations obtained with ESO’s Very Large Telescope have shown that the halo of stars around Messier 87 has been truncated, possibly because of some interaction with Messier 84. The observations also reveal that Messier 87 and 86 are moving towards each other. Credit: European Southern Observatory.
How to measure the motions of stars in the haloes of distant galaxies? Researchers in this study used a spectrograph on the European Southern Observatory’s Very Large Telescope in Chile to measure planetary nebulae on the outskirts of M87 and also in intergalactic space within the surrounding Virgo Cluster. Such nebulae show strong emission lines, making them easier to detect at great distances and allowing for precise radial velocity measurements. They offer clues to the number and type of stars in outer galaxy regions and help us account for their motions.
We don’t know that dark matter is the culprit here, but it’s interesting to note that similar effects believed to be caused by dark matter have been found in other galaxies, detected through gravitational lensing. The authors point out, however, that the data gathered to this point are insufficient to draw a conclusion between dark matter and other scenarios. Their thinking is that further study of planetary nebulae with a much larger sample will be needed to get an accurate picture of what is going on.
The paper is Doherty et al., “The Edge of the M87 Halo and the Kinematics of the Diffuse Light in the Virgo Cluster Core,” accepted for publication in Astronomy & Astrophysics and available online. More in this European Southern Observatory news release.
Excuse my lack of knowledge but what does the study of planetary nebulae have to do with the halo?
Len, it’s a reasonable question. As I understand it, planetary nebulae are useful because they help us understand the motion of stars in the outer halo of galaxies like M87. Their strong emission lines make it easier to detect and measure their velocities, so they’re a kind of window into what is going on in the outer galactic regions. The paper cited here used these planetary nebulae to project what we would expect to see around M87 and compare it to what we actually find, leading to these differing scenarios.
Hi Folks;
The above observations are interesting. If indeed a dark matter collapse was the culprit, such points to the complexity and variety of the structures within our universe. I am interested in dark matter phenomenon, not only because of the mysterious nature of these forms of matter, but also because the existence of CDM implies an increase in the number of types of thermodynamic degrees of freedom that exist at the quantum levels.
Because baryonic matter accounts for only about 15 percent of the total mass of the universe, not counting the mass equivalence of Dark Energy, when one goes out to view the night sky with the unaided eye in a dark country location, it is easy to have a sense of mystery restoked regarding the size and rich variety of phenomenon within the cosmos. Such mystery just becons us to develop plans for manned interstellar travel all the sooner, perhaps with a crash Manhattan Project-like effort to develop the hard ware required for the mission. With all of the Pop Culture Hollywood stuff that has been the subject of science fiction fantasy, and paranormal fantasy, over the past couple of decades, what would really unit the people of the United States, and indeed the Global Community, would involve a concerted effort to launch a real manned mission to one of our stellar neighboors.
Finding the potential collapse of Cold Dark Matter associated with M-87 will be just one piece in a large puzzle to determine the exact nature, variety, and distribution of CDM and the richness of the set of phenomenon the existence of the CDM will present us.
http://www.technologyreview.com/blog/arxiv/23607/
New kind of quantum tunnelling experiment goes live
Posted: 04 Jun 2009 09:10 PM PDT
Physicists may be able to prove the existence of dark matter by watching a blank wall
Almost every particle physics experiment ever performed can be explained by a single theory called the standard model of elementary particles. But while that’s a great triumph for particle physicists, it also makes life rather boring. So the search is on for experiments that will reveal physics beyond the standard model and one of the most exciting has just been switched on at the DESY particle accelerator in Germany.
The new experiment is searching for an entirely new form of tunnelling, the weird quantum process by which a quantum particle passes through a potential barrier that a classical particle could not traverse. This behaviour is the result of the Heisenberg uncertainty principle which gives a finite probability for a quantum particle to cross any barrier of specific height and thickness.
But there are other kinds of tunnelling too, as we recently saw. One of the most intriguing is that quantum mechanics allows photons to change into particles and then change back again. If those particles can pass through a barrier, then turn back into photons, it would look as if the photons were tunnelling.
Experiments to measure this shining-a-light-through-a-wall effect are exciting because they could reveal paticles that are not predicted by the standard model. For example, the particle that the folks at DESY are looking for is called the WISP (weakly interacting sub-eV particles) which could be a major component of dark matter. The standard model does not account for dark matter which is why any discovery in this area would be a jaw dropper for particle physicists.
The DESY experiment announced on the arXiv is essentially a hi-tech wall. On one side of the wall is a laser and on the other a detector. The trouble is that photons transform into WISPS only very rarely. So you need a huge number of photons to see the effect. The DESY team have a powerful laser producing some 10^19 photons per second but even that isn’t enough. So they’ve built themselves a couple of mirrors to reflect the photons back and forth, so each photon approaches the wall around 200 times.
The DESY team call themselves the Axion-Like Particle Search Collaboration or ALPs Collaboration. Having built their experiment, their task now is to sit back and watch a blank wall.
And if they spot one or two photons popping through, you’ll be hearing a whole lot more about ALPS.
Ref: http://arxiv.org/abs/0905.4159: Resonant laser power build-up in ALPS: a light-shining-through-walls experiment
June 8, 2009
Super-Size Me: Black Hole Bigger Than Previously Thought
Written by Nancy Atkinson
Using a new computer model, astronomers have determined that the black hole in the center of the M87 galaxy is at least twice as big as previously thought. Weighing in at 6.4 billion times the Sun’s mass, it is the most massive black hole yet measured, and this new model suggest that the accepted black hole masses in other large nearby galaxies may be off by similar amounts. This has consequences for theories of how galaxies form and grow, and might even solve a long-standing astronomical paradox.
Astronomers Karl Gebhardt from the University of Texas at Austin and Jens Thomas from the Max Planck Institute for Extraterrestrial Physics detailed their findings Monday at the American Astronomical Society conference in Pasadena, California.
To try to understand how galaxies form and grow, astronomers start with basic information about the galaxies today, such as what they are made of, how big they are and how much they weigh. Astronomers measure this last category, galaxy mass, by clocking the speed of stars orbiting within the galaxy.
Full article here:
http://www.universetoday.com/2009/06/08/super-size-me-black-hole-bigger-than-previously-thought/
July 2, 2009
Messier 87 Shows Off for Hundreds of Earth-bound Astronomers
Written by Anne Minard
When the giant radio galaxy Messier 87 (M 87) unleashed a torrent of gamma radiation and radio flux, an international collaboration of 390 scientists happened to be watching. They’re reporting the discovery in this week’s issue of Science Express.
The results give first experimental evidence that particles are accelerated to extremely high energies in the immediate vicinity of a supermassive black hole and then emit the observed gamma rays. The gamma rays have energies a trillion times higher than the energy of visible light.
Matthias Beilicke and Henric Krawczynski, both physicists at Washington University in St. Louis, coordinated the project using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration. The effort involved three arrays of 12-meter (39-foot) to 17-meter (56-foot) telescopes, which detect very high-energy gamma rays, and the Very Long Baseline Array (VLBA) that detects radio waves with high spatial precision
Full article here:
http://www.universetoday.com/2009/07/02/messier-87-shows-off-for-hundreds-of-earth-bound-astronomers/
A first joint M87 campaign in 2008 from radio to TeV gamma-rays
Authors: R. M. Wagner (1), M. Beilicke (2), F. Davies (3), H. Krawczynski (2), D. Mazin (4), M. Raue (5), S. Wagner (6), R. C. Walker (3) for the H.E.S.S. Collaboration, MAGIC Collaboration, VERITAS Collaboration, the VLBA 43 GHz M87 monitoring team ((1) Max-Planck-Institut für Physik, (2) Washington University in St. Louis/McDonnell Center for the Space Sciences, (3) National Radio Astronomy Observatory, (4) IFAE, (5) Max-Planck-Institut für Kernphysik, (6) Landessternwarte Heidelberg)
(Submitted on 9 Jul 2009)
Abstract: M87, the central galaxy of the Virgo cluster, is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing toward the line of sight, is spatially resolved in X-ray (by Chandra), in optical and in radio observations. Time correlation between the TeV flux and emission at other wavelengths provides a unique opportunity to localize the very high energy gamma-ray emission process occurring in AGN.
For 10 years, M87 has been monitored in the TeV band by atmospheric Cherenkov telescopes. In 2008, the three main atmospheric Cherenkov telescope observatories (H.E.S.S., MAGIC and VERITAS) coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. The campaign largely overlapped with an intensive VLBA project monitoring the core of M87 at 43 GHz every 5 days.
In February, high TeV activities with rapid flares have been detected. Contemporaneously, M87 was observed with high spatial resolution instruments in X-rays (Chandra). We discuss the results of the joint observation campaign in 2008.
Comments: 4 pages, 4 figures, contribution to the 31st ICRC, Lodz, Poland, July 2009
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Report number: MPP-2009-111
Cite as: arXiv:0907.1465v1 [astro-ph.CO]
Submission history
From: Robert Wagner [view email]
[v1] Thu, 9 Jul 2009 09:15:28 GMT (1496kb)
http://arxiv.org/abs/0907.1465
A New Probe of Dark Matter and High-Energy Universe Using Microlensing
Authors: M. Ricotti, A. Gould
(Submitted on 5 Aug 2009 (v1), last revised 7 Aug 2009 (this version, v2))
Abstract: We propose the existence of ultracompact minihalos as a new type of massive compact halo object (MACHO) and suggest an observational test to discover them. These new MACHOs are a powerful probe into the nature of dark matter and physics in the high energy Universe.
Non-Gaussian energy-density fluctuations produced at phase transitions (e.g., QCD) or by features in the inflaton potential can trigger primordial black hole (PBH) formation if their amplitudes are delta > 30%.
We show that a PBH accumulates over time a sufficiently massive and compact minihalo to be able to modify or dominate its microlensing magnification light curve. Perturbations of amplitude 0.03% < delta < 30% are too small to form PBHs, but can nonetheless seed the growth of ultracompact minihalos. Thus, the likelihood of ultracompact minihalos as MACHOs is greater than that of PBHs.
In addition, depending on their mass, they may be sites of formation of the first PopIII stars. Ultracompact minihalos and PBHs produce a microlensing light curve that can be distinguished from that of a "point-like" object if high-quality photometric data are taken for a sufficiently long time after the peak of the magnification event. This enables them to be detected below the stellar-lensing "background" toward both the Magellanic Clouds and the Galactic bulge.
Comments: 9 pages, 3 figure, submitted to ApJ (v2 corrects only a typo in arXiv abstract)
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)
Cite as: arXiv:0908.0735v2 [astro-ph.CO]
Submission history
From: Massimo Ricotti [view email]
[v1] Wed, 5 Aug 2009 20:03:54 GMT (207kb)
[v2] Fri, 7 Aug 2009 03:09:01 GMT (207kb)
http://arxiv.org/abs/0908.0735