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Milky Way Re-Sized

If you want to understand the size of the Milky Way, you have to know something about how fast stars move. Measuring the velocities of stars in the galaxy’s stellar halo — a spherical halo of old stars and globular clusters surrounding the disk — you can figure out the mass of the whole by examining the gravity needed to keep these stars in their orbits. The Milky Way’s stars are a part of that mass, of course, but so is the extended distribution of dark matter, about which we know all too little.

This is where the so-called ‘blue horizontal branch’ stars (BHB) come into play. These ancient objects have evolved past their red giant phase and now burn helium. Because they tend to be both distant and bright (BHB stars are generally of spectral class B or A), they make useful markers for measuring stellar velocities out to a distance of 180,000 light years from the Sun, far beyond the confines of the primary galaxy. The huge star survey called SEGUE (a part of the Sloan Digital Sky Survey) has been using 2400 BHB stars to take such readings.

The results suggested by the observed stellar velocities: The Milky Way is not as massive as we believed. So says team leader Xiangxiang Xue (National Astronomical Observatories of China):

“The Galaxy is slimmer than we thought. That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars.”

Milky Way and dark matter

Image: Our sun lies about 25,000 light years from the center of the Galaxy, roughly halfway out in the Galactic disk. The new mass determination is based on the measured motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from Galactic center, roughly the edge of the region illustrated above. The visible part of our Milky Way is embedded into its much more massive and more extended dark matter halo, indicated in dim red. The ‘blue horizontal branch stars’ that were found and measured in the SDSS-II study are orbiting the galaxy at large distances. Credit: Axel Quetz, Max Planck Institute for Astrophysics (Heidelberg), SDSS-II Collaboration.

The findings are useful because SEGUE’s large sample allows the method to be calibrated against existing computer simulations, giving us a better understanding of the Milky Way’s total mass. How the galaxy compares to distant galaxies that we see from without rather than within is a study that can help us in the quest to understand the broader principles of galactic formation. Just as significant, such work offers a valuable perspective on how the visible galaxy interacts with its dark matter halo.

More in this Sloan Digital Sky Survey news release. The paper, due to run in the Astrophysical Journal this fall, is Hue et al., “The Milky Way’s Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2500 SDSS Blue Horizontal Branch Stars,” available online.

Comments on this entry are closed.

  • Adam May 28, 2008, 17:47

    Hi Paul

    Seems Local Group astronomy has some surprises still. Though I was a bit surprised the current mass estimate, prior to the revision, was 2 trillion solar masses. Latest I had read was ~ 1.2 trillion, which now seems correct after all.

    Interesting to work out the mass ratio between the visible Galaxy – out to ~60,000 ly – and the invisible. Based on the flat rotation curve and Kepler’s equation we know there’s ~ 90 billion solar masses between Sol and the Central black-hole. Thus there’s ~ 200 billion solar masses between the visible disk’s extremes and the Core. So the Dark Galaxy, out to ~200,000 ly, at 1 trillion solar masses is 5 times more massive and the visible spiral is just 20% of the Galaxy’s mass. Because the mass density declines rapidly away from the disk (1/e for every 6,000 ly above or below) the inner 20% occupies a surprisingly small volume and is roughly 100 times denser than the rest.

  • Zeroth May 29, 2008, 18:15

    So there’s less of that hypothetical non-baryonic matter than previously thought, eh? Alert me when it dawns on the orthodoxy that less = zero.

  • James M. Essig May 30, 2008, 22:25

    Hi Adam and Zeroth;

    This is an interesting discovery. If there is significantly less non-baryonic matter than thought, then naturally, there would seem to be more baryonic matter within our universe than most current cold dark matter Big Bang models predict. This would add some uncertainty to our understanding of Big Bang evolution, which in a way, could be exciting in so far as the mystery and intrigue that would then present itself. Even more exiting might be the new theoretical and computational developments in Big Bang Cosmology, galaxy evolution and the like that may result as a result of the above findings.

    This might be good news as there would be additional matter in which stars could form. Perhaps the rate of time dependent star formation will increase again as such galactic halo baryonic matter settles into protostar formation configurations. At the very least, the rate of star formation might not decline as fast as originally proposed and perhaps it will experience an increase in some distant era of galactic evolution when needed for human and ETI civilization future extensions, say perhaps within trilions of years when current red dwarfs start burning out.

    Another cool aspect of the galactic halo is that there is more reaction mass and mattergy to power intergalctic space craft. I can see that sometypes of highly relativistic electrodynamic-plasma-hydrodynamic-drive, ISR, or other types of intergalactic space craft might utilize this extra baryonic matter for their propulsion systems.



  • philw1776 June 1, 2008, 13:07

    Put me as a beliver in non-baryonic matter. The Kepler/Newton rotation curves plus Occam’s Razor indicates that there is non-luminous, non light absorbing, gravitation generating mass. Dark baryonic matter in those quantities would block light. What’s problematic is the distribution of the cold dark matter.

  • ljk June 2, 2008, 11:39

    Baryons: What, When and Where?

    Authors: Jason X. Prochaska (1), Jason Tumlinson (2) ((1) UCO/Lick Observatory, UC Santa Cruz, (2) Yale University)

    (Submitted on 29 May 2008)

    Abstract: We review the current state of empirical knowledge of the total budget of baryonic matter in the Universe as observed since the epoch of reionization. Our summary examines on three milestone redshifts since the reionization of H in the IGM, z = 3, 1, and 0, with emphasis on the endpoints. We review the observational techniques used to discover and characterize the phases of baryons.

    In the spirit of the meeting, the level is aimed at a diverse and non-expert audience and additional attention is given to describe how space missions expected to launch within the next decade will impact this scientific field.

    Comments: Proceedings Review for “Astrophysics in the Next Decade: JWST and Concurrent Facilities”, ed. X. Tielens, 38 pages, 10 color figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.4635v1 [astro-ph]

    Submission history

    From: Jason X. Prochaska [view email]

    [v1] Thu, 29 May 2008 22:06:40 GMT (263kb)


  • Adam June 4, 2008, 4:58

    Flat Galactic rotation curves might be due to non-baryonic matter, but they might also be due to some weird gravity effect – like MOND or a dozen other concepts. But the issue won’t go away until we bag some dark-matter or until someone makes a rigorous MOND observation that dark-matter can’t explain.

  • ljk June 5, 2008, 10:33

    Nearby galaxies are chock-full of dark matter

    From New Scientist Space, June 4, 2008

    THE universe’s darkest secret may be hiding not far from us. Three dwarf galaxies near the Milky Way appear to contain a higher proportion of invisible dark matter than any stellar system so far studied. If so, they are the ideal place to look to figure out what the stuff consists of.

    Over the past three years, the Sloan Digital Sky Survey has identified Ursa Major II, Willman I and Coma Berenices Dwarf as small satellite galaxies of the Milky Way. Louis Strigari of the University of California, Irvine, analysed the motion of their stars and found that they appear to be subject to a gravitational field equivalent to that of at least 1 million solar masses distributed around each galaxy. Yet each of these galaxies only shines as bright as 1000 suns, a discrepancy which leads Strigari to suggest that these galaxies are rich in unseen dark matter.

    Full article here: