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Milky Way Dark Matter Halo Loses ‘Weight’


640px-Milky_Way_ArchMilky Way Arch, CC BY 3.0Bruno Gilli/ESOhttp://www.eso.org/public/images/milkyway/

Mass estimates for our Milky Way vary widely, from less than 1 trillion, to as high as 4 trillion, times the mass of the Sun.

A recent paper by a group of astronomers in Australia argues for a mass that is very much at the low end of this range. Prajawal Kafle and collaborators present a kinematic analysis and build a model of the Milky Way that incorporates a disk, a bulge, and a dark matter halo. The analysis utilizes K giant and horizontal branch star catalogs.

The disk component – in which our Sun resides – contains stars and gas and active star formation from this gas. The spheroidal bulge contains the oldest stellar population of the galaxy, including globular clusters. The spherical halo, significantly larger and more massive than both the other components, is dominated by dark matter. It is chiefly responsible for the overall gravitational potential of the Milky Way, and is evidenced by the high rotational velocity of our galaxy in its outer regions.

The result of their analysis is that the dark matter halo “weighs in” at about 800 billion solar masses, the disk is about 100 billion solar masses, and the bulge is only about 10 billion solar masses. They also find a dark matter density in the solar neighborhood equivalent to about 1/3 of a proton per cubic centimeter, consistent with other estimates. (This number is important for calibrating Earth-bound direct detection experiments for dark matter.)

The relatively low mass they determine for the dark matter halo implies fewer satellite galaxies in close proximity to the Milky Way. We see only 3, the two Magellanic Clouds and the Sagittarius Dwarf Galaxy. In the past this has been seen as an issue for the favored Lambda – Cold Dark Matter (ΛCDM) Cosmology.

However their lower halo mass is actually consistent with the Milky Way gathering only 3 so-called sub-halos (satellite galaxies) and thus there may be no Missing Satellite Problem with ΛCDM. Some had suggested warm dark matter, rather than cold dark matter, may be necessary because of the putative missing satellite problem, but this may not be the case, with a lighter Milky Way dark matter halo.

Another team, Penarrubia and collaborators, has recently modeled the dynamics of the Local Group of galaxies. They are thus using a different methodology to determine the total mass of the Milky Way. They find a total mass for the Local Group of 2.3 trillion solar masses. The Local Group mass is almost entirely due to the Andromeda Galaxy and our Milky Way. They also determine a Milky Way to Amdromeda mass ratio of about 1/2. This implies a mass of about 0.8 trillion solar masses for our Milky Way, consistent with the Australian team’s result.

These two latest measurements of the Milky Way mass seem to indicate that the total mass of the Milky Way galaxy is less than 1 trillion solar masses. And these two results thus suggest that the ΛCDM cosmology is in fact consistent with the small number of satellite galaxies around our Milky Way. Another success for ΛCDM, it seems.

References:

http://article.wn.com/view/2014/10/10/Milky_Way_has_only_half_of_the_Dark_Matter_than_thought_earl/

http://arxiv.org/pdf/1408.1787v1.pdf – P. Kafle et al. 2014, “On the shoulders of giants: Properties of the stellar halo and Milky Way mass distribution”

J. Penarrubia et al. 2014, Monthly Notices of the Royal Astronomical Society, 443, 2204, “A dynamical model of the local cosmic expansion”

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2 responses to “Milky Way Dark Matter Halo Loses ‘Weight’

  • mpc755

    Aether has mass. Aether physically occupies three dimensional space. Aether is physically displaced by the particles of matter which exist in it and move through it.

    The Milky Way’s halo is not a clump of stuff anchored to the Milky Way. The Milky Way is moving through and displacing the aether.

    The Milky Way’s halo is the state of displacement of the aether.

    The Milky Way’s halo is the deformation of spacetime.

    What is referred to geometrically as the deformation of spacetime physically exists in nature as the state of displacement of the aether.

    A moving particle has an associated aether displacement wave. In a double slit experiment the particle travels through a single slit and the associated wave in the aether passes through both.

    Q. Why is the particle always detected traveling through a single slit in a double slit experiment?
    A. The particle always travels through a single slit. It is the associated wave in the aether which passes through both.

    What ripples when galaxy clusters collide is what waves in a double slit experiment; the aether.

    Einstein’s gravitational wave is de Broglie’s wave of wave-particle duality; both are waves in the aether.

    Aether displaced by matter relates general relativity and quantum mechanics.

    • darkmatterdarkenergy

      You appear to be conflating dark energy and dark matter, but they have very different properties. One drives acceleration of the universe, while the other pulls back on the expansion. One has a constant density per unit volume even as the universe’s volume grows, while the other has a decreasing density.

      It appears that dark energy is the ‘new’ aether. Observations are consistent to date with a uniform – and unchanged since the Cosmic Microwave Backround radiation decoupled from matter- cosmological constant interpretation of the dark energy. That is, a constant residual energy of the vacuum. So this aether has pressure and drives acceleration of the universe’s expansion. Dark energy is uniformly distributed in each volume of space. This aether has mass-energy equivalence, but it manifests as energy, not as matter or particles with mass. Except that virtual particles are continuously popping in and out of this vacuum, “borrowing” their temporary existence and mass against the vacuum energy bank. (There is the possibility of a scalar field interpretation, but there is no evidence yet for anything beyond the simpler cosmological constant view).

      Dark matter is not distributed uniformly; it maps the filamentary cosmic web structure and clumps into galaxies, Galaxy groups and clusters and superclusters.

      What is the evidence for your assertions?

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