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What proportion of stars have proper motion

  1. Nov 1, 2015 #1

    wolram

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    I have just found this article and wondered how many bodies in the milky way have proper motion.
    And why our galaxy seems different to others in that it has such a massive black hole.
    http://arxiv.org/abs/astro-ph/9807210

    A. M. Ghez, B. L. Klein, M. Morris, E. E. Becklin (UCLA)
    (Submitted on 20 Jul 1998 (v1), last revised 21 Jul 1998 (this version, v2))
    Over a two year period (1995-1997), we have conducted a diffraction-limited imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar cluster using the Keck 10-m telescope. The K band images obtained reveal a large population of faint stars. We use an unbiased approach for identifying and selecting stars to be included in this proper motion study, which results in a sample of 90 stars with brightness ranging from K=9-17 and velocities as large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997; Genzel et al. 1997), the source confusion is reduced by a factor of 9, the number of stars with proper motion measurement in the central 25 arcsec^2 of our galaxy is doubled, and the accuracy of the velocity measurements in the central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar surface density and the velocity dispersion are consistent with the position of the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a function of distance from Sgr A*, the velocity dispersion displays a falloff well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo confined to a volume of at most 10^-6 pc^3, consistent with earlier results. Although uncertainties in the measurements mathematically allow for the matter to be distributed over this volume as a cluster, no realistic cluster is physically tenable. Thus, independent of the presence of Sgr A*, the large inferred central density of at least 10^12 Mo/pc^3, which exceeds the volume-averaged mass densities found at the center of any other galaxy, leads us to the conclusion that our Galaxy harbors a massive central black hole
     
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  3. Nov 1, 2015 #2

    mfb

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    Exactly the same number of stars that have a position.
    All of them.

    Do you mean "measured proper motion"? That number is smaller.
    Nearly all galaxies have such a black hole.
     
  4. Nov 1, 2015 #3

    wolram

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    I meant measured proper motion mfb
     
  5. Nov 1, 2015 #4

    mfb

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    Hipparcos measured about 100 thousand stars precisely, Tycho-2 has 2.5 million stars with a lower accuracy (~2.5 mas/year).
    Gaia aims for an accurary of 20 to 200 µas per year depending on the brightness, for 1 billion stars. 200 µas/year is ~10 km/s over 30.000 light years, so the measured value will be significantly different from zero for most observable stars across the whole galaxy.
     
  6. Nov 1, 2015 #5
    Neither of the Magellanic Clouds has a black hole nucleus. Triangulum has a bright core, but no black hole there either.
    How about the other galaxy groups? Local group result is 2 galaxies out of 5 have nuclei (Milky Way and Andromeda), 3 don´t (the two Magellanic clouds and Triangulum). In nearby galaxy clusters, what is the pattern as to which galaxies have nuclei and which don´t?
     
  7. Nov 1, 2015 #6

    mfb

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    Here are two references given - I'm not the only one with access to google...
    Tiny objects where the word "galaxy" is questionable might be different.
     
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