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Thread: Gaia DR2: Milky Way open clusters

  1. #1
    Join Date
    Feb 2006
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    516

    Gaia DR2: Milky Way open clusters

    I've created a rather simplistic Addon for Celestia which shows the locations of 1,229 open clusters and 397,897 of their member stars as published in "A Gaia DR2 view of the Open Cluster population in the Milky Way" by T. Cantat-Gaudin1 et.al. (2018). (See https://arxiv.org/abs/1805.08726 ) The resulting catalog (J/A+A/618/A93) was downloaded from VizieR.

    My Addon can be downloaded from https://www.classe.cornell.edu/~seb/...n_clusters.zip (20 MB expands to 25 MB)

    Although the members of nearby clusters, like the Pleiades, form nicely spherical aggregations, more distant clusters, like Herschel's Jewel Box, are quite elongated, presumably due to parallax measurement errors. (See the attached screengrab. Herschel's is at the top, Pleiades at the bottom)
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    Selden

  2. #2
    Join Date
    Sep 2006
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    Very nice! The elongated clusters remind me of the "Fingers of God" one sees in redshift catalogs. In both cases, the elongation is due to errors of measurement in the radial -- but not the tangential -- directions.

  3. #3
    Join Date
    Mar 2004
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    3,117
    Slight rewording, if I may - with clusters of galaxies, it's not error of measurement but internal motions that give the Finger of God effect. A large cluster may have a genuine Doppler spread with standard deviation (velocity dispersion) greater than 1000 km/second, a typical value which gives a completely bogus radial extent of 1000/H0 ~ 14 megaparsecs. (Trivium: in the very early Universe, as structures were still collapsing toward virial equilibrium, redshift-inferred thicknesses along the line of sight would have gone through a minimum value which would be zero for an idealized structure at turnaround between expansion and collapse).

    This diagram, following the layout of one from a review by A.J.S. Hamilton, shows how this effect would change with cosmological time for galaxy systems. A bit more detail is here.
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  4. #4
    Join Date
    Sep 2006
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    D'oh! Yes, of course, it's the motions of the galaxies within the cluster which creates the spread of radial velocities.

    Thanks for catching that.

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