Discussions under Mass of the stellar cluster (not the whole galaxy) lead me to mention that when an actual temperature is inferred from a Doppler-broadened line profile (rather than just a linewidth expressed as an effective temperature), we assume that the Maxwell-Boltzoman velocity distribution in the directions perpendicular to the line of sight is the same as that the one measured along the line of sight.

Have there been any examples of this kind of assumption turning out to be invalid later? Some situation where the distribution of velocities turned out to be substantially anisotropic?

I'm not asking for examples of anisotropic velocity distributions, I'm asking for a case where it seemed like a good assumption at the time, but later it turned out something more interesting was going on and the line-of-sight Doppler width did not represent the spread in perpendicular velocities.

  • $\begingroup$ Something like this, but for star clusters instead of galaxies? academic.oup.com/mnrasl/article/413/1/L81/1747626 $\endgroup$ May 14 at 12:50
  • $\begingroup$ This group found non-Gaussianity from the spectra of this system, but I think they say it's rather dubious iopscience.iop.org/article/10.1086/382721/fulltext $\endgroup$ May 14 at 12:52
  • $\begingroup$ @DaddyKropotkin thanks for your comments! My question is explicitly about anisotropy (mentioned twice) rather than departure from a Gaussian shape, but of course both can happen at the same time. Also "I'm asking for a case where (anisotropy) seemed like a good assumption at the time, but..." $\endgroup$
    – uhoh
    May 14 at 13:02
  • $\begingroup$ Is "gaussian" in this context not the same as "isotropic"? I'm not sure I see the difference here $\endgroup$ May 14 at 13:34
  • $\begingroup$ @DaddyKropotkin 20 minutes and I'll be at a real keyboard... $\endgroup$
    – uhoh
    May 14 at 15:35

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