The recently released map of galactic neutral hydrogen density and velocity is really beautiful. The work has been relased by the HI4PI collaboration (HI = neutral hydrogen, $4\pi$ = complete spherical coverage).

My question is fairly simple. Looking at the image near the galactic plane, why do I see near the center a green bump on the left and a blue bump on the right, then near the edges of the image a much stronger blue bump on the left and a green bump on the right. In other words, there is a major and a minor peak in the positive radial velocity gas, and a major and a minor peak in the negative radial velocity gas.

Does this have some simple geometrical explanation?

enter image description here

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above x2: neutral hydrogen density/velocity map from here with inset indicating color scale expanded.

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above: for those who can not distinguish the green and blue in the image, this plots the average green (solid) and blue (dashed) intensity within the central equatorial band.

There is a nice article at phys.org with a video, which can also be seen in YouTube. It shows the intensity of HI as a cut in radial velocity is slowly scanned through the data set.


Since the stackexchange YouTube option is not turned on in astronomy stackexchange, here is a GIF of screenshots at -30, -20, -10, 0, 10, 20, and 30 km/sec:

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1 Answer 1


The reason is detailed in depth in this pdf, which contains the following diagram:

enter image description here

Some key quantities:

  • $R_0$: Distance from the observer to the center of the Milky Way
  • $R$: Distance from target gas to the center of the Milky Way
  • $V_0$: Velocity of the observer with respect to a certain reference frame
  • $V$: Velocity of target gas with respect to the same reference frame
  • $l$: Angle between $R_0$ and $\mathbf{SM}$, the distance to the gas

When measuring the radial velocity, we look for the projection of $V$ onto our line of sight. Some geometry results in the relation $$V_r=V_0\left(\frac{R_0}{R}-1\right)\sin l$$ for the radial velocity $V_r$, assuming $V\approx V_0$ (which is not always true!). The $\sin l$ term, in addition to the asymmetrical rotation of the Milky Way, leads to the peculiar data you asked about.

  • $\begingroup$ OK I need a day to dig into this. What caught my eye is the $2\theta$-ness of the distribution - two blues and two greens per 360 degrees. The $sin(l)$ isn't getting me there, but I need to look again. $\endgroup$
    – uhoh
    Oct 23, 2016 at 17:09
  • 1
    $\begingroup$ @uhoh The reason for that is the factor of $\left(\frac{R_0}{R}-1\right)$, which creates another zero at $R_0=R$ and on either side of that. See several slides here. $\endgroup$
    – HDE 226868
    Oct 23, 2016 at 17:13
  • $\begingroup$ Oh right!! OK I've got it. Those are beautiful slides by the way - someone has put a lot of work into them! This will keep me busy for a while, thanks! $\endgroup$
    – uhoh
    Oct 23, 2016 at 17:30

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