<p>The reason is detailed in depth in <a href="http://euhou.obspm.fr/public/Milky_Way_Rotation_A4.pdf" rel="nofollow noreferrer">this pdf</a>, which contains the following diagram:</p>

<p><a href="https://i.sstatic.net/MQBLE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MQBLE.png" alt="enter image description here"></a></p>

<p>Some key quantities:</p>

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

<p>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.</p>

CC BY-SA 3.0
2016-10-23T17:09:37.103Z
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.
uhoh
CC BY-SA 3.0
2016-10-23T17:13:30.773Z
@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](https://www.astro.rug.nl/~sctrager/teaching/PoG/2012/HIintheMW.pdf).
HDE 226868
CC BY-SA 3.0
2016-10-23T17:30:13.417Z
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!
uhoh
CC BY-SA 3.0
2016-10-23T12:58:57.857Z
2016-10-23T12:58:57.857Z
HDE 226868
<p>The recently released map of galactic neutral hydrogen density and velocity is really beautiful. The work has been relased by the <a href="http://www.icrar.org/hi4pi/" rel="nofollow noreferrer">HI4PI collaboration</a> (HI = neutral hydrogen, $4\pi$ = complete spherical coverage).</p>

<p>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. </p>

<p>Does this have some simple geometrical explanation?</p>

<p><a href="https://i.sstatic.net/mfmgp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mfmgp.png" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/P9bIg.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/P9bIg.jpg" alt="enter image description here"></a></p>

<p><strong>above x2:</strong> neutral hydrogen density/velocity map <a href="http://www.icrar.org/hi4pi/" rel="nofollow noreferrer">from here</a> with inset indicating color scale expanded.</p>

<p><a href="https://i.sstatic.net/X6sM3.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/X6sM3.png" alt="enter image description here"></a></p>

<p><strong>above:</strong> 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.</p>

<p>There is a nice article at <a href="http://phys.org/news/2016-10-scientists-milky.html" rel="nofollow noreferrer">phys.org</a> with a video, which can also be <a href="https://www.youtube.com/watch?v=Q2mgpsTFuV8" rel="nofollow noreferrer">seen in YouTube</a>. It shows the intensity of HI as a cut in radial velocity is slowly scanned through the data set.</p>

<p><a href="https://www.youtube.com/watch?v=Q2mgpsTFuV8" rel="nofollow noreferrer">https://www.youtube.com/watch?v=Q2mgpsTFuV8</a></p>

<p>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:</p>

<p><a href="https://i.sstatic.net/53pPa.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/53pPa.gif" alt="enter image description here"></a></p>

<radio-astronomy><data-analysis><galactic-dynamics><velocity>
Why does the neutral hydrogen velocity have this characteristic behavior in the galactic plane?
uhoh
# Why does the neutral hydrogen velocity have this characteristic behavior in the galactic plane?

(Asked by: username_0 on 2016-10-23)

The recently released map of galactic neutral hydrogen density and velocity is really beautiful. The work has been relased by the [HI4PI collaboration](http://www.icrar.org/hi4pi/) (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](https://i.sstatic.net/mfmgp.png)](https://i.sstatic.net/mfmgp.png)

[![enter image description here](https://i.sstatic.net/P9bIg.jpg)](https://i.sstatic.net/P9bIg.jpg)

**above x2:** neutral hydrogen density/velocity map [from here](http://www.icrar.org/hi4pi/) with inset indicating color scale expanded.

[![enter image description here](https://i.sstatic.net/X6sM3.png)](https://i.sstatic.net/X6sM3.png)

**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](http://phys.org/news/2016-10-scientists-milky.html) with a video, which can also be [seen in YouTube](https://www.youtube.com/watch?v=Q2mgpsTFuV8). It shows the intensity of HI as a cut in radial velocity is slowly scanned through the data set.

<https://www.youtube.com/watch?v=Q2mgpsTFuV8>

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:

[![enter image description here](https://i.sstatic.net/53pPa.gif)](https://i.sstatic.net/53pPa.gif)

**Tags:** radio-astronomy, data-analysis, galactic-dynamics, velocity

---

## 1 answers

### Answer by username_1 on 2016-10-23. Score: 5 | Accepted answer

The reason is detailed in depth in [this pdf](http://euhou.obspm.fr/public/Milky_Way_Rotation_A4.pdf), which contains the following diagram:

[![enter image description here](https://i.sstatic.net/MQBLE.png)](https://i.sstatic.net/MQBLE.png)

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.

> **Comments:**
> **username_0**: 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.  
> **username_1**: @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](https://www.astro.rug.nl/~sctrager/teaching/PoG/2012/HIintheMW.pdf).  
> **username_0**: 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!  

---

