Finding the Spiral Arms of the Galaxy

Joachim Köppen Strasbourg 2010

To find the spiral arms of the Milky Way, we first have to do a complete survey of the Galactic Plane. From the Northern Hemisphere we can only observe Galactic longitudes between about l=0 and l=240, that's three quarters. For every position in the Plane, we should get a good spectrum, covering radial velocities between -150 and +150 km/s. Since the antenna's HPBW is about 5, it would be quite sufficient to observe every 5, but it does not harm to cover the Plane with a finer stepwidth, because one would execute these observations by batch operations. At every position, we should collect a sufficient number of good spectra, from which we can obtain a good quality average spectrum. Plotting these average spectra as a function of galactic longitude in the same way as we plot the waterfall map of any data, we would get a false colour map like this:

For the analysis, such a plot is very nice, but you can also deduce the spiral arms by simply looking at the averaged spectra for each longitude (see below). We can see that at some position there are distinct emission features at several different radial velocities. These features can be traced over a large range of longitudes. Each such feature is emission coming from the hydrogen gas in a spiral arm. They are quite distinct, so let us try to identify them and trace them in the Galaxy.

The JavaScript tool: Modeling of the Spiral Arms in the Milky Way allows you to construct a model of the spirals in the Galactic Plane, such as depicted in the left hand image below, and then you can show how the emission by the spirals would look like in the longitude-radial velocity map (right panel)

What is behind this tool, and behind such an analysis? The rendering below shows our situation in the Galactic Plane:

The Sun is at a position x0=0 and y0=8.5 kpc. When we look at Galactic Longitude l, we get emission from any distance d on the line of sight indicated. Therefore we can compute the position x,y of that point
x = x0 + d sin l
y = y0 - d cos l
From this, we know its distance from the Galactic Centre
r = sqrt(x + y)
Let us assume that we have determined the rotation curve, or that we simply guess it by the rotational speed v0 = 220 km/s at the position of the Sun, and the slope dvdr = 0 km/s/kpc, say, with which it increases with distance from the centre. So we can compute the rotational speed at that point in the Galaxy
vrot(r) = v0 + dvdr (r-r0)
Finally, ff one works out the radial velocity of this point as the projected rotational velocity, one obtains Oort's equation
vrad(l,d) = (vrot(r) r0/r - v0) sin l
This means that we can associate each position given by Galactic Longitude and distance with a spot in the Longitude-radial velocity diagram. However, the inverse is not generally true: As explained in Galactic Rotation, on any line-of sight towards the interior (|l| < 90) there are two distances whose emission appears at the same radial velocity. Thus, we cannot attribute a single distance to a specific radial velocity. But nonetheless, we can map our observed longitude-radial velocity plot into linear galactic positions:
We can distinguish two or three spiral arms: the one in which the Sun is located, one further out, at about 12 kpc distance from the Centre, and perhaps another one even further out. Perhaps there is another one inside the orbit of the Sun, but note the ambiguity for this inner part!

However, such a direct mapping is a bit complicated to program, and one has the ambiguity for the regions inside the Sun's orbit. Therefore, it is better and more comprehensible to do the analysis in the forward way: assume some spiral pattern and try to match with it your observed l-vrad map. One can use this tool: Modeling of the Spiral Arms in the Milky Way

There is another, even simpler way, which can be implemented even in Excel:

Important note
For this analysis it is not really vital to make a false colour map of longitude and radial velocity ... Simply inspect your averaged spectra at each longitude, identify the features, and try to match the measured radial velocity by guessing the distance, with the longitude being given.

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last update: Feb. 2010 J.Köppen