Blog #30: Lab Intro

Giant molecular clouds (GMCs) are some of the largest structures in the Galaxy, composed mostly of hydrogen and helium atoms, though other atoms and molecules are present as well. GMCs are where matter comes together to form new planets and stars.

An example of a GMC

In this lab, our goal is to determine the radial velocities of various GMCs in our Galaxy to see how the Galaxy's rotation speed varies by radius. We'll be doing this by measuring the Doppler shifts (specifically by using the emission spectrum of CO) of the GMCs in the radio frequency. This will allow us to see past the gas and dust that would obscure the GMCs were we to look for them using the visible spectrum. We will be observing in the Galactic plane, at Galactic longitudes from 10˚-70˚.

The GMC moving fastest from us will be those whose orbit is tangential to our line of sight. It will be the one with the tightest orbit, and if it is at the tangent point, its entire velocity (\(V_r(max)\)) will be directly away from us. However, we also need to take into account that the Sun, because of its own rotation, will be moving towards that GMC. Therefore, the total circular velocity \(V_{cir}=V_r(max)+V_{\odot}sinl\), where the sin(l) term corrects for the direction of the Sun's rotation.

Our ultimate goal with this lab is to create a Galactic rotation curve. We have talked a bit about the Galactic rotation curve earlier in the class--specifically in the context of how we know about the existence of dark matter. Along with that, we'll calculate/plot the orbital period, the total mass enclosed within each GMC's radius, the number of stars interior to the Sun, and the Sun's Galactic age.

In this diagram, GMC (b) is the one moving fastest from us, so it corresponds to the rightmost peak in the emissions spectrum. (From WS9.2)

Sources: 
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/molecular_clouds.html
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/images/horsehead.jpg