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Spectral Line Graphs

M82: radiomap

Here is a radiomap of M82 taken in the carbon monoxide (CO) line . The crosses denote the locations within M82 to which the spectral line graphs below correspond.

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M82 spectral line graphs

These graphs are simple line plots of intensity verses velocity of the incoming radio signals originating from each of the location within M82 that are indicated above, moving from left to right.

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What are intensity and velocity? How are they related to one another?

First, each gas molecule emits radiation in the form of spectral lines. The larger the number of molecules present in a given volume, the more radiation emitted and the greater the intensity of their corresponding spectral lines. In other words, intensity of emission here indicates the concentration of gas at a specific location in space. These graphs show the intensity of carbon monoxide (CO) gas emissions over a range of velocities measured in kilometers per second (km/s).

The velocity of the CO gas is represented along the horizontal axis and is determined using the Doppler effect. The spectral lines emitted by CO have characteristic frequencies, but due to the motions of the gas with respect to Earth, these frequencies are Doppler-shifted. Their shifts are translated into radial velocities, i.e. those towards and away from Earth.

Unfortunately, Doppler shifts cannot be used to determine traverse velocities, i.e. motions across the sky. What this means is that, from our vantage point on Earth, we are unable to decipher fully the attributes in all three dimensions of most objects located beyond the Solar System. However, because astonomers are able to distinguish between the different velocities of gas associated with an object, they can capture part of this three dimensional information.

Margaret Meixner, University of Illinois, on-camera
Picture of Margeret Meixner
QuickTime Movie (1.7 MB); Sound File (1.0 MB); Text

What Do the Spectral Line Graphs Show?

For each of three locations within M82, the above three spectral line graphs tell us how much CO gas is present, how fast this gas is moving, and whether the motion is towards or away from us.

The graph corresponding to the left side of M82 , as viewed in the top radiomap, shows the CO gas is moving away from us at about 300 km/s (kilometers per second). At the opposite (right) side of the galaxy, CO gas is moving away from us at approximately 100 km/s , as indicated in the bottom graph (D). In between these locations (see middle graph), the gas appears to be moving away from us at a velocity of 200 km/s, i.e. roughly half-way between the other velocities.

From this scenario, we can infer that the gas in M82 is rotating clockwise around the center of that galaxy, which, as a whole, is moving away from us at 200 km/s. With respect to the center of the galaxy, the gas at A is moving at a net 300 - 200 = 100 km/s, i.e., here the rotation of the gas causes it to move away from us at an additional 100 km/s over that of the galaxy as a whole (resulting in a total velocity away from Earth of 300km/s). On the other side of M82, gas is moving in the opposite direction (i.e. towards us) at a net velocity of 100 - 200 = - 100 km/s.

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NCSA. Last modified 11/13/95