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Channel Maps

M82 emits radiowaves containing a multitude of frequencies because of the many different gases present in M82. To make sense of this multitude of emissions astronomers focus on one frequency at a time using channel maps.

Channel Map of M82

Channel maps show the intensity of radio emissions from one particular clump of gas, possessing narrow range of frequencies, emanating from a chosen area within an object. This channel map shows the intensity of radio energy emitted from CO gas moving away from us at a velocity of 102 km/s. The bright regions are where large amounts of gas reside while the opposite is true for the dim regions.
JPEG Image (23K); Credits and Copyrights

Recall that each gas emits spectral lines with characteristic frequencies. However, due to the motion of the gas, the spectral lines are Doppler-shifted. Thus, carbon monoxide (CO) gas moving at some velocity in one area of M82 will show spectral lines shifted to a slightly different frequency than CO gas moving at a slightly different velocity in another area of M82. Each channel map depicts a gas moving at only one of those velocities.

The chosen area for a channel map is delineated by two spatial coordinates: right ascension (from left to right across an object); and declination (from bottom to top of an object) represented along the two axes of the channel map. The observed intensity of M82 is indicated by either a color code on a radiomap where the brightness represents the observed intensity, or by contours on a contour map where each contour represents a certain observed intensity value.

Contour Map of M82

Here's a contour representation of radiomap shown above.
JPEG Image (27K); Credits and Copyrights

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NCSA. Last modified 11/13/95