Carbon monoxide (CO) is a compound with molecules each consisting of one oxygen and one carbon atom.
Its detectable emission lines are used as a signature for cool hydrogen, i.e., hydrogen molecules in clouds. Cool hydrogen does not give off strong lines that can be seen from a distance in the infrared or radio ranges. The assumption is made that since molecular clouds sufficiently near that their hydrogen can be detected also show CO, that its presence at further distances indicates a likely molecular cloud.
One set of lines are due to the steps in possible rotational energy, i.e.,
J(J+1)h2 Erot = ———————— 2I
where I is the moment of inertial, h is Planck constant, and J is 0, 1, 3, etc. The lines' frequencies are generally multiples of ~115GHz. Interesting lines include:
These suggest gas clouds, i.e., starburst galaxies and possibly mergers, and can be detected in distant galaxies (e.g., z=1) in the millimeter/submillimeter range (e.g., submillimeter galaxy designator).
CO lines, such CO(1-0) are useful for determining redshifts.
CO(6-5) is used for observing protoplanetary disks and very distant (z>6) molecular clouds.
All the these CO lines together are known as the CO ladder (CO rotational ladder or CO J ladder) and when observed together (yielding their spectral line energy distribution (SLED)), can yield qualities of the gas, in particular, its temperature.
From CO line widths, using the virial theorem, if virial stability is assumed, the mass of the cloud can be calculated. Assuming a particular CO to H2 factor, the mass of a distant molecular cloud can be calculated.
CO incorporating rarer isotopes, such as C17O, C18O, or 13CO, are sometimes preserved better or otherwise easier to observe, thus are sometimes measured, using normal ratios of isotopes to extrapolate the mass of CO, and from that, the mass of H2. Large clouds have sufficient CO to saturate the lines of the most abundant isotope combination, 12C16O, so the observer may determine from it no more than a minimum amount of CO present. Inclusion of another isotope affects the wavelength of the CO lines sufficiently that they can be independently measured, thus the value in detecting isotopes in analyzing molecular clouds above a certain size.