Figure: [CII] and CO in Trumpler 14/Eta Carina Region

Abstract:

Fine structure lines have been employed as probes of the interstellar medium for more than 40 years, but it is really only with the extensive, high-sensitivity observations carried out with the Herschel Space Observatory and with the SOFIA airborne observatory that submillimeter fine structure transitions are being widely utilized as probes of conditions in the interstellar medium and of star formation on a range of spatial scales. These investigations extend from the boundaries of clouds in the Milky Way to ultraluminous galaxies at high redshifts. In this talk I will review some of the results from these varied programs, with an emphasis on modeling of emission from the most widely observed species, C⁺ and N⁺. The [CII] fine structure line at 158 microns wavelength is being imaged over relatively large areas, giving us new information on the “CO-dark molecular gas,” which adds substantially to the molecular mass of the Milky Way, since hydrogen there is molecular (although essentially unobservable directly). [CII] also probes the diffuse ISM, the evolution of atomic to molecular clouds, and the feedback produced by massive young stars. The 205 micron and 122 micron lines of ionized nitrogen arise in fully ionized regions and their ratio is a direct measure of the electron density there. Comparing [CII] and [NII] emission allows determination of the fraction of [CII] coming from photon dominated regions, and better understanding of the relationship between [CII] emission and the rate of star formation. With the availability of multi-pixel high spectral resolution systems, we can look forward to even more extensive imaging in these (and other) fine structure lines from airborne and balloon suborbital platforms as well as future space missions.