Understanding the processes of star formation is one of the major goals of extragalactic astronomy. One important piece of this goal is determining how the energy inputted into the interstellar medium (ISM) by young stars is regulated and eventually dissipated through emission of far-infrared cooling lines. Measurements of cooling lines can also provide insight on the potential feedback processes that could be enhancing or limiting star formation. The brightest of these cooling lines is often the 158 μm line of singly ionized carbon (Luhman et. al. 2003). As singly ionized carbon is the predominant phase of carbon spanning multiple phases of the ISM, the [CII] 158 μm line can originate in a wide variety of galactic environments (Herrera-Camus et. al. 2015). This multiphase origin makes the [CII] line both an intriguing and complex indicator of ISM conditions, with the potential to be used as a star-formation rate indicator, to trace CO-dark molecular gas, and probe the energy balance throughout the ISM. In order to test these different uses, detailed studies of velocity resolved [CII] measurements are necessary.
One source of velocity resolved [CII] measurements is the GREAT receiver on SOFIA. [CII] maps observed with GREAT can be used to isolate [CII] emission from different phases of the ISM (see, for example, Röllig et. al. 2012 and Fahrion et. al 2017). Supplementing these velocity-resolved [CII] maps, the origins of the [CII] emission can be further probed by comparing the strength of the [CII] emission to the emission from singly ionized nitrogen at 205 μm. The ratio of these two lines can be used to estimate the fraction of [CII] 158 μm emission arising in phases of the ISM where hydrogen is ionized, like HII regions surrounding young stars (Croxall et. al. 2017).
Spatially resolved maps of the [CII] 158 μm and [NII] 205 μm lines in four HII regions in M33 obtained using GREAT are available on the IRS SOFIA Archive (PI 03_0005 and PI 04_0038). PI 03_0005 contains [CII] 158 μm and [NII] 205 μm maps for HII regions NGC592, NGC595 and NGC604, while PI 04_0038 contains maps of NGC558 and additional maps of NGC604. These HII regions are in local galaxy M33 (D~0.9 Mpc) and are widely studied (Ubeda et al 2009, and references within). The combination of archival GREAT and other multiwavelength survey data could be used to study the star formation efficiencies within these nearby HII regions.