ADS: Gullberg et al. 2015
arXiv: Gullberg et al. 2015
This paper presents part of our comprehensive follow-up of the SPT dusty galaxies. In this case, we were interested in looking at the emission from ionized carbon atoms, the [CII] fine structure transition at 158um. This line is usually the brightest one in dusty star-forming galaxies, and can make up over 1% of the entire luminosity of the galaxy. Since it’s so bright, there’s been a lot of interest in figuring out what, exactly, this line can tell you about what’s happening inside the galaxy. In this paper, we used [CII] observations of 20 SPT galaxies to try to answer this.We combined our [CII] observations with observations of low-excitation CO transitions, and determined that the combination of the two lines is best described by a medium in which the CO line is optically thick (i.e., opaque beyond the surface layer) and the [CII] line is moderately optically thick (you can think of it as being translucent, like sun through fog or frosted glass). This goes against the usual picture of [CII], which is that it’s optically thin (sort of like it’s transparent). But, this is perfectly okay, given what we know about the structure of gas clouds exposed to UV light from young, massive stars. Having optically thick [CII] could explain the “CII deficit”, where the [CII] line gets weaker as you look at brighter galaxies, since you only see [CII] light coming from the outer, surface layers of clouds.