부산대학교 도서관

We present ALMA [CI]($1-0$) (rest frequency 492 GHz) observations for asample of 13 strongly-lensed dusty star-forming galaxies originally discoveredat 1.4mm in a blank-field survey by the South Pole Telescope. We compare thesenew data with available [CI] observations from the literature, allowing a studyof the ISM properties of $\sim 30$ extreme dusty star-forming galaxies spanninga redshift range $2 < z < 5$. Using the [CI] line as a tracer of the molecularISM, we find a mean molecular gas mass for SPT-DSFGs of $6.6 \times 10^{10}$M$_{\odot}$. This is in tension with gas masses derived via low-$J$ $^{12}$COand dust masses; bringing the estimates into accordance requires either (a) anelevated CO-to-H$_2$ conversion factor for our sample of $\alpha_{\rm CO} \sim2.5$ and a gas-to-dust ratio $\sim200$, or (b) an high carbon abundance $X_{\rmCI} \sim 7\times10^{-5}$. Using observations of a range of additional atomicand molecular lines (including [CI], [CII], and multiple transitions of CO), weuse a modern Photodissociation Region code (3D-PDR) to assess the physicalconditions (including the density, UV radiation field strength, and gastemperature) within the ISM of the DSFGs in our sample. We find that the ISMwithin our DSFGs is characterised by dense gas permeated by strong UV fields.We note that previous efforts to characterise PDR regions in DSFGs may havesignificantly underestimated the density of the ISM. Combined, our analysissuggests that the ISM of extreme dusty starbursts at high redshift consists ofdense, carbon-rich gas not directly comparable to the ISM of starbursts in thelocal Universe.