Left: [CI] 1–0/ CO J = 4–3 luminosity ratio of 3D-HST GS30274 as a function of its [CI] 1–0/ FIR luminosity ratio. In this case the luminosities were taken to be in solar units. Coloured lines represent the density and UV radiation field contours as presented in Alaghband-Zadeh et al. (2013), based on the Kaufman et al. (1999, 2006) PDR models. These line ratios provide us with a good constraint on the UV radiation field and add an additional constraint on the gas density. Right: density and UV radiation field of 3D-HST GS30274 compared to the densities and radiation fields of main-sequence galaxies (Malhotra et al. 2001), local starburst galaxies (Stacey et al. 1991), and local ULIRGS (Davies et al. 2003). The density of 3D-HST GS30274 is similar to densities seen in local starbursts and ULIRGS, whereas the UV radiation field is similar to local ULIRGS.