Constraints on the hydrogen density–UV field plane from the excitation of C I fine-structure levels. The probability density is represented by the colour intensity, and the 68% and 30% confidence level contours are shown as solid and dashed green lines, respectively. The extragalactic UV background sets a lower limit to I_UV as shown by the horizontal black line with upward arrows. Attenuation effects are not taken into account and the actual incident UV field can be a few times higher.