f_coll factor accounting for the density effects. The red line represents the 3-parameter fit obtained on the basis of the photoionization modeling to be used in formula (8) to calculate L(Lyα). At each density, vertical lines show the spread in values arising from the different T_bb and U models used: models with higher T_bb and lower U have higher values. The fit has been made for primordial nebular abundances and the black dotted line is f_coll from Panagia (1973).