On the x axis we find the temperature of the exciting background radiation and on the y axis the column density of H₃O⁺. For each galaxy, the brightness temperature of the H₃O⁺ transition is shown in a gray scale map. The thick black contour marks the observed value, with one sigma errors marked by dashed lines. Background temperatures estimated by IR observations are also reported and used to derive a column density consistent with the observed brightness temperature. Note that the column density fits include only the para column.