POL-2 polarisation fraction and column density. Panel a shows the column density map N₅ (H₂) at 40′′ resolution. The contours are drawn at 0.5, 1, and 2 times 10²² cm⁻² and the red circles denote the locations of the point sources of Table 2. In panel b, p_mas is plotted as a function of column density for data with N(H₂) > 10²² cm⁻². The pixels coinciding with point sources (red circles of panel a) are plotted in red. The blue bars are representative error estimates. The result of the weighted least squares fit to data at N(H₂)> 2 × 10²² cm⁻² is givenin the panel and is shown as a black solid line. The shading corresponds to N(H₂)< 2 × 10²² cm⁻² where the p_mas estimates may be biased.