a) Distribution of background cloud column densities for the population of 446 candidate prestellar cores identified with Herschel in the whole Aquila field. The vertical dashed line marks a fiducial threshold at an equivalent visual extinction level mag (cf. Heiderman et al. 2010; Lada et al. 2010; André et al. 2010, 2014). b) Normalized “probability” of finding a Herschel prestellar core as a function of background column density (blue histogram with error bars), obtained by dividing the number of prestellar cores detected with Herschel below a given background column density level by the total surface area covered by the HGBS survey below the same level. The red curve shows a simple fit of the form P_core(A_V) = 1 − exp(a × A_V + b) with a = −0.17 and b = 0.86. The vertical dashed line marks mag as in the left panel. (Color figure is available in the online version.)