H₂ column density map of the south-western part of the Orion B giant molecular cloud, derived from Herschel Gould Belt Survey observations (André et al. 2010; Schneider et al. 2013). The field observed by the Orion-B collaboration and used for this work is overlaid in red. The blue squares mark the nebulae NGC 2024, NGC 2023 and the Horsehead, and the pink star symbols mark the stars Alnitak, HD 38087 and σ Ori (north to south).