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Context: The north-west photo-dissociation region (PDR) in the reflectionnebula NGC 7023 displays a complex structure. Filament-like condensations atthe edge of the cloud can be traced via the emission of the main cooling lines,offering a great opportunity to study the link between the morphology andenergetics of these regions. Aims: We study the spatial variation of thefar-infrared fine-structure lines of [C II] (158 um) and [O I] (63 and 145 um).These lines trace the local gas conditions across the PDR. Methods: We usedobservations from the Herschel/PACS instrument to map the spatial distributionof these fine-structure lines. The observed region covers a square area ofabout 110" x 110" with an angular resolution that varies from 4" to 11". Wecompared this emission with ground-based and Spitzer observations of H2 lines,Herschel/SPIRE observations of CO lines, and Spitzer/IRAC 3.6 um images thattrace the emission of polycyclic aromatic hydrocarbons. Results: The [C II](158 um) and [O I] (63 and 145 um) lines arise from the warm cloud surfacewhere the PDR is located and the gas is warm, cooling the region. We find thatalthough the relative contribution to the cooling budget over the observedregion is dominated by [O I]63 um (>30%), H2 contributes significantly in thePDR (35%), as does [C II]158 um outside the PDR (30%). Other species contributelittle to the cooling ([O I]145 um 9%, and CO 4%). The [O I] maps resolve thesecondensations into two structures and show that the peak of [O I] is slightlydisplaced from the molecular H2 emission. The size of these structures is about8" (0.015 pc) and in surface cover about 9% of the PDR emission. Finally, wedid not detect emission from [N II]122 um, suggesting that the cavity is mostlyfilled with non-ionised gas.

The Spatial Variation of the Cooling Lines in the Reflection Nebula NGC7023