ISM parameters in the normal galaxy NGC 5713

We report ISO Long Wavelength Spectrometer (LWS) observations of the Sbc galaxy NGC 5713. We have obtained strong detections of the fine-structure forbidden transitions [C II] 158 μm, [O I] 63 μm, and [O III] 88 μm, and significant upper limits for [N II] 122 μm, [O III] 52 μm, and [N III] 57 μm, and have measured the galaxy's FIR dust continuum. These observations represent the first FIR line survey of a nearly normal galaxy and allow us to make comparisons with COBE results for the Milky Way, and with KAO observations of IR-bright starburst galaxies. We find that NGC 5713 displays a fairly high [C II]/FIR ratio (∼ 0.7%). Under the assumption that the disk emission is dominated by photodissociated molecular gas, we model the FIR continuum and the [C II] and [O I] line fluxes to derive the density of the warm atomic gas surrounding molecular clouds and the UV flux incident on the clouds. In this model, we find higher densities and lower UV fluxes in NGC 5713 than are typical for starburst galaxies, but higher UV fluxes than suggested for the disks of NGC 6946 and the Milky Way. We also discuss alternative origins for the strong [C II] emission, such as from a diffuse component of cold H I.