SHINING, A Survey of Far-infrared Lines in Nearby Galaxies. I. Survey Description, Observational Trends, and Line Diagnostics

We use the Herschel/PACS spectrometer to study the global and spatially resolved far-infrared (FIR) fine-structure line emission in a sample of 52 galaxies that constitute the SHINING survey. These galaxies include star-forming, active-galactic nuclei (AGNs), and luminous infrared galaxies (LIRGs). We find an increasing number of galaxies (and kiloparsec-size regions within galaxies) with low line-to-FIR continuum ratios as a function of increasing FIR luminosity (L-FIR), dust infrared color, L-FIR to molecular gas mass ratio (L-FIR/M-mol), and FIR surface brightness (Sigma(FIR)). The correlations between the [C II]/FIR or [O I]/FIR ratios with Sigma(FIR) are remarkably tight (similar to 0.3 dex scatter over almost four orders of magnitude in Sigma(FIR)). We observe that galaxies with L-FIR/M-mol greater than or similar to 80 L-circle dot M-circle dot(-1) and Sigma(FIR) greater than or similar to 10(11) L-circle dot kpc(-2) tend to have weak fine-structure line-to-FIR continuum ratios, and that LIRGs with infrared sizes greater than or similar to 1 kpc have line-to-FIR ratios comparable to those observed in typical star-forming galaxies. We analyze the physical mechanisms driving these trends in Paper II. The combined analysis of the [C II], [N II] 122 mu m, and [O III] 88 mu m lines reveals that the fraction of the [C II] line emission that arises from neutral gas increases from 60% to 90% in the most active star-forming regions and that the emission originating in the ionized gas is associated with low-ionization, diffuse gas rather than with dense gas in H II regions. Finally, we report the global and spatially resolved line fluxes of the SHINING galaxies to enable the comparison and planning of future local and high-z studies.