  We report initial results from the far-infrared fine structure line
observations of a sample of 44 local starbursts, Seyfert galaxies and infrared
luminous galaxies obtained with the PACS spectrometer on board Herschel. We
show that the ratio between the far-infrared luminosity and the molecular gas
mass, Lfir/MH2, is a much better proxy for the relative brightness of the
far-infrared lines than Lfir alone. Galaxies with high Lfir/MH2 ratios tend to
have weaker fine structure lines relative to their far-infrared continuum than
galaxies with Lfir/MH2 < 80 Lsun/Msun. A deficit of the [C II] 158 micron line
relative to Lfir was previously found with the ISO satellite, but now we show
for the first time that this is a general aspect of all far-infrared fine
structure lines, regardless of their origin in the ionized or neutral phase of
the interstellar medium. The Lfir/MH2 value where these line deficits start to
manifest is similar to the limit that separates between the two modes of star
formation recently found in galaxies on the basis of studies of their gas-star
formation relations. Our finding that the properties of the interstellar medium
are also significantly different in these regimes provides independent support
for the different star forming relations in normal disk galaxies and major
merger systems. We use the spectral synthesis code Cloudy to model the emission
of the lines. The expected increase of the ionization parameter with Lfir/MH2
can simultaneously explain the line deficits in the [C II], [N II] and [O I]
lines.
