The full ISO Long Wavelength Spectrometer spectra of six IR-bright galaxies from Fischer et al. (1999) showing a dramatic sequential progression in ionic/atomic fine-structure emission line and molecular/atomic absorption line characteristics extending from strong [O III]52,88 μm and [N III]57 μm line emission to detection of only faint [C II]158 μm line emission from gas in photodissociation regions. The molecular absorption spectra show varying excitation as well, extending from galaxies in which the molecular population mainly occupies the ground state to the nearby ultraluminous galaxy Arp 220 in which there is significant population in higher levels.
The spectra have been shifted and ordered vertically according to the equivalent width of the [O III] 88 m m line. The excitation potential, the energy required to create the species, is given in eV at the bottom of the figure. Using the temperature-insensitive [O III] 52 μm [O III] 88 μm line ratio as a probe of density, we find no clear dependence of the [O III]88/FIR ratio on electron density and all of the measured [O III] line ratios were found to fall within the range 0.6 - 1.2, consistent with electron densities between 100 - 500 cm-3. From this we infer that neither density nor far-infrared differential extinction (between 52 - 88 μm) to the ionized gas appears to be the single dominant parameter in the observed sequence. Rather, since both of the line ratios [N III]57/[N II]122 and [O III]52/[N III]57 increase as a function of the [OIII]88/FIR ratio, we infer that [O III]88/FIR correlates with excitation. We find that progression to high ionization parameters (the dust-bounded case) and extreme optical depths can explain these effects. The molecules seen in absorption are located in dense photodissociation or X-ray dissociation regions, where they are radiatively pumped by the far-infrared emission from warm dust. If extinction plays a role in this sequence it appears from this analysis that the affected regions are very heavily obscured even in the far-infrared, while the detected line emission is relatively unobscured. In this case, the progression to low excitation could be a result of total obscuration of the youngest starburst population and/or the central AGN.
Our new Herschel Space Observatory observations have shown that in many of the most luminous IR-bright galaxies, the ultraluminous infrared galaxies, known to be mergers of gas-rich galaxies, the molecular gas is being swept out of the galaxies, possibly by the intense radiation pressure from the central source, indicating for the first time how these systems rid themselves of star-forming molecular gas on their evolutionary path to gas-poor elliptical galaxies.