AAS 195th Meeting, January 2000
Session 53. ISM: Ionized Gas
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

## [53.11] Far Infrared spectroscopy of normal galaxies with ISO-LWS: Physical conditions in the Interstellar Medium

S Malhotra (N.O.A.O), M Kaufman (S.J.S.U.), D Hollenbach (NASA, Ames), G Helou (IPAC), R Rubin (NASA, Ames), J Brauher (IPAC), D Dale (IPAC), N.Y. Lu (IPAC), S.D. Lord (IPAC), A Contursi (IPAC), T Jarrett (IPAC), D Hunter (Lowell Observatory)

We present measurements of FIR cooling lines from neutral and ionized ISM of 60 normal, star-forming galaxies, selected to span a range in morphology, FIR colors indicating dust temperatures, and FIR/Blue ratios (indicating star-formation activity, optical depth). From the trends seen in the data and comparison with theoretical models we draw the following main conclusions:\\ (1) The ratio of [CII](158 \mu m)/FIR declines with increasing dust temperature and star-forming activity in galaxies; spannning a factor of more than 50 in [CII]/FIR with [CII] deficient galaxies at its extreme end.\\ (2) The ratio of [OI]/[CII] lines increases for galaxies with higher F60/F100. In galaxies with warmer dust, there is less cooling via the [CII] line, while [OI] at 63 \mu m remains a major coolant. This trend is explained in PDR models by an increase in UV radiation field G0, which raises both the gas and dust temperature. \\ (3) [NII] (122 \mu m) from ionized regions, shows behavior similar to [CII], i.e. the ratio [NII]/FIR declines with F60/F100 and L(FIR)/L(B). This suggests that a fair fraction of [CII] comes from ionized gas. We derive theoretical scaling between [NII](122 \mu m) and [CII] from ionized gas and use that scaling to estimate the [CII] flux from neutral regions [CII]c.\\ (4) By comparing the observed ratios of [OI](63\mu m) and [CII]c lines and the ratio of line to the total FIR continuum from dust with a grid of PDR models, we derive the UV radiation density G0 and gas densities n in the sample galaxies. G0 and n correlate with each other and G0 increases as n1.4. Since G0 can be used as a measure of current star-formation, this directly confirms Schmidt law of star-formation where the star-formation increases with gas density to some power.\\