Probing the interstellar medium in early-type galaxies with Infrared SpaceObservatory observations

Four IRAS-detected early-type galaxies were observed with the Infrared Spac e Observatory (ISO) With the exception of the 15 mum image of NGC 1052, the mid-IR images of NGC 1052, NGC 1155, NGC 5866, and NGC 6958 at 4.5, 7, and 15 pm show extended emission. Mid-IR emission from NGC 1052, NGC 1155, and NGC 6958 follows a de Vaucouleurs profile. The ratio of 15 mum/7 mum flux decreases with radius in these galaxies, approaching the values empirically observed for purely stellar systems. In NGC 5866, the 7 and 15 pm emission is concentrated in the edge-on dust lane. All the galaxies are detected in the CC nl (158 mum) line, and the SOs NGC 1155 and NGC 5866 are detected i n the [O I] (63 mum) line as well. Previous detections of neutral interstel lar medium (ISM) are sparse: only NGC 1052 had been detected in H I and NGC 5866 in CO. The ISO long-wavelength spectrograph observations of the CC nr l line are more sensitive measures of cool neutral ISM than H I and CO by a bout a factor of 10-100. Comparison of [C II] with H alpha shows that [C II ] does not arise in H II regions and therefore must arise in the neutral me dium. Three of four early-type galaxies, namely, NGC 1052, NGC 6958, and NG C 5866, have low ratios of far-infrared to blue luminosity and show a lower L-[C II]/L-FIR, which is explained by postulating a softer radiation field from old stellar populations in early-type galaxies, compared to spirals a nd irregulars, where young stars are present. While optical photons are eff ective in heating the dust, UV radiation is needed to heat the gas by the g rain photoelectric mechanism. The low [C II]/CO ratio in NGC 5866 [L-[C II] /L-CO(1-0) less than or equal to 570] confirms this scenario. We estimate t he UV radiation expected from the old stellar populations in these galaxies and compare it to that needed to heat the gas to account for the cooling o bserved [C II] and [O I] lines. In three out of four galaxies, NGC 1052, NG C 5866, and NGC 6958, the predicted UV radiation falls short by a factor of 2-3 of that required to sufficiently heat the gas. In view of the observed intrinsic scatter in the "UV upturn" in elliptical galaxies and its great sensitivity to age and metallicity effects, this difference is not signific ant. However, the much larger difference (about a factor of 20) between the UV radiation from old stars and that needed to produce the far-infrared li nes for NGC 1155 is strong evidence for the presence of an additional UV so urce, probably young stars, in NGC 1155.