Probing the interstellar medium in early-type galaxies with infrared space observatory observations

Four IRAS-detected early-type galaxies were observed with the Infrared Space Observatory (ISO). With the exception of the 15 μm image of NGC 1052, the mid-IR images of NGC 1052, NGC 1155, NGC 5866, and NGC 6958 at 4.5, 7, and 15 μm show extended emission. Mid-IR emission from NGC 1052, NGC 1155, and NGC 6958 follows a de Vaucouleurs profile. The ratio of 15 μm/7 μm flux decreases with radius in these galaxies, approaching the values empirically observed for purely stellar systems. In NGC 5866, the 7 and 15 μm emission is concentrated in the edge-on dust lane. All the galaxies are detected in the [C II] (158 μm) line, and the SOs NGC 1155 and NGC 5866 are detected in the [O I] (63 μm) line as well. Previous detections of neutral interstellar 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 [C II] line are more sensitive measures of cool neutral ISM than H I and CO by about a factor of 10-100. Comparison of [C II] with Hα shows that [C II] does not arise in H II regions and therefore must arise in the neutral medium. Three of four early-type galaxies, namely, NGC 1052, NGC 6958, and NGC 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 and irregulars, where young stars are present. While optical photons are effective in heating the dust, UV radiation is needed to heat the gas by the grain photoelectric mechanism. The low [C II]/CO ratio in NGC 5866 [L_{[C II]}/L_CO(1-0) ≤ 570] confirms this scenario. We estimate the 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 observed [C II] and [O I] lines. In three out of four galaxies, NGC 1052, NGC 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 significant. 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 lines for NGC 1155 is strong evidence for the presence of an additional UV source, probably young stars, in NGC 1155.