2000A&A...359...41B

We present the results of far infrared (FIR) spectroscopy of the luminous blue compact galaxy (BCG) Haro 11 (ESO 350-IG38) obtained with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO) in low resolution mode. This metal poor dwarf merger is an extremely hot IRAS source. We discuss the balance between dust and line cooling in the photodissociated regions (PDR), in particular the role of the [CII]λ158µ line, and derive the basic properties of the PDR gas and estimates of the gas and dust masses. The mass of the PDRs, 2⁺²_–1x10⁸M_☉, is comparable to that of the ionized gas and exceeds the observed upper limit of the HI mass. The gas/dust mass ratio is low, indicating that the galaxy contains little cold dust. The low metallicity, the intense radiation field and the low column density of Haro 11 results in an extremely high [CII]/CO flux ratio and probably also a very high M(H₂)/L_CO conversion factor. Therefore CO is a poor indicator of the H₂ mass in starburst dwarf galaxies. After a reanalysis we confirm the claimed correlation (Malhotra et al., 1997ApJ...491L..27M) between the [CII]158µ/FIR flux ratio and the IRAS f₆₀/f₁₀₀ dust temperature and reduce the scatter. We find that Haro 11 deviates from the relationship being brighter in [CII] than what would be expected, if the mechanism proposed by Malhotra et al. is dominant. As an alternative (or complementary) explanation we propose that the [CII]158µ/FIR versus f₆₀/f₁₀₀ relationship is caused by an increasing optical depth with increasing IRAS temperature. The low metallicity of Haro 11 and its extreme starburst properties probably allows the medium to be thin despite its high f₆₀/f₁₀₀ ratio. This leaves room for a more optimistic view on the possibilities to detect massive starforming mergers at high redshifts, using the [CII] line.