2001A&A...370..868M

We observed the blue compact dwarf galaxy I Zw 36 in the [CII] 158µm line with the Infrared Space Observatory obtaining an upper limit for the line flux, F_[CII]≤1.1x10^–20W/cm² (3 σ). A comparison with previous CO observations yields an upper limit for the [CII]/CO J=1-0 line flux ratio, F_[CII]/F_CO≤1.3x10³ (3σ). This limit indicates that the [CII]/CO J=1-0 line ratio in I Zw 36 is not higher than those in our Galactic plane (I_[CII]/I_CO=1.1x10³) and is at least one order of magnitude lower than those in irregular galaxies (I_[CII]/I_CO≃2x10⁴) with low metallicities, in spite of the lower metallicity in I Zw 36 (12+log[O/H]=7.9) than in the irregulars. The observed difference in the line ratio between I Zw 36 and the irregulars can be accounted for by a higher gas density (10³cm^–3≲n_H≲10⁴cm^–3) in I Zw 36, because the higher CO-formation rate at the higher density enables CO molecules to survive even against the photodissociation enhanced by the low dust abundance. The expected higher gas density in I Zw 36 may be related to change in large-scale gravitational potential with galactic evolution, if blue compact dwarfs and irregulars have evolutionary links.