5 Conclusions

We observed the BCDG I Zw 36 ( $\rm 12 + \log[ O / H ] = 7.9$) in the [C II] $158\ \mu \rm m$ fine-structure line with the ISO/LWS. The conclusions are summarized as follows.

1.

An upper limit for the [C II] flux in the LWS beam (68 $^{\prime\prime}$ in FWHM) was obtained: $F_{\rm [C{\sc ii}]} \leq 1.1~10^{-20}\ \rm W\ cm^{-2}$ ($3 \sigma$).

2.

A comparison with the previous CO observations provides an upper limit for the [C II]/CO J = 1-0 line flux ratio of $F_{\rm [C{\sc ii}]} / F_{\rm CO} \leq 1.3~10^3$. This indicates that the line ratio in I Zw 36 is at least one order of magnitude lower than those observed in irregular galaxies ( $I_{\rm [C{\sc ii}]} / I_{\rm CO} = 2.3~10^4$ in the LMC), in spite of the lower metallicity of I Zw 36.

3.

A comparison with PDR models shows that is required in I Zw 36 to account for the observed difference in the [C II]/CO line ratio between I Zw 36 and irregular galaxies, under the assumption of $N_{\rm H}^{\rm cloud} \sim 10^{22}\ \rm cm^{-2}$in both the types of galaxies. Taking into account a possible larger $N_{\rm H}^{\rm cloud}$ in I Zw 36 than in the previously observed irregulars, we conclude that $n_{\rm H}$ in I Zw 36 is between that in the irregulars and the estimate for the constant $N_{\rm H}^{\rm cloud}$ case: .

Acknowledgements

We are much indebted to I. Yamamura, T. Nakagawa, and T. Tohya for their stimulating discussions at the Institute of Space and Astronautical Science. We are grateful to the members of Japanese ISO Guaranteed Time observation team for constructing the observation plan and supporting the data reduction, especially to H. Okuda, K. Kawara, and Y. Sato. We would also like to thank C. P. Pearson for his helpful comments. K. M. was financially supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.