3 Results

The [C II] emission was not detected in any of the on-source, off-source, or foreground-subtracted spectrum. Figure 1 shows the foreground-subtracted spectrum; a linear baseline was also subtracted. An upper limit of $F_{\rm [C{\sc ii}]} \leq 7.2~10^{-21}\ \rm W\ cm^{-2}$ (statistical uncertainty of a $3 \sigma$ level) was derived for the [C II] line flux from this spectrum. Including the LWS calibration uncertainty (Sect. 2), we obtain the final upper limit $F_{\rm [C{\sc ii}]} \leq 1.1~10^{-20}\ \rm W\ cm^{-2}$as listed in Table 1.

The ¹²CO J = 1-0 line emission from I Zw 36 was detected in the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey (Young et al. 1995). The integrated main-beam intensity of the emission is $\int T_{\rm MB} {\rm d}v = 0.82 \pm 0.18\ \rm K\ km\ s^{-1}$with the uncertainty of a $1 \sigma$ level (including the calibration uncertainty) at a beam size of $45\hbox{$^{\prime\prime}$ }$ (HPBW). This yields a CO J = 1-0 flux of $F_{\rm CO} = (6.9 \pm 1.5)~10^{-24}\ \rm W\ cm^{-2}$in the beam slightly smaller than that of the LWS, under the assumption of a Gaussian profile for the CO beam. Thus, the upper limit ($3 \sigma$) for the [C II]/CO line flux ratio is $F_{\rm [C{\sc ii}]} / F_{\rm CO} \leq 1.3~10^3$. This limit for the flux ratio can also be regarded as an upper limit for the [C II]/CO intensity ratio ( $I_{\rm [C{\sc ii}]} / I_{\rm CO}$) in the CO beam, whose size is close to the optical diameter ( $0\hbox{$.\mkern-4mu^\prime$ }92$; de Vaucouleurs et al. 1991) of the galaxy. Thus, $I_{\rm [C{\sc ii}]} / I_{\rm CO}$ in I Zw 36 is not higher than those observed in the inner Galactic plane, $I_{\rm [C{\sc ii}]} / I_{\rm CO} \simeq 1.1~10^3$(Nakagawa et al. 1998; Dame et al. 1987, corrected as described in Bronfman et al. 1988).

Figure 2 shows the [C II]/¹²CO J = 1-0line intensity ratio observed in galaxies as a function of oxygen abundance, $\rm 12 + \log[ O / H ]$, derived from optical-UV emission line observations of H II regions in the galaxies. The [C II] and CO data are, respectively, from Nakagawa et al. (1998) and Dame et al. (1987) for the Galaxy; Mochizuki et al. (1994) and Cohen et al. (1988) for the Large Magellanic Cloud (LMC); Madden et al. (1997) and Becker (1990) for IC 10. The CO intensity scale of Dame et al. (1987) was corrected as described in Bronfman et al. (1988). The [C II]/CO line ratios for NGC 4449 and NGC 1569 are from Lord et al. (1995). The references for the oxygen abundance are as follows: the inner Galaxy, Deharveng et al. (2000); the LMC, Russell & Dopita (1990); NGC 4449 and IC 10, Hidalgo-Gámez & Olofsson (1998) based on observations of McCall et al. (1985) and Lequeux et al. (1979); NGC 1569, Kobulnicky & Skillman (1997); I Zw 36, Viallefond & Thuan (1983). The radial variation of metallicity in our Galaxy is taken into account; the lower and upper limits in the figure correspond to the abundances at the Galactocentric distance of the Sun and the extrapolation of the relation between the abundance and the Galactocentric distance to the Galactic center, respectively.

Irregular galaxies with low metallicities have [C II]/CO line ratios higher than those in the inner Galaxy and also than the starburst limit obtained by Stacey et al. (1991). These extremely high ratios have been accounted for by the enhanced photodissociation of interstellar CO molecules because of the low dust abundances due to the low metallicities (Maloney & Black 1988). However, the BCDG I Zw 36 does not show a [C II]/CO line ratio higher than those in normal spirals in spite of its low metallicity.