1 Introduction

The [C II] 158 $\mu$m fine-structure ( $^2P_{3/2} \rightarrow\, ^2$P_1/2) emission is radiated from interstellar C⁺ ions. The line ratio of the [C II] to the ¹²CO J = 1-0 transitions can be used as a probe of the interstellar ultraviolet (UV) intensity in galaxies (Stacey et al. 1991; Pierini et al. 1999), because the interstellar UV radiation field dissociates CO molecules and ionizes C atoms near the surfaces of molecular clouds. Nevertheless, Stacey et al. (1991) found that even starburst galaxies, which have intense interstellar UV fields, cannot have [C II]/¹²CO J = 1-0 line intensity ratios higher than 5 10³(ratios when the main-beam temperature scale is adopted for the CO intensity instead of the $T_{\rm R}^\ast$ scale). Line ratios exceeding this limit ( $[C{\sc ii}] / {\rm CO} > 10^4$) have been observed on a galactic scale only toward irregular galaxies (Mochizuki et al. 1994; Lord et al. 1995; Madden et al. 1997) and a few spirals in the Virgo cluster (Smith & Madden 1997).

The extremely high [C II]/CO line ratios in the irregulars are accounted for by their lower metallicities; the lower dust abundance due to the lower metallicity allows CO-dissociating UV photons to penetrate deeper into the molecular clouds (Maloney & Black 1988). Low metallicities are also suggested for the spirals with the extremely high [C II]/CO ratios (Smith & Madden 1997), although the metallicities have not been measured in these galaxies.

In the present paper, we report observations of a blue compact dwarf galaxy (BCDG) in the [C II] line. In order to obtain the [C II]/CO line ratio, we selected an object previously detected in the CO J = 1-0 emission: I Zw 36 (Mrk 209; UGCA 281). This nearby ($\simeq$ $4.6\ \rm Mpc$; e.g., Viallefond & Thuan 1983) galaxy has a lower metallicity ( $\rm 12 + \log[ O / H ] = 7.9$ in interstellar oxygen abundance; Viallefond & Thuan 1983) than the irregulars previously observed in the [C II] emission. The comparison of I Zw 36 with the irregulars enables us to investigate the effect of galactic morphology on the C⁺-CO chemical balance in the low-metallicity environments.

 

 

Table 1: Observation parameters of I Zw 36

Parameter	Value	Reference
Object	I Zw 36	1
$\alpha (2000)$ ( $\ \rm ^h\ ^m\ ^s$)	12 26 17.1	2
$\delta (2000)$ ( $\ \hbox{$^\circ$ }\ \hbox{$^\prime$ }\ \hbox{$^{\prime\prime}$ }$)	48 29 37	2
$\rm 12 + \log[ O / H ]$	$7.93 \pm 0.07$	3
$\int T_{\rm A}^\ast (\rm {CO}\ {\it J} = 1{-}0$)  d v $^{\rm a}$ ( $\rm K\ km\ s^{- 1}$)	$0.45 \pm 0.10$	4
$F_{\rm [C\, II]}\, ^{{\rm b}}$ ( $10^{-20}\ \rm W\ cm^{-2}$)	$\leq 1.1$	5
$F_{\rm [C\, II]}/F_{\rm CO}\, ^{{\rm c}}$	$\leq 1.3~10^3$	4, 5

$^{\rm a}$ The $45\hbox{$^{\prime\prime}$ }$ (HPBW) beam was centered on $\rm\alpha (2000) = 12^h 26^m 15 \hbox{$.\!\!^{\rm s}$ }9$ and $\rm\delta (2000) = 48\hbox{$^\circ$ }29\hbox{$^\prime$ }30\hbox{$^{\prime\prime}$ }$.

$^{\rm b}$ The beam size was 68'' in FWHM. The flux calibration uncertainty was taken into account as well as the statistical noise of a $3 \sigma$ level.

$^{\rm c}$ The upper limit results from that in $F_{\rm [C\, II]}$. The main-beam temperature scale is adopted for the CO intensity instead of the $T_{\rm R}^\ast$ scale.

References: 1. Zwicky et al. (1961); 2. Palumbo et al. (1988); 3. Viallefond & Thuan (1983); 4. Young et al. (1995); 5. this work.