1995A&A...295..599I

Out of 25 dwarf galaxies searched for J=1-0 ¹²CO emission, four (NGC 55, Haro 2, NGC 6822 and NGC 7793) were unambiguously detected. Four more (DDO 68, 83, 99 and 165) showed doubtful emission at 2-5 times the rms noise level. These and previously obtained data yield 24 dwarf galaxies both (almost) completely covered by the CO observing beam and having known optical colours. The two detected galaxies NGC 1569 and Haro 2 are blue with (B-V)₀=0.3; the uncertain detections also have blue colours. The galaxies not detected cover the full range (B-V)₀=0.0-0.8. Inferred CO-to-FIR flux ratios of the generally metal-poor dwarf galaxies fall between those of high-metallicity centers of bright spiral galaxies and Galactic HII regions. This suggests that the CO emissivity of dwarf galaxies is more determined by ambient radiation field strengths than by metallicity. Except for Haro 2 and NGC 7793, inferred mass ratios M(H₂)/M(HI) are less than a few per cent, more than an order of magnitude lower than those of late-type spiral galaxies. This does not necessarily reflect a real lack of H₂ in dwarf galaxies. Instead, in these objects the CO to H₂ conversion factor may well be higher than the standard Galactic factor, thus allowing the presence of potentially large amounts of undetected molecular hydrogen gas.