CO OBSERVATIONS OF 25 DWARF GALAXIES

Out of 25 dwarf galaxies searched for J=1-0 12CO emission, four (NGC 5 5, Haro 2, NGC 6822 and NGC 7793) were unambiguously detected. Four mo re (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 gal axies both (almost) completely covered by the CO observing beam and ha ving known optical colours. The two detected galaxies NGC 1569 and Har o 2 are blue with (B-V)o = 0.3; the uncertain cover the full range (B- V)o = 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 t he CO emissivity of dwarf galaxies is more determined by ambient radia tion field strengths than by metallicity. Except for Haro 2 and NGC 77 93, inferred mass ratios M(H2)/M(HI) are less than a few per cent, mor e than an order of magnitude lower than those of late-type spiral gala xies. This does not necessarily reflect a real lack of H2 in dwarf gal axies. Instead, in these objects the CO to H2 conversion factor may we ll be higher than the standard Galactic factor, thus allowing the pres ence of potentially large amounts of undetected molecular hydrogen gas .