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For refcode 1998AJ....116.2746T:
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Copyright by American Astronomical Society. Reproduced by permission
1998AJ....116.2746T CO EMISSION IN LOW-LUMINOSITY, H I-RICH GALAXIES CHRISTOPHER L. TAYLOR Ruhr-Universitat Bochum, Astronomisches Institut, Universitatsstrasse 150, D-44870 Bochum, Germany; taylorc@astro.ruhr-uni-bochum.de HENRY A. KOBULNICKY University of California, Santa Cruz, Lick Observatory/Board of Studies in Astronomy, Santa Cruz, CA 95064; chip@ucolick.org AND EVAN D. SKILLMAN University of Minnesota, Department of Astronomy, 116 Church Street, SE, Minneapolis, MN 55455; skillman@zon.spa.umn.edu Received 1998 July 17; revised 1998 August 18 ABSTRACT We present ^12^CO 1 - 0 observations of 11 low-luminosity (M_B_ > -18), H I-rich dwarf galaxies. Only the three most metal-rich galaxies, with 12 + log (O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely metal- poor systems [12 + log (O/H) <= 7.5] yield only low upper limits on the CO surface brightness, I_CO_ < 0.1 K km s^-1^. Three of these six have never before been observed in a CO line, while the others now have much more stringent upper limits. For the very low metallicity galaxy Leo A, we do not confirm a previously reported detection in CO, and the limits are consistent with another recent nondetection. We combine these new observations with data from the literature to form a sample of dwarf galaxies that all have CO observations and measured oxygen abundances. No known galaxies with 12 + log (O/H) < 7.9 (Z < 0.1 Z_sun_) have been detected in CO. Most of the star-forming galaxies with higher [12 + log (O/H) > 8.1] metallicities are detected at similar or higher I_CO_ surface brightnesses. The data are consistent with a strong dependence of the I_CO_/M_H_2__ = X_CO_ conversion factor on ambient metallicity. The strikingly low upper limits on some metal-poor galaxies lead us to predict that the conversion factor is nonlinear, increasing sharply below ~1/10 of the solar metallicity [12 + log (O/H) <= 7.9]. Key words: galaxies: dwarf-galaxies: ISM
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