CO deficiency in LSB galaxies. Clues to their star-formation history

An optical Hα, CO, and H I survey of six low surface-brightness galaxies from the Schombert and Bothun catalog is presented. These galaxies are all late-type, low H I mass disk systems with unusually low-luminosity densities. Overall, the sample exhibits several paradoxical properties such as no detectable CO emission (a mean upper limit of script M sign_H2/script M sign_HI.<0.10) despite normal H I surface density, very blue colors without evidence of strong star formation, weak H I regions lacking in massive stars and high script M sign/L's. These properties suggest two possible star-formation histories for this class of objects centering on (1) inefficient star formation in a very metal-poor environment and (2) star formation which occurs preferentially outside of the giant molecular cloud environment with an IMF operating in a suppressed high mass mode.