Giant Molecular Clouds in the Magellanic Clouds

摘要

It is widely accepted that almost all giant molecular clouds areactively forming massive stars as evidenced by observations in the solar vicinity.However, new millimeter CO observations of the Large Magellanic Cloud (LMC)have revealed that nearly 30 % of the giant molecular clouds in the LMC show nosigns of massive star formation, apparently remaining completely starless. Thissuggests that the timescale of star formation is considerably longer in the LMCthan in the Milky Way. We propose that the lower heavy element abundancein the LMC is the primary cause of this retarded star formation via a higherdegree of ionization and a lower degree of molecular cooling.