The self regulating star formation of gas rich dwarf galaxies in quiescent phase

The expected episodic or intermittent star formation histories (SFHs) of gas rich dwarf irregular galaxies (dIrrs) are the longstanding puzzles to understand their whole evolutional history. Solving this puzzle, we should grasp what physical mechanism causes the quiescent phase of star formation under the very gas rich condition after the first starburst phase. We consider that this quiescent phase is kept by lack of H-2, which can be important coolant to generate the next generation of stars in the low-metal environment like dIrrs. Furthermore, in dIrrs, H-2, formation through gas-phase reactions may dominate the one on dust-grain surfaces because their interstellar medium (ISM) are very plentiful and the typical dust-to-gas ratio of dIrrs (D-dIrrs=1.31x10(-2) D-MW, where D-MW is its value for the local ISM) is on the same order with a critical value D(cr)similar to10(-2) D-MW. We show that the lack of H-2 is mainly led by H- destruction when gas-phase H-2 formation dominates since H- is important intermediary of gas-phase H-2 formation. H- is destroyed by the radiation from all stars born in the previous starburst phase because H- destroying infrared photon can penetrate the whole ISM of dIrrs. Considering the physical process which timescale is the longest as main process in regulating global star formation, we can show this lack of H-2 leads the quiescent phase of star formation. Hence, we can say that the stellar radiation which destroys H- and leads low H-2 abundance should be properly treated in studying SFHs of dIrrs.