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 \mathrm { H _ { 2 } } , which can be important coolant to generate the next generation of stars in the low-metal environment like dIrrs .
Furthermore , in dIrrs , \mathrm { 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 _ { \mathrm { dIrrs } } = 1.31 \times 10 ^ { -2 } D _ { \mathrm { MW } } , where D _ { \mathrm { MW } } is its value for the local ISM ) is on the same order with a critical value D _ { \mathrm { cr } } \sim 10 ^ { -2 } D _ { \mathrm { MW } } .
We show that the lack of \mathrm { H _ { 2 } } is mainly led by \mathrm { H ^ { - } } destruction when gas-phase \mathrm { H _ { 2 } } formation dominates since \mathrm { H ^ { - } } is important intermediary of gas-phase \mathrm { H _ { 2 } } formation . \mathrm { H ^ { - } } is destroyed by the radiation from all stars born in the previous starburst phase because \mathrm { 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 \mathrm { H _ { 2 } } leads the quiescent phase of star formation .
Hence , we can say that the stellar radiation which destroys \mathrm { H ^ { - } } and leads low \mathrm { H _ { 2 } } abundance should be properly treated in studying SFHs of dIrrs .