Extreme Starbursts and Molecular Clouds in Galaxies

The extraordinary starbursts found in ultraluminous IR galaxies occur in molecular gas concentrated in compact very massive clouds which we call Extreme Starbursts. They have one thousand times the mass but are only a few times larger than GMCs. High-mass star formation in sufficiently dense and massive structures does not disrupt further star formation; it is a runaway process. Star formation remains embedded in the molecular gas and there is little or virtually no optical-UV radiation. In the early universe extreme starbursts may be more frequent and they may be the mode of star formation in high redshift submillimeter sources.