BREAKOUT - THE ORIGIN OF FAINT EXTENDED BROAD EMISSION-LINES ASSOCIATED WITH GIANT EXTRAGALACTIC H II REGIONS

The faint extended broad (greater than or equal to 1000 km s(-1)) opti cal emission lines associated with giant H II regions are shown here t o be produced in a shell of ISM material smoothly accelerated soon aft er breakout. Two-dimensional calculations of remnants caused by a stro ng energy deposit in a low metal abundance ISM are here shown to under go breakout once encountering a steep density gradient, leading to a f ast-moving shell capable of producing the broad and faint emission lin es. Energetic sources lead to fast, thick, and hot shells, and when ev olving in a low-metallicity ISM, to quasi-adiabatic shells that strong ly delay their fragmentation owing to Rayleigh-Taylor instabilities. A t the same time, these are smoothly accelerated to reach large distanc es from the breakout point. The shell acceleration is promoted by the passage of several shocks with small relative speeds, caused by the co ntinuous push exerted by the hot gas that steadily increases its speed to fill the deformed superbubble volume.