ARE GALAXIES OPTICALLY THIN TO THEIR OWN LYMAN-CONTINUUM RADIATION .1. M33

Previously published H alpha data and UBV photometry of blue stars in the inner kiloparsec of M33 are used to study the distribution of OB s tars and H II regions in the galaxy and to determine whether individua l regions of the galaxy are separately and/or collectively in a state of ionization balance. Based on the surface brightness of the H alpha emission, we identify three distinct ionized gas environments (bright, halo, and field). We find that similar to 50% of the OB stars are loc ated in the field, so that one-half of the lifetime of OB stars must b e spent outside recognizable H II regions. If OB stars escape from bri ght H II regions by destroying their parent molecular clouds, this res ult would imply that molecular cloud lifetimes after forming OB stars could be as low as similar to 5 x 10(6) yr or one-half the typical lif etime of OB stars. We show that a possible origin for the large field OB population is that they were born in and subsequently percolated ou t of the similar to 10(3) giant molecular clouds with masses greater t han or similar to 10(3) M. predicted to exist within the inner kilopar sec of the galaxy. Using ionization models, we predict H alpha fluxes in the bright, halo, and field regions and compare them to those obser ved to find that the regions, separately as well as collectively, are not in ionization balance: predicted fluxes are a factor of similar to 3-7 greater than observed. The heaviest loss of ionizing photons appe ars to be taking place in the field. Observed and predicted H alpha lu minosities in the field are in best agreement when case A recombinatio n is assumed. Therefore, our findings suggest that star formation rate s obtained from Her luminosities must underestimate the true star form ation rate in these regions of M33. We have performed a similar analys is of an individual, isolated region with bright and halo H alpha emis sion to find that comparable results apply and that the region, as a w hole, is also not in ionization balance.