ARE GALAXIES OPTICALLY THIN TO THEIR OWN LYMAN-CONTINUUM RADIATION .2. NGC-6822

In this paper we study OB stars, H II regions, and the state of ionization balance in the Local Group galaxy, NGC 6822. Using H alpha data and BV photometry of the blue stars in this dIrr galaxy, we investigate the distribution of OB stars and H II regions and determine whether individual areas of the galaxy are separately and/or collectively in a state of ionization balance. Four distinct components of the H alpha emission (bright, halo, diffuse, and field) differentiated by their surface brightnesses are identified. We find that approximately 1/2 of all OB stars in NGC 6822 are located in the field, while only a are found in the combined bright and halo regions, suggesting that OB stars spend roughly 1/4 of their lifetimes outside ''classical'' H II regions. If OB stars escape from bright H II regions by destroying their parent molecular clouds, then cloud lifetimes after forming OB stars could be as low as similar to 1-3 x 10(6) yr or 1/4 the typical main-sequence lifetimes of OB stars. However, if the stars are simply escaping from the clouds without destroying them, then these data place no limits on molecular cloud lifetimes. We find that the entire field population of OB stars cannot have originated in and percolated out of existing H II regions. Comparing the observed H alpha emission with that predicted from stellar ionizing flux models and hydrogen recombination theory, we find that although the bright, halo, and diffuse regions are probably in ionization balance, the field region is producing at least 6 times as much ionizing flux as is observed. The ionization balance results in NGC 6822 suggest that star formation rates obtained from H alpha luminosities must underestimate the true star formation rate in this galaxy by about 50%. Comparing our results for NGC 6822 with previous results for the Local Group spiral galaxy M33, we find that the inner kiloparsec of M33 is in a more serious state of ionization imbalance, perhaps as a result of its higher surface density of blue stars. Thus, the morphological class of a galaxy may be an important factor in how accurately we can determine star formation rates from H alpha luminosities.