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 ioniza tion balance in the Local Group galaxy, NGC 6822. Using H alpha data a nd BV photometry of the blue stars in this dIrr galaxy, we investigate the distribution of OB stars and H II regions and determine whether i ndividual areas of the galaxy are separately and/or collectively in a state of ionization balance. Four distinct components of the H alpha e mission (bright, halo, diffuse, and field) differentiated by their sur face brightnesses are identified. We find that approximately 1/2 of al l OB stars in NGC 6822 are located in the field, while only a are foun d in the combined bright and halo regions, suggesting that OB stars sp end roughly 1/4 of their lifetimes outside ''classical'' H II regions. If OB stars escape from bright H II regions by destroying their paren t 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-sequenc e lifetimes of OB stars. However, if the stars are simply escaping fro m 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 predic ted from stellar ionizing flux models and hydrogen recombination theor y, we find that although the bright, halo, and diffuse regions are pro bably in ionization balance, the field region is producing at least 6 times as much ionizing flux as is observed. The ionization balance res ults in NGC 6822 suggest that star formation rates obtained from H alp ha luminosities must underestimate the true star formation rate in thi s galaxy by about 50%. Comparing our results for NGC 6822 with previou s results for the Local Group spiral galaxy M33, we find that the inne r kiloparsec of M33 is in a more serious state of ionization imbalance , perhaps as a result of its higher surface density of blue stars. Thu s, the morphological class of a galaxy may be an important factor in h ow accurately we can determine star formation rates from H alpha lumin osities.