The star formation history of NGC 6822

Images of five fields in the Local Group dwarf irregular galaxy NGC 6822 obtained with the Hubble Space Telescope in the F555W and F814W filters are presented. Three of the fields lie mostly within the central bar of NGC 6822, where most of the sites of current star formation are concentrated, while two other fields sample more of the outer regions of the galaxy. Photometry for the stars in these images was extracted using the point-spread function-fitting program HSTPHOT/MULTIPHOT. The resulting color-magnitude diagrams reach down to V approximate to 26, a level well below the red clump, and were used to solve quantitatively for the star formation history of NGC 6822. Assuming that stars began forming in this galaxy from low-metallicity gas and that there is little variation in the metallicity at each age, the distribution of stars along the red giant branch is best fitted with star formation beginning in NGC 6822 12-15 Gyr ago. The best-fitting star formation histories for the old and intermediate-age stars are similar among the five fields and show a constant or somewhat increasing star formation rate from 15 Gyr ago to the present except for a possible dip in the star formation rate from 3 to 5 Gyr ago. The main differences among the five fields are in the higher overall star formation rate per area in the bar fields, as well as in the ratio of the recent star formation rate to the average past rate. These variations in the recent star formation rate imply that stars formed within the past 0.6 Gyr are not spatially very well mixed throughout the galaxy. The star formation histories in conjunction with a galaxy evolution code are used to infer the evolution of the integrated absolute magnitude of NGC 6822 as a function of look-back time and redshift. The results indicate that galaxies with star formation histories similar to that determined for NGC 6822 would have an absolute magnitude of M-B = -1.57 at a redshift z = 0.6, well below the detection limit of the current redshift surveys, and therefore are most likely not contributing to the evolving galaxy population detected in deep redshift surveys.