J/AJ/109/173 J/AJ/109/173 BVR photometry of NGC 6822 G Marconi M Tosi L Greggio P Focardi Astron. J. 109 173 1995 1995AJ....109..173M J/AJ/105/894 : Star formation in DDO 210 and NGC 3109 (Greggio+ 1993) Galaxies, photometry Photometry, CCD Our proposed method to study the star-formation histories in nearby irregulars is here applied to NGC 6822. To this purpose we have obtained accurate CCD photometry of three regions in the galaxy, reaching V=23.5 with the required accuracy of {sigma}_BVR_<=0.1mag. Major information on the stellar populations, star-formation rates, and initial mass functions of these regions in the last 1Gyr are derived from the comparison of the observational color-magnitude diagrams and luminosity functions with the corresponding synthetic diagrams and luminosity functions generated by a numerical simulation code based on stellar evolutionary tracks. We find that in the last 1Gyr the star-formation activity has been rather continuous, possibly occurring in long episodes of moderate activity separated by short quiescent periods, and that the initial mass function exponent a is in the vicinity of Salpeter's value, i.e., slightly flatter than in the solar neighborhood. The studied regions of NGC 6822 appear to contain different stellar populations. The preferred star formation regime for region A is a continuous activity coupled with a moderately flat {alpha}. For region C, instead, our best models suggest a discontinuous star formation and a moderately steep {alpha}. NGC 6822 is the last object of our original sample of five nearby irregulars. We can then draw the scenario resulting from this sample: most of the observed fields appear to have experienced a gasping regime of star formation, with average exponent {alpha}=2.35 for the initial mass function. Despite their small sizes, the majority of the sample galaxies contain different stellar populations in different regions.

NGC 682219 44 56.0-14 48 06

Region NGC 6822 region --- Star Star number in the region --- Xpos X position number=1 The field are centered at: RA(1950)=19h42m07s, DE(1950)=-14{deg}58'24" for field A RA(1950)=19h42m15s, DE(1950)=-14{deg}55'54" for field B RA(1950)=19h42m07s, DE(1950)=-14{deg}53'23" for field C North is down, East is right, 1 pix = 0.17" pix Ypos Y position number=1 The field are centered at: RA(1950)=19h42m07s, DE(1950)=-14{deg}58'24" for field A RA(1950)=19h42m15s, DE(1950)=-14{deg}55'54" for field B RA(1950)=19h42m07s, DE(1950)=-14{deg}53'23" for field C North is down, East is right, 1 pix = 0.17" pix Vmag V magnitude mag B-V B-V colour index mag V-R V-R colour index mag James Marcout, Patricia Bauer CDS 1997Sep16 UNKNOWN UNKNOWN Prepared via OCR at CDS. J_AJ_109_173.xml