Abstract
?????We have used the Hubble Space Telescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands: F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modified version of DoPHOT. Artificial star tests showed the data to be 50% complete to V = 26.3, B = 25.4, and I = 25.2. The color-magnitude diagrams contain well-defined populations, including a very young main sequence (MS) (<10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of ? = 26.75 ? 0.35 (2.2 Mpc), which we adopted. An extended stellar halo was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 M? Myr-1 kpc-2 with up to 60% variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100?200 pc), which lasted ~100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies: Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger MH I/LB. Also, the star formation pattern is similar in all four galaxies. Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.
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