The dwarf irregular galaxy sextans A. I. HST photometry of the resolved stars

We have used the Hubble Space Telescope to observe the resolved stars of the nearby (D=1.4 Mpc) dwarf irregular galaxy Sextans A (DDO 75, A 1008-04). The data consist of dithered WFPC2 images in 3 bands: F439W (1 hour), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted using the DoPHOT program developed by Schechter et al. (1993, PASP, 105, 1342) with modifications for WFPC2 data by Saha et al. (1996, ApJ, 466, 550). The dithering improved the photometric accuracy of faint stars since it produced stellar profiles that were more uniform than in undithered images. Also, the PSF fitting method used by DoPHOT was insensitive to the intra-pixel sensitivity variation of the WFPC2 chips, while simple aperture photometry was slightly sensitive to this variation. The photometry was calibrated to V, B, and I with the equations presented in Holtzman et al. (1995, PASP, 107, 1065). Comparisons with previously published ground-based data showed excellent agreement with stars brighter than V ≈ 21. Fainter stars are severely hampered by crowding from the ground. At the distance of Sextans A, the superior resolution of the HST overcomes this confusion, and enables very accurate and deep photometry. Artificial star tests showed the data to be 50% complete to V=25.6, B=25.4, and I=25.3. This high quality data produced color-magnitude diagrams (CMDs) with excellent detail. The main sequence (MS), red supergiants, and red giant branch (RGB) are all clearly distinct populations. Most striking is a population just redward of the MS, which had not been photometrically resolved from the MS in ground-based observations. These stars are massive core Helium burning stars (HeB) in the bluest extent of the so-called "blue-loop" phase. This population has never before been clearly identifiable in a low metallicity system.