HIGH-RESOLUTION IMAGING OF VIRGO CLUSTER GALAXIES .1. THE DISTANCE BASED ON THE BRIGHTEST STARS IN NGC-4571

Deep, high-resolution images (FWHM = 0".43) of a relatively face-on, late-type spiral galaxy (NGC 4571) in the Virgo Cluster have been obtained using the High-Resolution Camera (HRCam) and the Canada-France-Hawaii Telescope. The galaxy is apparently resolved into stars. The combined image has a limiting magnitude of m(R) approximately 26, enabling the brightest three magnitudes of the luminosity function to be examined, where the photometry is complete. Point-spread function fitting photometric techniques (i.e., DAOPHOT) are applied to the data allowing the application of quantitative and objective criteria for distinguishing stars from H II regions and compact clusters/associations. Approximately 50% of the brightest sources appear stellar (delta-pi less-than-or-equal-to 0".04), a fraction in reasonable agreement with that found elsewhere from spectroscopic studies of the brightest "stars" in nearby galaxies. These unresolved images are interpreted as the brightest supergiants in NGC 4571. The apparent magnitudes of the brightest blue and red supergiants are used to estimate the distance to the Virgo Cluster, assuming both classes of stars to be "standard candles." Multicolor, low-resolution data (FWHM approximately 1".4) are used to distinguish between the red and blue supergiants. We obtain a distance modulus for NGC 4571 of m - M = 30.9 +/- 0.2 mag, or 14.9 +/- 1.5 Mpc with both techniques. This estimate is in excellent agreement with results obtained from the majority of other modern techniques which imply a "short" distance for the Virgo Cluster, and hence large values for H0 (approximately 85 km s-1 Mpc-1). With this distance estimate for the Virgo Cluster we obtain an absolute calibration for Type la supernovae given the apparent magnitudes for those events in E/S0 galaxies in the cluster. We find M(B) = - 19.0 +/- 0.2 where the uncertainty is dominated by that in our distance estimate. The prospects for the detection of Cepheid and long-period variables in the Virgo Cluster using ground-based high-resolution imaging techniques are briefly discussed.