Resolved stars in external galaxies

Abstract

In this thesis we apply techniques based on the resolution and photometry of stars to estimate the distances to external galaxies. Two critical rungs of the distance ladder, the local group galaxies M31 and M33, and the Virgo Cluster are selected for study. The existing Cepheid distances to local group galaxies, which are important calibrators of secondary indicators, are still controversial, largely due to uncertainty in the correction for extinction due to dust. We apply modern techniques of crowded field photometry to CCD images in order to find new estimates of the reddening, and hence extinction, and also to recalibrate previous photometry. In the process, we consider several means of enhancing and evaluating the results from these methods. In M31 we present BVR photometry for ~ 2300 stars to a limit of V ~ 23.(^m)5. The majority of stars are red-giants from an old disk population, but there is also a clear main-sequence. After accounting for age and metallicity effects, we use the colour of the main-sequence to deduce a value for the reddening of E(_B)-v - 0.09 ± 0.05, where the error is largely from the uncertainties in the photometric zero-points. Correcting a scale error in the original photographic Cepheid photometry, and accounting for the low metallicity thought to apply to this field, we find a distance modulus of (_µ0) = 24.(^m)44±0.(^m)16. For M33 we find B and V photometry for ~ 4500 stars to ~ 22.(^m)5. In this case the field is characterised by a very young population of bright blue stars. By plotting colour magnitude diagrams for different portions of the field, we demonstrate that the reddening is a function of position. Applying the correct extinction to each Cepheid individually we get a distance modulus of (_µ0) = 24.(^m)5±0.(^m)25. We use the new technique of ground-based image-sharpening to study galaxies in the Virgo Cluster, which is, arguably, the point at which the main disagreement in the extragalactic distance scale hinges. The technique is assessed with regard to its potential for astronomical observations, and several developments described. The best results obtained during bright time tests achieved ~ 0.3 FWHM seeing. Moderately deep images of two galaxies have been obtained, both with 0.6 seeing. In the first, IC3583, we do not find any convincing candidates for brightest supergiants, and conclude a lower limit for the distance of = 30.(^m)7. Observations of the second galaxy, NGC4523, go deeper and are supplemented by colour and narrow-band information. Three good candidates are found for yellow supergiants, and we estimate a distance of (_µ0) = 30.(^m)6. The implications of such a low distance to Virgo, and hence high value of the Hubble constant, are discussed.