Uncertainty in the metal abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) to observe Cepheids in two fields in the nearby spiral galaxy M101, which span a range in oxygen abundance of 0.7 ± 0.15 dex. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ(m − M) 0 /δ[O/H] = −0.24 ± 0.16 mag dex −1 . The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisions of Cepheid distances with those derived from three other distance indicators, in particular the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.

This paper presents the calibration ofTully-Fisher relations based on Cepheid distances BV RIH~0 .5 to 21 galaxies within 25 Mpc and 23 clusters within 10,000 km s~1. These relations have been applied to several distant cluster surveys in order to derive a value for the Hubble constant, mainly concentrat-H 0 , ing on an I-band all-sky survey by Giovanelli and collaborators, consisting of total I magnitudes and 50% line width data for D550 galaxies in 16 clusters. For comparison, we also derive the values of H 0 using surveys in the B and V bands by Bothun and collaborators, and in H band by Aaronson and collaborators. Careful comparisons with various other databases from the literature suggest that the H-band data, which have isophotal magnitudes extrapolated from aperture magnitudes rather than total magnitudes, are subject to systematic uncertainties. Taking a weighted average of the estimates of Hubble constants from four surveys, we obtain (random)^7 (systematic). We have also H 0 \ 71^4 investigated how the value of is a †ected by various systematic uncertainties, such as the internal H 0 extinction correction method used, Tully-Fisher slopes and shapes, a possible metallicity dependence of the Cepheid period-luminosity relation, and cluster population incompleteness bias.

Deep V and I CCD images in a central and an outer field of the Local Group dwarf elliptical galaxy NGC 147 have been obtained with the Wide Field and Planetary Camera-2 (WFPC2) on board of the Hubble Space Telescope. The color-magnitude diagram shows a number of interesting features, including a well defined red giant branch (RGB), a red horizontal branch (HB), a strong red clump, and a small number of extended asymptotic giant branch (EAGB) stars. A mean distance modulus of (m-M) 0 =24.39 is derived based on both the HB and the RGB tip brightness. The metallicity [Fe/H] as determined from the RGB color has a mean value of-0.91 in the central field, and-1.0 in the outer field; and the outer field shows a weak tendency of increasing metallicity with galactocentric radius. A metallicity dispersion is also present in the galaxy, and it shows a clear radial variation in the sense that a larger dispersion is seen at smaller radii. The small population of EAGB stars indicates the presence of intermediate-aged (several Gyr) stars in the galaxy, while the absence of the main sequence stars with M v< 1 shows that star formation ceased at least 1 Gyr ago. The distribution of the EAGB stars indicates that the younger stars are more centrally concentrated than the majority of older stars. A similar age gradient is also implied by the relative distribution of the HB stars, which appear to be more populous at larger radii. These results are considered within the context of theoretical models for the evolution of dwarf elliptical galaxies.

Cl: isolatd; very noisy I light curve C2: in crowded region with brighter companion C3: isolated C4: close brighter companion, P = 14.5 days is also good C5: in crowded region with brighter companion; very noisy I light curve C6: brightest star in crowded region; close companion; 1' = 52.1 days is also good C7: nearby companion (W 0.7 arcscc) C8: isolated C9: close, failltcr companion Cl O: double (two stars found) ill crowded region C11: wry faint nearby companion Cl 2: companion stars w 0.8-1.0 arcsec away C] 3: in crowded region, 1' = 16.9 days is also goOd Cl 4: very faint nearby companions; very noisy I ligl)t curve, dots Ilot pliasc well Cl 5: brightest star in crowded region CIG: close companion; near frame edge, Lbcreforc OJIIY 16 data points Cl 7: elongated image C18: isolated; very faint nearby companions C19: brightesi star in crowded region; very noisy I light curve, does not phase WCII C20: isolated c21: fainter close companiOrl w 0.3 arcscc away; I' = 13.7 days is al~O gOOd C22: faint companion w 0.7 arcscc away C23: isolated; nearest companion w 0.8 arcscc away; very noisy 1 Iigllt curve C24: very faint close companioxl (W 0.4 arcscc)

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