The Cepheid distance scale provides the zero-point calibration for most of the relative distance indicators in use today: the Tully-Fisher relation for spiral galaxies, the surface-brightness fluctuation method for elliptical galaxies, the planetary nebula luminosity function, Types I and II supernovae, brightest stars in galaxies, and the globular cluster luminosity function (for a recent overview see Jacoby et al. 1992). The Cepheid distance scale also lies at the heart of the HST Key Project on the Extragalactic Distance Scale (Freedman 1994a,b,c; Kennicutt et al. 1995) and in several other HST distance scale programs (e.g., Sandage et al. 1994; Saha et al. 1994, 1995; and Tanvir et al. 1995).
The Key Project on the Extragalactic Distance Scale has been designed to use Cepheid variables to determine (Population I) primary distances to a representative sample of galaxies in the field, in small groups, and in major clusters. The galaxies were chosen so that each of the secondary distance indicators with measured high internal precisions could be accurately calibrated in zero point and intercompared on an absolute basis. These data will then be used as secondary calibrations and applied to independent galaxy samples at cosmologically significant distances. Cepheid distances to the Virgo and Fornax clusters provide an alternative route to the secondary calibrations. The intention is to derive a value for the expansion rate of the Universe, the Hubble constant, to an accuracy of 10% (for additional details see Kennicutt et al. 1995, Freedman, Madore & Kennicutt 1997).
A measurement of the Hubble constant to 10% provides an immense challenge given the history of systematic errors in the extragalactic distance scale. For this reason, the Key Project has been designed to allow many independent cross-checks of both the primary and secondary distance scales. The goals of the Key Project are described in more detail in Kennicutt et al. (1995) and Freedman et al. (1994a). Briefly, there are four primary goals: (1) To discover Cepheids, and thereby measure accurate distances to spiral galaxies located in the field and in small groups that are suitable for the calibration of several independent secondary methods. (2) To make direct Cepheid measurements of distances to three spiral galaxies in each of the Virgo and Fornax clusters. (3) To provide a check on potential systematic errors in the Cepheid distance scale via independent distance estimates to the nearby galaxies, M31, M33 and the Large Magellanic Cloud (LMC). And (4) to undertake an empirical test of the sensitivity of the zero point of the Cepheid PL relation to metallicity as described previously.
Prior to the 1994 repair mission, the Key Project team was still able to undertake a search for new Cepheids in the nearby galaxy M81. These observations were undertaken to provide a test of the new discovery algorithms (Madore & Freedman 1998) which were designed to optimally detect Cepheids with a range of (unknown) periods, using a minimum of spacecraft time, and further restricted by the small (60-day) observing windows available only once in any given year. 30 Cepheids were discovered in two fields searched in M81 and a reddening-corrected distance modulus of 27.80 ± 0.20 mag was derived (Freedman et al. 1994b). Previous ground-based attempts to discover variables in this galaxy yielded only two confirmed Cepheids, one of which was intentionally targeted by the Key Project as a test of the search procedure. This Cepheid was recovered and confirmed to have a period in agreement with the more extensive ground-based determination derived from decades worth of data.
Immediately following the December 1993 repair mission, BVR images of the Virgo spiral galaxy M100 were obtained as part of a collaboration between the WFPC2 IDT and the H0 Key Project teams. ALLFRAME photometry was obtained for over 30,000 stars. By overlaying the position of the mean Cepheid instability strip on the resulting color-magnitude diagrams, it was possible to demonstrate that stars were present with the magnitudes and colors expected for Cepheid variables at the distance of the Virgo cluster. Given this success, a sequence of 12 V and 4 I exposures was begun in April 1994. Twenty high signal-to-noise Cepheid variables were found, from which a reddening corrected distance of 17.1 ± 1.8 Mpc (or a distance modulus of 31.16 ± 0.20 mag) was determined to M100 (Freedman et al. 1994a). Allowing for the uncertainty in the position of M100 with respect to the Virgo cluster core, in addition to the uncertainty in the Virgo cluster recession velocity, a preliminary value of the Hubble constant of H0 = 80 ± 17 km/sec/Mpc was determined. A discussion of the random and systematic errors is found in Freedman et al. (1994a). Recently, a new determination of the distance to M100 has been made based on a larger sample of over 50 Cepheids and an improved calibration (Ferrarese et al. 1996). A value of 15.8 ± 1.5 Mpc is obtained, in good agreement, to within the measurement uncertainties, with the earlier value.
Other galaxies currently being analyzed as part of the HST Key Project include NGC 925 (Silbermann et al. 1996) in the NGC 1023 Group; NGC 3351 in the Leo I group (Graham et al. 1997); two fields in M101, discussed above in the context of extending the metallicity test (Kelson et al. 1996; Stetson et al. 1998, in preparation; Kennicutt et al. 1998, in preparation); NGC 7331 (Hughes et al. 1998, in preparation), a Tully-Fisher calibrator in the field; NGC 4414 (Turner et al. 1998, in preparation), a distant and fairly inclined early-type spiral useful for calibrating a number of secondary methods including type Ia supernovae. Recently, NGC 1365, a galaxy in the Fornax cluster (Silbermann et al. 1998; Madore et al. 1998, in preparation) has been observed for Cepheids, and the results of that survey are discussed in the closing sections of this lecture series. Other galaxies completed at the time of writing include NGC 3621 (Rawson et al. 1998) and NGC 2090 (Phelps et al. 1998).