The main aims of the Key Project were (Aaronson & Mould 1986; Freedman et al. 1994a; Kennicutt, Freedman & Mould 1995): (1) To use the high resolving power of HST to discover Cepheids in, and determine distances to, a sample of nearby ( 20 Mpc) galaxies, and establish an accurate local distance scale. (2) To determine H0 by applying the Cepheid calibration to several secondary distance indicators operating further out in the Hubble flow. (3) To intercompare the Cepheid and other distances to provide estimates of the external uncertainties for all of the methods. (4) To conduct tests of the universality of the Cepheid period-luminosity relation, in particular as a function of metal abundance. Finally, an ancillary aim was to measure Cepheid distances to a small number of galaxies in each of the two nearest clusters (Virgo and Fornax) as an independent check on other Hubble constant determinations.
Why was HST necessary for an accurate determination of H0 ? Atmospheric seeing sets the practical limit for resolving Cepheids and measuring well-defined period-luminosity relations to only a few megaparsecs. The superb and essentially non-varying image quality of HST extends that limit tenfold, and the effective search volume a thousandfold. Furthermore, HST offers a unique capability in that it can be scheduled optimally to facilitate the discovery of Cepheid variables. Observations can be scheduled independently of the phase of the Moon, the time of day, or weather, and there are no seeing variations. Before the launch of HST, most Cepheid searches were confined to our own Local Group of galaxies, and the very nearest surrounding groups (M101, Sculptor and M81 groups; see Madore & Freedman 1991; Jacoby et al. 1992). At that time, only 5 galaxies with well-measured Cepheid distances provided the absolute calibration of the Tully-Fisher relation (Freedman 1990) and a single Cepheid distance, that for M31, provided the calibration for the surface-brightness fluctuation method (Tonry 1991). Moreover, before HST no Cepheid calibrators were available for Type Ia supernovae (although one historical, nearby type Ia supernova, SN1885A, had been observed in M31).
2.2. Choice of Target Galaxies / Observing Strategy
In each nearby target spiral galaxy in the Key Project sample, Cepheid searches were undertaken in regions active in star formation, but low in apparent dust extinction, based on ground-based, photographic images (e.g., Sandage & Bedke 1988). To the largest extent possible, we avoided high-surface-brightness regions in order to minimize source confusion or crowding. For each galaxy, over a two-month time interval, HST images in the visual (V-band, 5550 Å), and in the near-infrared (I band, 8140 Å), were made using the corrected Wide Field and Planetary Camera 2 (WFPC2). Among the galaxies on the Key Project observing program, only M81 and an outer field in M101 were observed with the original Wide Field / Planetary camera (WF/PC), before the first HST servicing mission that restored the telescope capabilities. Two of the Type Ia supernova calibrators investigated by the Sandage, Tammann et al. team and rediscussed here were also observed with WF/PC: IC 4182 and NGC 5253. The field of view of the WFC2 is L-shaped with each of the 3 cameras covering 1.33 arcmin by 1.33 arcmin on the sky, and the PC 35 arcsec by 35 arcsec.
For the observations, two wavelength bands were chosen to enable corrections for dust extinction, following the precepts of Freedman (1988) and Madore & Freedman (1991). Initially, during the observing window, 12 epochs at V (F555W), and 4 observations at I (F814W), were obtained. For some of the galaxies observed early in the program, some B (F439W) data were also obtained. For the targets observed later in the program, observations were obtained at both V and I at each of the 12 epochs. An additional observation was generally made either one year earlier or later, to increase the time baseline and reduce aliasing errors, particularly for the longer-period stars. The time distribution of the observations was set to follow a power-law, enabling the detection and measurement of Cepheids with a range of periods optimized for minimum aliasing between 10 and 50 days (Freedman et al. 1994b).
Since each individual secondary method is likely to be affected by its own (independent) systematic uncertainties, to reach a final overall uncertainty of ± 10%, the numbers of calibrating galaxies for a given method were chosen initially so that the final (statistical) uncertainty on the zero point for that method would be only ~ 5%. (In practice, however, some methods end up having higher weight than other methods, owing to their smaller intrinsic scatter, as well as how far out into the Hubble flow they can be applied - see Section 7). In Table 1, each method is listed with its mean dispersion, the numbers of Cepheid calibrators pre- and post-HST, and the standard error of the mean. (We note that the fundamental plane for elliptical galaxies cannot be calibrated directly by Cepheids; this method was not included in our original proposal, and it has the largest uncertainties. As described in Section 6.3, it is calibrated by the Cepheid distances to 3 nearby groups and clusters.) The calibration of Type Ia supernovae was part of the original Key Project proposal, but time for this aspect of the program was awarded to a team led by Allan Sandage.
For the Key Project, Cepheid distances were obtained for 17 galaxies chosen to provide a calibration for secondary methods, and a determination of H0. These galaxies lie at distances between 3 and 25 Mpc. They are located in the general field, in small groups (for example, the M81 and the Leo I groups at 3 and 10 Mpc, respectively), and in major clusters (Virgo and Fornax). An additional target, the nearby spiral galaxy, M101, was chosen to enable a test of the effects of metallicity on the Cepheid period-luminosity relation. HST has also been used to measure Cepheid distances to 6 galaxies, targeted specifically to be useful for the calibration of Type Ia supernovae (e.g., Sandage et al. 1996). Finally, an HST distance to a single galaxy in the Leo I group, NGC 3368, was measured by Tanvir and collaborators (Tanvir et al. 1995, 1999). Subsequently and fortuitously, NGC 3368 was host to a Type Ia supernova, useful for calibrating H0 (Jha et al. 1999; Suntzeff et al. 1999). (17)
We list the galaxies which we have used in the calibration of H0 in Table 2, along with the methods that they calibrate. To summarize the total Cepheid calibration sample, as part of the Key Project, we have surveyed and analyzed data for 18 galaxies, in addition to reanalyzing HST archival data for 8 galaxies observed by other groups. When these distances are combined with those for 5 very nearby galaxies (M31, M33, IC 1613, NGC 300, and NGC 2403), it results in a total 31 galaxies, subsets of which calibrate individual secondary methods, as shown in Table 2.
2.3. Key Project Archival Database
As part of our original time allocation request for the Key Project, we proposed to provide all of our data in an archive that would be accessible for the general astronomical community. We envisaged that the Cepheid distances obtained as part of the Key Project would provide a database useful for the calibration of many secondary methods, including those that might be developed in the future. For each galaxy observed as part of the Key Project, the Cepheid positions, magnitudes, and periods are available at http://www.ipac.caltech.edu/H0kp/H0KeyProj.html. In addition, photometry for non-variable stars that can be used for photometry comparisons, as well as medianed (non-photometric) images for these galaxies are also available. These images are also archived in NED, and can be accessed on a galaxy-by-galaxy basis from http://nedwww.ipac.caltech.edu.
As a means of guarding against systematic errors specifically in the data reduction phase, each galaxy within the Key Project was analyzed by two independent groups within the team, each using different software packages: DoPHOT (Schechter et al. 1993; Saha et al. 1994), and ALLFRAME (Stetson 1994, 1996). The latter software was developed specifically for the optimal analysis of data sets like those of the Key Project, consisting of large numbers of observations of a single target field. Only at the end of the data reduction process (including the Cepheid selection and distance determinations) were the two groups' results intercompared. This "double-blind" procedure proved extremely valuable. First, it allowed us to catch simple (operator) errors. And, it also enabled us to provide a more realistic estimate of the external data reduction errors for each galaxy distance. The limit to the accuracy of the photometry that can be obtained in these galaxy fields is set by the sky (i.e., unresolved galaxy) background in the frames, and ultimately, the difficulty in determining aperture corrections. Each of the two packages deals with sky determination and aperture corrections in different ways, thereby providing a means of evaluating this systematic uncertainty in the Cepheid photometry. As discussed in Section 8.5, we also undertook a series of artificial star tests to better quantify the effects of crowding, and to understand the limits in each of these software packages (Ferrarese et al., 2000c).
The determination of accurate distances carries with it a requirement for an accurate, absolute photometric calibration. Ultimately, the uncertainty in the Hubble constant from this effort rests directly on the accuracy of the Cepheid magnitudes themselves, and hence, systematically on the CCD zero-point calibration. In view of the importance of this issue for the Key Project, we undertook our own program to provide an independent calibration of both the WF/PC and WFPC2 zero points, complementary to the efforts of the teams who built these instruments, and the Space Telescope Science Institute. These calibrations have been described in Freedman et al. (1994b) and Kelson et al. (1995) for WF/PC and Hill et al. (1998), Stetson (1998), and Mould et al. (2000a) for WFPC2.
As part of an HST program to study Galactic globular clusters, but also extremely valuable for the photometric calibration of WFPC2, hundreds of images of Cen, NGC 2419, and M92 have been obtained both on the ground and with HST over the last several years (Stetson 1998; Mould et al. 2000a). Despite this extensive effort, the calibration of WFPC2 remains a significant source of systematic uncertainty in the determination of H0. This lingering uncertainty results from the difficulty in characterizing the charge transfer efficiency (CTE) properties of the WFPC2, which turn out to be a complicated function of position on the chip, the brightness of the object, the brightness of the sky, and the wavelength of the observations (presumably because of the differing background levels; Stetson 1998; Whitmore, Heyer & Casertano 1999; Saha et al. 2000; Dolphin 2000).
Recent WFPC2 calibrations (Stetson 1998; Dolphin 2000) differ from our earlier calibration based on Hill et al. (1998). Based on the reference star photometry published in papers IV to XXI in the Key Project series, Mould et al. (2000a) found that the reddening-corrected distance moduli on the Stetson (1998) system were 0.07 ± 0.02 mag closer, in the mean, than those published based on the Hill et al. (1998) system. This difference in the reddening-corrected distance moduli results from a 0.02 mag mean offset in the V-band, and a 0.04 mag mean offset in the I-band. The more recent calibrations are based on a more extensive calibration data set than that available in the Hill et al. or the Saha et al. analyses, and they result in galaxy distance moduli that are closer. The main reason for this difference is that the earlier Hill et al. "long" versus "short" zero points determined for globular clusters (bright stars on faint sky) turned out to be inappropriate for the Cepheid fields (faint stars on bright sky) because the combinations of flux dependence and background dependence were different in the two situations. Stetson (private communication) indicates that a 0.02-0.03 mag uncertainty remains due to this effect. The Stetson CTE correction is in agreement with Dolphin (2000) and Whitmore et al. (1999): the Stetson zero point results in reddening-corrected distance moduli that agree within 1.5% (0.03 mag) of the new calibration by Dolphin (2000). Although Stetson did not find a significant time dependence as seen in the more recent studies, in all studies, the temporal variation of the CTE ramps are found to be negligible for the high background long exposures for the Key Project.
In this paper, we have adopted the WFPC2 calibration due to Stetson (1998), and applied a -0.07 ± 0.04 mag correction to the reddening-corrected distance moduli. The uncertainty reflects the remaining differences in the published WFPC2 calibrations, and their impact on the distance moduli, when corrected for reddening (Equations 3,4). As we shall see later in Section 8, the uncertainty due to the WFPC2 photometric zero point remains a significant systematic error affecting the measurement of H0. Unfortunately, until linear, well-calibrated detectors can be applied to the Key Project reference stars, this uncertainty is unlikely to be eliminated.
17 In addition, recently, SN1999by occurred in NGC 2841, a galaxy for which Cepheid observations have been taken in Cycle 9 (GO-8322). Back.