The Hubble Space Telescope Key Project on the Extragalactic Distance Scale

Postcript version of this chapter

Goals of the HST H₀ Key Project

Cepheid distances lie at the heart of the HST Key Project on the Extragalactic Distance Scale (Freedman et al. 1994a,b; Kennicutt, Freedman & Mould 1995). The Key Project has been designed to use Cepheid variables to determine 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 can be accurately calibrated in zero point, and then intercompared on an absolute basis. The Cepheid distances can then be used for secondary calibrations and applied to independent galaxy samples at cosmologically significant distances. Cepheid distances to the Virgo and Fornax clusters provide a consistency check of the secondary calibrations. The aim is to derive a value for the expansion rate of the Universe, the Hubble constant, to an accuracy of 10%.

Briefly, there are three 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 both in the Cepheid distance scale and the secondary methods. We briefly review the progress to date of the Key Project in the next section, before moving on to a discussion of our most recent results.

Summary of Recent Results

All three aspects of the Key Project are now well underway. Most of the galaxies in our sample were chosen to provide distances critical to the calibration of secondary distance methods. In the case of the face-on spiral galaxy M101, we have measured samples of Cepheids at two different radial positions in the disk, allowing us to begin undertaking a test of the level of sensitivity of the Cepheid period-luminosity relation to chemical composition.

Data have been acquired for several galaxies: M81, M101 (an inner and outer field), the Virgo cluster galaxies M100, NGC 4548 and NGC 4535; seven inclined spiral galaxies: NGC 925 (a member of the NGC 1023 group), NGC 7331, NGC 3621, NGC 2541, NGC 2090, NGC 3351 (a member of the Leo I Group), NGC 4414 (a host galaxy for a type Ia supernova); as well as NGC 1365, a barred spiral galaxy located in the southern hemisphere cluster, Fornax. Two additional Fornax galaxies will be observed in the upcoming months: NGC 1326A and NGC 1425, in addition to the field spiral NGC 3319. To date the H₀ key project results have been published for 6 galaxies in the following papers: M81 (Freedman et al. 1994b), M100 (Freedman et al. 1994a; Ferrarese et al. 1996); M101 (Kelson et al. 1996), NGC 925 (Silbermann et al. 1996), NGC 3621 (Rawson et al. 1996), and NGC 3351 (Graham et al. 1997).

In Freedman, Kennicutt & Madore (1997), Mould et al. (1997) (the proceedings of the STSCI conference on the Extragalactic Distance Scale, CUP, in press), and Madore et al. (1997), Nature, we have presented a status report of our results to date which yield a value of H₀ = 73 ± 6 (statistical) ± 8 (systematic) km/sec/Mpc. The systematic error takes into account a number of factors including: the present uncertainty in the zero point of the Cepheid period-luminosity relation of ±5% (or equivalently the uncertainty in the distance to the LMC), the potential uncertainty due to metallicity, also in the Cepheid period-luminosity relation at a level of ±5%, an uncertainty which allows for the possibility that the locally measured H₀ out to say, 10,000 km/sec may not be the global value of H₀ of ±7%, plus an allowance for a scale error in the photometry that could afect all of the results of ±3%. Our current adopted value for H₀ is 73 ± 10 km/sec/Mpc. At the present time, the total uncertainties amount to about ±15%. This result is based on a variety of methods, including a Cepheid calibration of the Tully-Fisher relation, type Ia supernovae, a calibration of distant clusters tied to Fornax, and direct Cepheid distances out to ~ 20 Mpc. We summarize below the values of H₀ based on these various methods.