Astron. Astrophys. 322, 730-746 (1997)

4. Primary calibrator galaxies

In order to determine the absolute zero-points for the TF relations, and consequently, the absolute distance scale and the value of , one needs a sample of calibrators. Furthermore, this sample should be statistically equivalent to a volume-limited sample, providing unbiased or . The galaxies which can be used to build such a sample come from rather heterogeneous observational programmes, with different distance indicators applied, and not always collected with the requirement of volume-limitedness in mind.

4.1. Galaxies with Cepheid distances

The number of galaxies with Cepheid distances is increasing rapidly. The on-going Hubble Space Telescope programmes (cf. Kennicutt et al. 1995, and Sandage et al. 1996) aim at measuring Cepheids in 25 galaxies. 18 of these galaxies have been chosen as optimal calibrators for the IR Tully-Fisher relation. Together with Cepheid observations with advanced ground-based telescopes (e.g. Pierce et al. 1994) we now have a larger sample of calibrator galaxies with more accurate distances than in BGPT86. The range of distances has increased considerably; the first Cepheids in Virgo galaxies have been successfully detected.

We have collected all the galaxies with Cepheid distances and excluded face-on ones, and those being very peculiar or outside the type range 1 - 8.

Though we excluded the low inclination galaxies N4571 and N5457 (=M101), distance to M101 was used for its close companions (cf. Table 2). The very peculiar "Sombrero" galaxy (M104) was excluded together with several galaxies of irregular types 9-10 (I10, I4182, I1613, N6822, Sex A, Sex B).

We are left with 15 galaxies for our primary calibrator sample. In Table 1, the Cepheid distances, corrected apparent diameters and B -magnitudes, rotational velocities, types, and radial velocities (), of these galaxies are listed. Sources for distances are given, other values are from our KLUN-sample or from LEDA.

Table 1. Primary calibrator sample; galaxies with Cepheid distances. The values in columns 3-7 are taken from KLUN-sample or from LEDA database. Sources for the distance moduli are listed in column 8.

In Table 2 we list possible members of groups with distances known from primary calibrators. For these we attach the mean group distance, except in the case of Sculptor group, where the distance differences are defined from resolution into brightest stars (Tammann, 1987). The 15 galaxies in Table 2 form our secondary calibrator sample. Combining these with the primary sample we enlarge the number of calibrators, which increases statistical reliability even though the distances in Table 2 are obviously not as accurate as in the primary sample. All the types 1-8 are represented in the combined sample.

Table 2. Secondary calibrator sample; galaxies with group distances. Distances to Sculptor Group galaxies are determined from the Cepheid distance to group member N300 and from resolution arguments (Tammann 1987). M81 Group distance is defined as intermediate between two group members N3031 (=M81) and N2403. Cepheid distance of to N5457 (=M101) has been recently measured with HST (Kelson et al. 1996), but being a face-on galaxy, it was not included in our primary calibrators. Leo Group distance is adopted from the member galaxy N3368.

For the convenience of the reader we list in Table 3 the rest of the Hubble Space Telescope Key Project galaxies. As the project continues, these galaxies may easily be added to the previous calibrator list. In the Appendix, we derive an algorithm for re-evaluating the value of with the more complete calibrator sample.

Table 3. A list of Hubble Space Telescope extragalactic distance scale project galaxies. As the project continues, the reader may fill the gaps and add these galaxies in the primary calibrators. Following methods explained in the text, the TF relations may then be re-calibrated and value of re-evaluated.

4.2. BGPT86 calibrators

The calibrators used in BGPT86 had distances from several different methods - Cepheids, brightest stars, H II regions, luminosity indices, novae, etc. In the present work we use only Cepheid distances, which are now sufficiently numerous and rest on a firm physical and observational basis.

Nine of the BGPT86 calibrators with new distances are found in Tables 1 and 2. Only the Magellanic Clouds, being of irregular type, have been dropped out. Ten years ago we used three different sets of distances: de Vaucouleurs scale, and "old" and "new" Sandage and Tammann scales (Table 4 in BGPT86). On average the distance moduli of the deV-scale were smaller by , "old" and "new" ST-scales were larger than the present values, and -0.06 respectively.

In terms of the Hubble constant, deV-scale gives that is larger, "old" and "new" ST-scales give values of that are and smaller than when the present distances are used.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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