2 Selection of primary calibrators

The distance moduli of calibrators come from two independent calibrations of Cepheid Period-luminosity relations (Paturel et al. 2002a,b) based on the sample by Gieren et al. (1998) and on the HIPPARCOS Cepheid sample (Lanoix et al. 1999). The apparent magnitudes come from the LEDA database (http://www-obs.univ-lyon1.fr). They are corrected for galactic extinction and inclination effects following the precepts of Schlegel et al. (1998) and Bottinelli et al. (1995), respectively.

The Malmquist bias introduces a major difficulty in estimating distances of astronomical objects. It is caused to the fact that faint galaxies are missing in the sample because of the limiting apparent magnitude (see the review paper by Teerikorpi 1997). To reach large distances with limited bias, we have to consider only intrinsically bright galaxies. Then, because the method of sosie galaxies selects galaxies having almost the same absolute magnitude as calibrators, we have to consider the brightest calibrators. On the other hand, we need a large sample and should not reject too many calibrators. The best compromise was judged from histograms of B- and I-absolute magnitudes. We kept only calibrators satisfying either M_B < -19 or M_I < -21. One calibrator (NGC 4603) was rejected because of the large uncertainty on its distance modulus (0.86 mag). The 21 remaining calibrators are presented in Table 1 as follows:
Column 1: PGC number from LEDA.
Column 2: NGC number.
Column 3: Distance modulus and its mean error (Paturel et al. 2002a,b).
Column 4: Morphological type from LEDA.
Column 5: Adopted inclination from LEDA following Fouqué et al. (1990).
Column 6: Internal extinction in B following Bottinelli et al. (1995).
Column 7: Galactic Extinction from Schlegel et al. (1998).
Column 8: $B_{\rm o}$, corrected B-magnitude from LEDA with its actual uncertainty (Paturel et al. 1997).
Column 9: Same as Col. 8 for I-band magnitudes. The corrections are 0.44 times the B-band ones (Cols. 6 and 7). This 0.44 factor should be slightly larger for the internal extinction (Han 1992) but for the method of sosie this correction vanishes because the inclination is the same for the calibrator and its sosies.
Column 10: log of maximum rotation velocity and its actual uncertainty taken from LEDA. It is calculated as a weighted mean of $\log V_M$ from both the 21-cm line width and $H_{\alpha}$ rotation curve.

 

 

Table 1: List of calibrating galaxies. The distance moduli come from the Cepheid Period-Luminosity relation calibrated with two independent samples. The rest of the data is extracted from LEDA2002.

PGC	NGC	$\mu$	Type	i	Ai	Ag	$B_{\rm o}$	$I_{\rm o}$	$\log ~V_{\rm M}$
0002557	NGC 224	$24.47\pm0.23$	Sb	78.0	0.67	0.46	$3.18\pm0.30$	$1.47\pm0.10$	$2.39\pm0.01$
0005818	NGC 598	$24.77\pm0.39$	Sc	55.0	0.38	0.18	$5.73\pm0.12$		$2.02\pm0.02$
0013179	NGC 1365	$31.31\pm0.15$	SBb	57.7	0.32	0.09	$9.90\pm0.18$	$8.18\pm0.04$	$2.34\pm0.03$
0013602	NGC 1425	$31.68\pm0.17$	Sb	69.5	0.54	0.06	$10.83\pm0.24$	$9.56\pm0.02$	$2.25\pm0.01$
0017819	NGC 2090	$30.36\pm0.15$	Sc	68.3	0.54	0.17	$10.94\pm0.39$	$9.35\pm0.11$	$2.17\pm0.02$
0028630	NGC 3031	$27.70\pm0.16$	Sab	59.0	0.38	0.35	$7.12\pm0.10$	$5.41\pm0.10$	$2.36\pm0.01$
0030197	NGC 3198	$31.11\pm0.23$	SBc	70.0	0.80	0.05	$10.22\pm0.32$	$9.22\pm0.06$	$2.19\pm0.01$
0031671	NGC 3319	$30.83\pm0.18$	SBc	59.1	0.48	0.06	$11.33\pm0.64$	$10.39\pm0.06$	$2.03\pm0.03$
0032007	NGC 3351	$29.85\pm0.16$	SBb	41.5	0.33	0.12	$10.05\pm0.12$	$8.32\pm0.06$	$2.27\pm0.03$
0032192	NGC 3368	$30.08\pm0.17$	SBab	54.7	0.21	0.11	$9.75\pm0.13$	$7.99\pm0.05$	$2.30\pm0.04$
0034554	NGC 3621	$29.14\pm0.15$	SBcd	65.6	0.64	0.35	$9.11\pm0.22$	$8.01\pm0.06$	$2.13\pm0.01$
0034695	NGC 3627	$29.79\pm0.15$	SBb	57.3	0.48	0.14	$8.97\pm0.19$	$7.54\pm0.06$	$2.30\pm0.02$
0039600	NGC 4258	$29.48\pm0.16$	SBbc	72.0	0.65	0.07	$8.37\pm0.12$	$7.04\pm0.08$	$2.34\pm0.01$
0040692	NGC 4414	$31.35\pm0.26$	Sc	54.0	0.41	0.08	$10.65\pm0.55$	$8.87\pm0.10$	$2.35\pm0.02$
0041471	NGC 4496A	$30.89\pm0.18$	SBd	48.1	0.23	0.11	$11.78\pm0.20$		$1.97\pm0.06$
0041812	NGC 4535	$30.98\pm0.20$	SBc	44.0	0.20	0.08	$10.35\pm0.15$	$8.89\pm0.06$	$2.29\pm0.06$
0041823	NGC 4536	$30.93\pm0.19$	SBbc	58.9	0.62	0.08	$10.43\pm0.26$	$9.15\pm0.08$	$2.25\pm0.01$
0041934	NGC 4548	$30.94\pm0.17$	SBb	37.0	0.12	0.16	$10.66\pm0.10$	$8.89\pm0.06$	$2.27\pm0.08$
0042741	NGC 4639	$31.71\pm0.21$	SBbc	52.0	0.30	0.11	$11.81\pm0.07$	$10.27\pm0.06$	$2.25\pm0.05$
0043451	NGC 4725	$30.46\pm0.18$	SBab	54.4	0.23	0.05	$9.74\pm0.15$	$8.04\pm0.06$	$2.36\pm0.04$
0069327	NGC 7331	$30.86\pm0.22$	Sbc	75.0	0.62	0.39	$9.24\pm0.17$	$7.70\pm0.05$	$2.40\pm0.01$

The use of some calibrators (e.g., NGC 598 and NGC 4496A) is debatable because they are faint with poor photometry. This point is discussed in Sect. 5.