2.5. Cross-Check of the Cepheid Distances
2.5.1. Tests with Geometric Methods
NGC4258 (M106) is a Seyfert 2 galaxy with H2O maser emission from clouds orbiting around a black hole of mass 4 x 107 M located at the centre. Precise VLBA measurements of Doppler velocities show that the motion of the clouds is very close to Keplerian and is perturbed very little (Miyoshi et al. 1995). A complete determination is made for the orbital parameters including centripetal acceleration and a bulk proper motion of the emission system. This yields a geometric distance to NGC4258 to be 7.2 ± 0.3 Mpc (Herrnstein et al. 1999).
Maoz et al. (1999) measured the distance to NGC4258 using the conventional Cepheid PL relation, and gave 8.1 ± 0.4 Mpc with (m - M)LMC = 18.5. This distance is 13% longer than that from the maser measurement. The short LMC distance would bring the Cepheid distance in a perfect agreement with the geometric distance. This is, however, only one example, and it can merely be a statistical effect: the deviation is only twice the error, so it may happen with a chance probability of 5%.