2.3.9. Summary of Virgo Distance Estimates

An objective summary of these 6 independent distance measurements to Virgo immediately suggests that one or more of them has a systematic error in the zeropoint calibration. Deciding on which of the methods has the most serious systematic error is an arduous chore which often leads to circular reasoning. Hence, we wish to leave finger pointing to others and go where the data leads. In this case, the data lead to the conclusion that the distance to the Virgo cluster is still unknown at the 20% level. What we have indicated to be the more reliable indicators, do converge on a value of (m - M) = 30.9 - 31.2 though a distance as large as (m - M) = 31.5 can not be rigorously ruled out.

Unfortunately, even though we now have a distance determination to the Virgo cluster, we are not yet in a position to determine H₀. To derive H₀ from the distance to Virgo requires a determination of the proper cosmic velocity of Virgo. We derive this cosmic velocity in Chapter 3 after discussing the nature of galaxy clustering in the observable universe. The large scale clustering of galaxies causes gravitationally induced deviations from pure expansion (cosmic) velocity. These velocity deviations seriously impact the determination of H₀ at the 10-20% level. The situation is worse than 10-20% for Virgo, however, as the velocity of Virgo must be corrected for both the infall of the Local Group and the possible large scale motion of Virgo itself toward other mass concentrations. Moreover, it is likely that the Virgo cluster has substructure which causes the mean velocity of Virgo to become dependent upon which sample of Virgo members is used. Because of these complications it seems unlikely that even a precise measurement of the distance to Virgo would yield H₀. As we will see, a better way to determine H₀ is to use the relative distance between the Virgo and Coma clusters of galaxies in combination with the cosmic velocity of Coma. This is done at the end of Chapter 3.