Authors: Bruce Rout
The accurate measurement of extragalactic distances is a central challenge of modern astronomy, being required for any realistic description of the age, geometry and fate of the Universe. The measurement of relative extragalactic distances has become fairly routine, but estimates of absolute distances are rare. In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but heretofore such techniques have been difficult to apply to other galaxies. As a result, uncertainties in the expansion rate and age of the Universe are dominated by uncertainties in the absolute calibration of the extragalactic distance ladder. Here we compare previous distance measurements to the galaxy NGC 4258 from both an estimate of Hubble's constant and a direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy to a direct measurement using a model of constant rotational velocity and galactic spiral morphology. The results of the comparison help validate methods of direct measurement of spiral galaxies to much greater distances.
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