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A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk


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 rare1. In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but such techniques have hitherto 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 ladder2. Here we report a geometric distance to the galaxy NGC4258, which we infer from the direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy. The distance so determined—7.2 ± 0.3 Mpc—is the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.

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Figure 1: The NGC4258 water maser.
Figure 2: Line-of-sight (LOS) velocities (a) and right ascensions (b) at the peaks of the systemic maser spectrum.
Figure 3: Systemic maser bulk proper motion (〈θ̇ x〉; a) and acceleration (〈 LOS〉; b) probability density functions.


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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Correspondence to J. R. Herrnstein.

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Herrnstein, J., Moran, J., Greenhill, L. et al. A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk. Nature 400, 539–541 (1999).

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