Omega, Age, and H_0 Implications of Recent Hubble Space Telescope Data in the Coma Cluster

Details Omega, Age, and H_0 Implications of Recent Hubble Space Telescope Data in the Coma Cluster Omega, Age, and H_0 Implications of Recent Hubble Space Telescope Data in the Coma Cluster

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aj....116...31b&link_type=abstract

The Astronomical Journal, Volume 116, Issue 1, pp. 31-36.

Astronomy and Astrophysics

Astronomy

3

Cosmology: Observations, Galaxies: Clusters: Individual: Name: Coma, Cosmology: Large-Scale Structure Of Universe

Scientific paper

This paper presents a way of finding mutually consistent values of Omega_M, t_0, and H_0. This approach is illustrated using recent Coma Cluster distance data, together with recent Hipparcos data for RR Lyrae stars. The age of the universe is derived by two completely independent methods, both linked to the RR Lyrae stars. The two methods can be made to yield the same answer by adjustment of the RR Lyrae magnitude zero point. One of these two age determinations is based on stellar evolution in globular clusters, and the other is based on the Hubble constant derived from globular clusters as distance indicators calibrated in the Milky Way. If, for example, RR Lyrae stars are brighter than previously thought, the stellar evolution age is shortened, whereas the Hubble age is increased, so one can determine the RR Lyrae magnitude zero point that would make the stellar evolution age coincide with the Hubble age, and ask whether it is a reasonable value. The answer depends strongly on the mass density of the universe, Omega_M, which can therefore be derived simply by demanding that the two age determinations agree. For each straw-man value of Omega_M, we find the RR Lyrae zero point M_V(RR)_0 for which the two age determinations coincide. We then choose the Omega_M value for which the implied RR Lyrae zero point best agrees with observations. In the present paper, the mean of six recent Hipparcos values, M_V(RR)_0 = 0.61 +/- 0.15, is used for illustration. For that choice, the coinciding of ages occurs at 12.5 +/- 1.5 Gyr. The corresponding value of the Hubble constant is H_0 = 61 +/- 5 km s^-1 Mpc^-1. In a zero-Lambda universe, the implied mass density is Omega_M = 0.41 with only weak constraints on either higher or lower Omega_M values. In a flat universe with Omega_M + Omega_Lambda = 1, however, the implied mass density is Omega_M = 0.62, and the uncertainty in it is asymmetric. For example, a critical-density universe with Omega_M = 1.0 and Omega_Lambda = 0.0 (for which H_0 = 57 km s^-1 Mpc^-1 and t_0 = 11.4 Gyr) differs by only 0.7 sigma, and is therefore not strongly negated, whereas a low-density universe with Omega_M = 0.1 and Omega_Lambda = 0.9 (for which H_0 = 74 km s^-1 Mpc^-1 and t_0 = 17.0 Gyr) differs by 3 sigma, and therefore appears unlikely. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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