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For refcode 1995ApJ...446....1S:
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Copyright by American Astronomical Society. Reproduced by permission
1995ApJ...446....1S STEPS TOWARD THE HUBBLE CONSTANT. X. THE DISTANCE OF THE VIRGO CLUSTER CORE USING GLOBULAR CLUSTERS ALLAN SANDAGE The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101 AND G. A. TAMMANN Astronomisches Institut der Universitat Basel, Venusstrasse 7, CH4102 Binningen, Switzerland Received 1994 May 31; accepted 1994 December 20 ABSTRACT New data for Secker's (1992) unbiased sample of 100 globular clusters in the Milky Way exhibit a nearly Gaussian luminosity function with peak (turnover) luminosities of <M_B_>_0_ = -6.90+/-0.11, {sigma}(M) = 1.07 mag, and <M_V_>_0_ = - 7.60 +/- 0.11, {sigma}(M) = 1.07. These are derived from revised Galactic globular cluster distances determined from an RR Lyrae calibration of M_V_(RR) = 0.30([Fe/H]) + 0.94 based on the Oosterhoff-Arp-Preston period-metallicity effect. The new calibration averages 0.25 mag brighter than is generally in current use. Halo clusters in M31 give <M_B_>_0_ = -7.01 +/- 0.20, {sigma}(M) = 0.89, and <M_V_>_0_ = -7.70 +/- 0.20, {sigma}(M) = 0.89 for the globular cluster luminosity function (GCLF) using a Cepheid modulus for M31 of (m-M)^0^ = 24.44 (Madore & Freedman 1991) with a foreground reddening of E(B- V) = 0.08. Use of the brighter RR Lyrae magnitudes for the MW calibrators nearly eliminates the marginal 0.2-0.3 mag difference between the MW and the M31 GCLFs obtained earlier by Racine & Harris (1992) and Secker (1992). The combined globular cluster luminosity function for the MW plus M31 gives <M_B_>_0_ (turnover) = -6.93 +/- 0.08 and <M_V_>_0_(turnover) = - 7.62 +/- 0.08, with a realistic external error of ~0.2 mag. Applying this local calibration to the E galaxy Virgo cluster data gives the modulus of the Virgo cluster core as (m-M)^0^ = 31.64 +/-0.25 (D = 21.3 +/- 2.7 Mpc; external error). This value is based on the observed mean GCLF turnover luminosity of <B>_0_ = 24.64 +/- 0.07 for three galaxies as determined by Harris et al. (1991). The modulus is only marginally increased if NGC 4365 is accepted as a likely Virgo cluster member and if new data for NGC 4636 are included. We have used the precept that the mean absolute turnover luminosity of the GCLF is universal, that is, does not depend on a second parameter. The suggestion (Secker & Harris 1993) that the turnover luminosity varies with the dispersion of the GCLF, based on their a priori adoption of a short-distance-scale modulus of the Virgo cluster as (m - M)^0^ = 30.89, is discussed. Contrary, independent evidence that the modulus of the Virgo cluster core is (m-M)^0^ = 31.7 is reviewed. The Hubble constant, based on (1) the distance to the Virgo cluster core determined from the calibration of the globular cluster method given here, and (2) the redshift of the Virgo core of {nu}(cosmic) = 1179+/-17 km s^-1^ relative to the Machian frame of the cosmic microwave background, freed from all local streaming motions (Jerirn & Tammann 1993), is H_0_(global) = 5517 km s^-1^ Mpc^-1^ (external error). Subject headings: distance scale - galaxies: individual (Virgo) - galaxies: distances and redshifts - galaxies: star clusters
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