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Published September 3, 1998 | public
Journal Article

A Cepheid distance to the Fornax cluster and the local expansion rate of the Universe


Both galaxy distances and velocities are required for the determination of the expansion rate of the Universe, as described by the Hubble constant H_0. The radial velocities of galaxies arise not just from this expansion but also from random components and large-scale flows. To reach out to distances dominated by the overall cosmic expansion, it is necessary to probe large physical scales where galaxy–galaxy and galaxy–cluster interactions become less important. But accurate distances of nearby galaxies and clusters (commonly measured using Cepheid variable stars) are nevertheless required to calibrate the indirect distance indicators generally used to measure these large scales. Here we report a Cepheid distance of 18.6 ± 1.9 (statistical error) for the galaxy NGC1365 in Fornax, a cluster of galaxies in the Southern Hemisphere. We find a value of H_0 = 70 km s^(-1) Mpc^(-1) from Fornax alone, and 73 km s^(-1) Mpc^(-1) from the intervening galaxy flow, each corrected for infall into the Virgo cluster. These values are consistent with the Hubble constant measured in the far field using secondary methods. Our data support previous suggestions that the local small-scale velocity field has very small scatter (~ ± 70 km s^(-1)).

Additional Information

© 1998 Macmillan Publishers Ltd. Received 18 March; accepted 1 June 1998. This work was supported by NASA and the NSF, and made extensive use of the NASA/IPAC Extragalactic Database (NED). Observations are based on data obtained using the Hubble Space Telescope which is operated by the Space Telescope Science Institute under contract from the Association of Universities for Research in Astronomy. L.F. acknowledges support by NASA through a Hubble Fellowship grant awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA.

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