A Map with a Scale: The Las Campanas Redshift Survey

Abstract

New techniques for measuring galaxy redshifts and new approaches to measuring the Hubble Constant make it possible to map the spatial distribution of galaxies on very large scales. At the Las Campanas Observatory, a fiber-fed spectrograph on the 2.5 m telescope gathers large samples of galaxy redshifts. A sketch of the technique and preliminary results are reported here. We have spectra for over 20 000 galaxies, at a typical redshift near 30 000kms-1, with velocity accuracy of about 75kms-1. The redshift distribution of these galaxies suggests that the pattern of voids and filaments seen in nearby samples such as the CfA survey is repeated with the same scale even though this survey could see structures which are 5 times larger. Perhaps we have begun to probe deeply enough to see the end of large scale structure and the onset of homogeneity. While the patterns are determined by redshift measurements, the scale of these redshift maps is not determined without knowledge of the Hubble constant, where Supernovae have opened some new approaches. New work on geometric measurement of the circumstellar ring of SN 1987A using IUE spectra and HST images provides an independent test of the Cepheid distance scale to the Large Magellanic Cloud. HST Calibration of the SN Ia standard candle has been carried out by Sandage et al. who observed Cepheids in galaxies with well-observed supernovae. The Expanding Photosphere Method (EPM) allows direct measurement to each type II supernova that has adequate photometry and spectroscopy. There are now 18 such objects. The sample of EPM distances from 4.5 Mpc to 180 Mpc indicates H 0 = 73 ± 6(statistical) ± 7(systematic) krns-1 Mpc-1. With both redshift maps and a determination of the Hubble constant, we can begin to learn the true spatial distribution of galaxies.