A New Method for Measuring Extragalactic Distances

摘要

We have pioneered a new method for the measurement of extragalacticdistances. This method uses the time-lag between variations in the shortwavelength and long wavelength light from an active galactic nucleus (AGN),based on a quantitative physical model of dust reverberation that relates thetime-lag to the absolute luminosity of the AGN. We use the large homogeneousdata set from intensive monitoring observations in optical and near-infraredwavelength bands with the dedicated 2-m MAGNUM telescope to obtain thedistances to 17 AGNs in the redshift range z=0.0024 to z=0.0353. These distancemeasurements are compared with distances measured using Cepheid variable stars,and are used to infer that H_0= 73 +- 3 (random) km/s/Mpc. The systematic errorin H_0 is examined, and the uncertainty in the size distribution of dust grainsis the largest source of the systematic error, which is much reduced for asample of AGNs for which their parameter values in the model of dustreverberation are individually measured. This AGN time-lag method can be usedbeyond 30 Mpc, the farthest distance reached by extragalactic Cepheids, and canbe extended to high-redshift quasi-stellar objects.