This photo mosaic, which shows a field of distant galaxies, is a computer enhanced reproduction of a picture taken 4 March 1994 with the repaired Hubble Space Telescope. It combines 16 exposures of 15 minutes each, taken through two filters (F555W and F814W) with the Wide Field Planetary Camera 2. The HST WFPC2 field is chevron-shaped, because it is a mosaic of images recorded with three Wide Field cameras and one higher resolution camera (Planetary Camera) in the upper left.
The brightest object in this picture is NGC 4881, approximately centered here in the Planetary Camera (the small quadrant). It is a 13th-magnitude elliptical galaxy in the outskirts of the Coma Cluster, a great cluster of galaxies more than 5 times farther away than the Virgo Cluster. The radical velocity (redshift) of NGC 4881, based on the Doppler displacement of lines in its spectrum, is about 7000 km/sec. Except for a 16th-magnitude Coma spiral at the right and a few foreground stars of the Milky Way, nearly everything else in this field lies far beyond the Coma Cluster. There is a fascinating assortment of background galaxies, including an apparent galaxian merger in progress.
Purpose: This HST-WFPC2 observation was made to explore the use the globular star clusters surrounding NGC 4881 as distance indicators for inferring the distance to the Coma Cluster. They are barely visible point sources in this reproduction. The distance to the Coma Cluster is an important cosmic yardstick for scaling the over all size of the universe, because Coma (unlike Virgo) is far enough away that regional departures from a smooth expansion of the universe should not be a major source of uncertainty if Coma is used for estimating the age and rate of expansion (the Hubble Constant).
The brightness distribution of globular clusters has been studied in a number of nearer galaxies. They are most numerous between -7 and -8 absolute magnitude. In the Milky Way they peak at -7.6 absolute magnitude. We must find that peak ("turnover") in NGC 4881 in order to judge the distance. Within statistical uncertainties, the number of globulars per magnitude in NGC 4881 increases down to our present threshold of 27.6 magnitude. We do not yet see evidence of the turnover in NGC 4881, which suggests that the Coma Cluster may be more than 100 megaparsecs away and that the Hubble Constant may therefore be less than 70 km/sec per megaparsec. The adding together of more exposures will evidently be needed to reach a fainter threshold and find the turnover. Though not yet definitive in itself, this exploratory observation of NGC 4881 shows that it is within the reach of HST to obtain a definitive globular-cluster distance to the Coma Cluster and an associated value of the Hubble Constant.
Contact: William A. Baum
University of Washington
Seattle, WA 98195