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.
Credits
Credit: Hubble Space Telescope WFPC Team, NASA, STScI| About The Object | |
|---|---|
| Object Name | NGC 4881 |
| R.A. Position | 12h 59m 57.79s |
| Dec. Position | 28° 14' 47.69" |
| About The Object | |
|---|---|
| Object Name | A name or catalog number that astronomers use to identify an astronomical object. |
| Object Description | The type of astronomical object. |
| R.A. Position | Right ascension – analogous to longitude – is one component of an object's position. |
| Dec. Position | Declination – analogous to latitude – is one component of an object's position. |
| Constellation | One of 88 recognized regions of the celestial sphere in which the object appears. |
| Distance | The physical distance from Earth to the astronomical object. Distances within our solar system are usually measured in Astronomical Units (AU). Distances between stars are usually measured in light-years. Interstellar distances can also be measured in parsecs. |
| Dimensions | The physical size of the object or the apparent angle it subtends on the sky. |
| About The Data | |
| Data Description |
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| Instrument | The science instrument used to produce the data. |
| Exposure Dates | The date(s) that the telescope made its observations and the total exposure time. |
| Filters | The camera filters that were used in the science observations. |
| About The Image | |
| Image Credit | The primary individuals and institutions responsible for the content. |
| Publication Date | The date and time the release content became public. |
| Color Info | A brief description of the methods used to convert telescope data into the color image being presented. |
| Orientation | The rotation of the image on the sky with respect to the north pole of the celestial sphere. |
