Chandra Release - October 26, 2004

Visual Description: NGC 4555

Two images side by side show an elliptical galaxy, NGC 4555.  The Chandra image on the left revealed that this large, isolated, elliptical galaxy is embedded in a cloud of 10-million-degree Celsius gas. The hot gas cloud has a diameter of about 400,000 light years, roughly twice that of the visible galaxy (shown at right).  The Chandra image (blue) is roughly shaped like a large ethereal gaseous bat. The optical image (yellow and white) shows a larger blobby structure at the center with a few smaller bright spots sprinkled around the field of view.  An enormous envelope, or halo, of dark matter is needed to hold the hot gas cloud to the galaxy. The total mass of the required dark matter halo is about ten times the combined mass of the stars in the galaxy, and 300 times the mass of the hot gas cloud. Most large, elliptical galaxies are found in groups and clusters of galaxies where they can gain or lose dark matter through collisions with other galaxies, so it is difficult to determine how much dark matter they originally possessed. The Chandra observation of NGC 4555 confirms that an isolated, elliptical galaxy can possess a dark matter halo of its own.