Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

January 4, 1999
See Explanation.  Clicking on the picture will download 
 the highest resolution version available.

Ring Around the Cluster
Credit: ISAAC, 8.2-meter VLT, ESO

Explanation: It is difficult to hide a galaxy behind a cluster of galaxies. The closer cluster's gravity will act like a huge lens, pulling images of the distant galaxy around the sides and greatly distorting them. This is just the case observed in the above recently released image from the VLT. The cluster CL2244-02 is composed of many yellow galaxies and is lensing the image of a blue-white background galaxy into a huge arc. Careful inspection of the image will reveal at least one other lensed background galaxy appearing in red. The foreground cluster can only create such a smooth arc if most of its mass is smoothly distributed dark matter - and therefore not concentrated in the yellow galaxies visible. Analyzing these gravitational arcs gives astronomers a method to estimate the dark matter distribution in clusters of galaxies.

Tomorrow's picture: The Jupiter Jiggle

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
&: Michigan Tech. U.