Galaxy Cluster Abell 370
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Galaxy Cluster Abell 370

April 28, 2005
This photo shows a composite colour image of the Abell 370 cluster of galaxies . This cluster hosts one of the most prominent gravitational arcs known to date.

The light of a very distant object is deformed by the foreground cluster. Abell 370, at the center of the photo, contains a large number of galaxies - most of them are ellipticals. The two brightest red galaxies slightly above the gravitational arc are the most massive in the cluster and give an important contribution to the lensing event.

This gravitational arc - in combination with the cluster of galaxies - provides a natural laboratory to study the physics of gravity. The deformation of "space-time" by the large mass of Abell 370 can be derived and modeled from the slope of the gravitational arc. The exact shape of the arc allows to estimate the mass of the cluster through the equations of general relativity. This estimate agrees with independent determinations from the X-ray emission and the motion of the galaxies in the cluster.

Technical Information: The photo is a combination of three exposures through R- (red; centered at 657 nm and with FWHM 150 nm), B- (blue; 429 nm and FWHM 88 nm) and U-band (ultraviolet; 366 nm and FWHM 36 nm) filters and with exposure times of 240, 300 and 500 sec, respectively. Only one quarter (1050 x 1050 pix) of the full image is displayed here. The seeing was 0.9 arcsec in R and B, and 1.1 arcsec in the U-band filter. The field measures 3.5 x 3.5 arcmin with North at the top and East to the left.

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