Faint, distant cluster of galaxies

The image shows the sky region near the powerful radio galaxy TN J1338-1942 at a redshift of 4.1. The photo is a "negative" rendering (the objects are dark on a bright background) of an image obtained with the FORS2 multi-mode instrument on the 8.2-m VLT KUEYEN telescope (ESO Paranal Observatory, Chile) through a narrow-band optical filter, centered at the wavelength of the redshifted Lyman-alpha line.

The 20 galaxies that have been confirmed to be emitting the sharp colours due to glowing hydrogen gas at the distance of the radio galaxy are encircled in blue.

The green rectangle marks the radio galaxy, from which a stream of hydrogen gas stretches to the northwest, over a distance of about 300,000 light-years. The size of the sky field corresponds to about 10 million light-years at the distance of these galaxies. North is up and East is left.

The image is derived from from FORS2-exposures, obtained on March 25 and 26, 2001, using a narrow-band optical filter (peak at 619.5 nm with transmission 80%, FWHM 6.0 nm). The total exposure time was 33300 seconds (9 hrs 15 min). The field-of-view of the final image is 6.4 x 6.2 arcmin², corresponding to about 3 Mpc on each side. The frames were obtained in photometric conditions, and the image quality in the combined frame is 0.65 arcsec.