Lyman-alpha contours at radio galaxy 1138-262

This is a false-colour picture of the ionized hydrogen gas surrounding 1138-262 (Lyman-alpha). The size of this cloud is about 5 times larger than the optical extent of the Milky Way Galaxy. A contour plot, as observed with VLT ANTU + FORS1 in a narrow-band filter around the wavelength of the redshifted Lyman-alpha line, is superposed on a false-colour representation of the same image. The contour levels are a geometric progression in steps of 2 1/2. The image has not been flux calibrated, so the first contour level is arbitrary. The field measures 35 x 25 arcsec 2, corresponding to about 910,000 x 650,000 light-years (280 x 200 kpc). The linear scale is indicated at the lower left. North is up and East is left.

Narrow and broad-band imaging was carried out on April 12 and 13, 1999, with the ESO VLT ANTU (UT1), using the FORS1 multi-mode instrument in imaging mode. A narrow-band filter was used which has a central wavelength of 381.4 nm and a bandpass of 6.5 nm. For 1138-262 (redshift z = 2.2), the emission of Lyman-alpha at 121.6 nm is redshifted to 383.8 nm, which falls in this narrow band. The broad-band filter was a Bessel-B with central wavelength of 429.0 nm. The detector was a Tektronix CCD with 2048 x 2046 pixels and an image scale of 0.20 arcsec/pixel. Eight separate 30-min exposures were taken in the narrow band and six 5-min in the broad band, shifted by about 20 arcsec with respect to each other to minimize problems due to flat-fielding and to facilitate cosmic ray removal. The average seeing was 1.0 arcsec. Image reduction was carried out by means of the IRAF reduction package. The individual images were bias subtracted and flat-fielded using twilight exposures (narrow band) or an average of the unregistered science exposures (broad-band). The images were then registered by shifting them in position by an amount determined from the location of several stars on the CCD. The registered images were co-added and dark pixels from cosmic rays were cleaned. To improve the signal-to-noise ratio, the resulting images were smoothed with a Gaussian function having full-width-at half-maximum (FWHM) = 1 arcsec (5 pixels).