ESO Education and Public Relations Dept.

23 June 1998

First VLT Colour Photo of a Strange Galaxy

The first VLT 8.2-m telescope (UT1) is now undergoing a "Commissioning Phase" during which all systems are thoroughly tested and further tuned. Although priority is given to technical work, some astronomical images have been obtained during the recent weeks.

ESO Press Photo 19/98

This is a colour photo from the VLT that shows NGC 4650A, a member of the so-called Centaurus chain of galaxies in the southern constellation of that name.

NGC 4650A is a complex system that is located at a distance of about 50 Megaparsec (165 million light-years). As it is clear from this high-resolution picture, there are two main components, a lenticular-shaped galaxy (of type S0), surrounded by a knotty extended ring-like distribution of stars, dust and gas, nearly perpendicular to each other.

The stars in the central, structureless region and the material (stars, gas, and dust) in the outer disk rotate in two different planes that are nearly perpendicular to each other. The observed motions indicate the presence of a significant halo of dark matter within the system, although it is still unclear to which of the two components this matter belongs. Because the velocities of stars and gas can be measured in two independent planes, NGC 4650A serves as a prototype for detailed studies of the shape of dark matter halos.

The origin of this strange spatial configuration is not known with certainty. It is likely, however, that an encounter between two galaxies may have played a significant role.

The central component and the highly inclined, polar disk have different stellar populations -- this is clearly indicated by the different colours. The S0-component mainly contains a population of old (reddish) stars, while the polar ring harbours many young (blue) stars and gaseous nebulae.

The photo is based on a series of seven exposures through three different optical filtres, obtained under mediocre observing conditions during a small break in the technical work. They have been combined to produce the present photo. In this case (contrary to the VLT astronomical photos published earlier), a slight amount of image processing has been applied to balance the individual exposures.

Technical information: Combination of three 10-min B (blue) exposures (seeing 0.68 - 0.82 arcsec), two 10-min V (green-yellow) exposures (0.55 and 0.77 arcsec), and one 4-min and one 10-min R (red) exposures (0.55 and 0.52 arcsec) with the VLT Test Camera. Individual frames were flat-fielded and cleaned for cosmics, combined and deconvolved with the Richardson-Lucy algorithm to produce a final FWHM = 0.53 arcsec before colour combination. The field measures 1.5 x 1.5 arcmin. North is to the upper left; East is to the lower left.

Further images of astronomical objects from the VLT UT1 will be published at irregular intervals.

This is the caption to ESO PR Photo 19/98 [JPG, 112k]. It is also available in a high-resolution version [JPEG, 728k]. It may be reproduced, if credit is given to the European Southern Observatory.

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