eso9916 — Photo Release
Unusual VLT Views of a Spiral Galaxy
The Colours of NGC 1232
27 February 1999
NGC 1232 is a prominent southern Sc spiral galaxy in the constellation Eridanus (The River). With a diameter of nearly 200,000 lightyears, it is about twice the size of the Milky Way galaxy. The distance is about 100 million light-years, but the excellent optical quality of the VLT and FORS allows us to see an incredible wealth of details.
Spectacular images of this object from FORS have been published in ESO Press Release eso9845 (eso9845e). A true-colour poster of NGC 1232 is also available. More exposures of this galaxy were made at that time, some of which have been used to study the distribution of various types of objects (stars, nebulae) in more detail.
The photo is based on two individual images of NGC 1232 obtained by the FORS1 Commissioning Team at the ESO VLT UT1 on 20 September 1998. They were taken in ultraviolet ("U-band") and blue ("B-band") light. The exposure times were 10 min and 6 min, respectively. To obtain this colour-index image, the two images were carefully recentered and then divided numerically.
Star formation regions that emit strong ultraviolet light therefore appear as particularly bright areas in this colour-index image. They trace the shock fronts of the gas in the spiral arms.
The field measures about 5.5 x 5.5 arcmin 2. North is up and East is left.
Dust lanes in the spiral arms absorb more green light and therefore appear redder than their surroundings. In this differential image, they are seen as dark regions. They are exceptionally sharp and particularly strong in the inner part of the galaxy.
The first image is a computer processed "(U-B) colour-index image" of NGC 1232. This technique allows to show the "difference" between images of the galaxy, as seen in different wavebands. Since different types of objects have different brightness in different colours, this method is very useful to locate objects of a particular type and to obtain an overview of their distribution in the galaxy.
The second image: This "(V-I) colour-index image" was produced in the same way as eso9916a. It is based on two other exposures, taken on the same date, through a visual-green filter (V) and a near-infrared-red filter (I), respectively. The exposure time was 3 min for both. The field size and orientation is the same. The round, dark spot at the upper left is an artefact.