Return to Heritage Home Page Current Image Gallery Archive Information Center Hubble Art Search
Return to Heritage Home Page Current Release Home Page Caption Fast Facts Biographies Supplemental Material Original Images

NGC 1316, normal (left) and with intrinsic stellar profile model subtracted (right).
Images courtesy of P. Goudfrooij and the Hubble Heritage Team (STScI).

Elliptical Galaxies: To model or not to model?

Elliptical galaxies are very smooth, featureless entities. Unlike their spiral cousins with heavy dust lanes, dense spiral arms, and intense star-forming activity, ellipticals have a uniform light output that gradually fades as one moves out from the nucleus towards the edge of the galaxy. Or so we see from the outside. Astronomers fit a model containing elliptical isophotes or areas of constant brightness to the data in each filter. They avoided areas where dust absorption was obvious, so that the model fit the intrinsic stellar light distribution (areas uncontaminated by dust). Each HST filter image was then divided by this smooth model galaxy in order to improve the visual appearance of intricate details in the galaxy morphology, such as dust lanes, ripples, etc., while still retaining an impression of the smooth stellar light distribution of the galaxy.

The final result looks very different from the non-modeled image, but it gives a visually clearer representation of what is going on inside this galaxy The image on the left is the normal composition of filters taken by the Hubble telescope. The image on the right is a composition of filters that have been divided by the model galaxy. The inner regions of the galaxy shown in the Hubble image reveal a complicated system of dust lanes and patches. These are thought to be the remains of the interstellar medium associated with one or more of the spiral galaxies swallowed by NGC 1316.

Images of the same field of NGC 1316/1317 from the UK Schmidt telescope at Siding Springs Observatory (left) and the Cerro Tololo Interamerican Observatory (center and right) taken by François Schweizer from Carnegie Observatories (images courtesy of P. Goudfrooij from STScI). Each image is 20 arcminutes wide.

Taking a Long, Hard Look...

The assertion that NGC 1316 has had a violent history is evident in various ways. The left most image was taken with the UK Schmidt telescope in Australia and is part of the Digitized Sky Survey collection. NGC 1316 is the large galaxy in the middle of the image. NGC 1317 is the smaller compact galaxy at the top center. The images at center and right were taken with the Blanco 4-m telescope at Cerro Tololo Interamerican Observatory (CTIO) in Chile. All three images are of the same field, but of longer and longer exposure times. By lengthing the exposure, one is able to bring out fainter features in the image at the cost of saturating the brightest objects (stars and the galaxy's nuclei). The longer exposure images taken at CTIO show a bewildering variety of ripples, loops and plumes immersed in its outer envelope of these two galaxies. Amongst these so-called "tidal" features shown in these over-exposed images, the narrow ones are believed to be the stellar remains of spiral galaxies that merged together some time during the last few billion years.