Centaurus A

The peculiar radio galaxy Centaurus A (NGC 5128) provides a striking example of how dust can affect our view of a galaxy, and how going to redder wavelengths helps penetrate interstellar dust. This color-composite CCD image (from B, V, and R-band frames obtained with the 1.5-meter telescope at Cerro Tololo) shows at once the reddening effects of dust in the strong dust lane, some red emission from ionized gas at the H-alpha line, and blue regions of star formation along the dust lane. The dust blocks most of the light in the blue, and progressively less going through the other wavelengths (and new images deeper in the infrared show even less of the dust). This effect against stars in our own galaxy is known as interstellar reddening, though the effect in galaxies is complicated by the fact that some of the light is not in fact absorbed but scattered in different directions upon encountering a dust grain. Centaurus A is an unusually colorful galaxy; note the very blue star clusters along the edges of the dust lane, most prominent in the blue-light picture. This is widely held to be an elliptical galaxy that has acquired a spiral by merger, with the broad and gas-rich dust lane being the most visible debris of the collision. This intricate dust structure, newly-formed star clusters, and the obscured active nucleus are beautifully shown in the HST images released from WFPC2 and NICMOS data.

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