galaxy - What's this red stuff in some astronomical photos, e.g. Centaurus A?

"Clean the image", how apt, because most of what you can see is caused by dust.

It is a bit difficult to comment specifically because I think you have chosen an example which is a combination of images and not all the images are in the visible part of the spectrum and so have had a "false colour" applied. In fact it is a combination of optical, X-ray and sub-mm images.

In general terms red is present in false colour astronomical images, usually used to representing the coolest or longest wavelength light in the image.

Red is also present in true-colour images of nebulae and is caused by light emitted due to transitions of atoms and ions between excited states separated by particular energies. Red light is predominantly due to Hydrogen alpha and ionised nitrogen emission.

The coolest stars are also red in true colour images, simply because their pseudo-blackbody radiation spectra peak in the red part of the spectrum for temperatures lower than 5000 K. The integrated light from old stellar populations (predominantly cool main sequence stars and red giants) would therefore have a reddish tinge - for example in elliptical galaxies.

As I said, it is hard to say in the case of this particular composite image, but the band across the centre of the galaxy is dust. It obscures (makes dark), the optical image, but warm dust glows in the sub-mm image. The structures coming out of the galaxy at right angles are jets from the active galactic nucleus traced in the X-ray.