My question is inspired by this question about red shift.

When viewing photos of distant galaxies, especially quasars, they are often shown as white. However, the light would have been red-shifted. Are the photos,

a) post-processed to cancel out the red-shift and show the galaxy as it would appear "close by"?

b) showing us the galaxy's ultra-violet emissions, i.e. the photo is in false color?

c) accurate because, for most stars, uv emissions are mostly similar to visible light emissions?

d) accurate because the red shift is not significant enough to affect the visible light appearance?

  • 3
    $\begingroup$ These days, most astronomy "photos" are in false colour. Unless the photo is actually taken in visible light, it has to be so. Also, in many cases, they are a combination of several images take with different sensors (visible, IR, UV, etc). $\endgroup$
    – hdhondt
    Dec 21, 2015 at 0:01

1 Answer 1


Distant galaxies are most definitely red, or indeed infrared. The fact that their rest frame UV light is heavily absorbed, leading to a so-called Lyman break when it is then redshifted, is one of the main ways of finding high redshift galaxies.

Quasars can have a very large amount of UV continuum (from hot accreting gas close to their centres) that gets to us relatively redshifted into the visible region, especially for redshifts of 1-3. At higher redshifts, even quasars do start to become very red because UV absorption that is intrinsic to the quasar galaxy, and also in intervening space, is a highly effective absorber of UV light shortward of 91.2 nm. This leads to objects with increasing redshifts having almost no emission in the blue, then green and then even the red part of the spectrum at redshifts of 6 or more.

Indeed a major source of confusion in finding distant quasars is mixing them up with cool brown dwarfs in photometric surveys.

So $z<3$ quasars may have flux at all visible wavelengths, but higher redshift objects should become increasingly red as the Lyman break is redshifted further into the visible range.


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