"When there are intervening small particles along our line of sight, this makes things behind them look dimmer," Martin Gaskell, a research associate in astronomy and astrophysics at University California Santa Cruz and lead author of the paper, said in aThe dimming of the evening sun and the reddening of the solar disk both have the same cause. The same effect applies to the distant galactic centers, which appear redder than they actually are.
In the case of the, however, astronomers can easily compare the dimmed light with the actual wavelength composition of the sun's radiation and its intensity. To do the same for distant galaxies is much more complicated, as estimates of the intensity of the various wavelength components of undimmed radiation from distant active galactic nuclei are mostly based on theoretical predictions.