NASA released this image of deep space. The way light shifting vis-a-vis universe expansion is supposed to work is by making the furthest galaxies appear mostly red-shifted. But NASA's image shows somewhat of an even balance between blue and red shift. Why is that?

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    $\begingroup$ Is it already redshift corrected? $\endgroup$ Aug 17, 2018 at 14:02

4 Answers 4


You cannot gauge the redshift of a galaxy by looking at a false colour image. The images taken through different filters are stacked and colourised to suit. You can say that the blue galaxies are indeed bluer than the red galaxies, but there is no absolute scale with which to judge redshift by eye.

Secondly, there is no detail in the NASA web page, but the ACS and WFC3 cameras have near infrared capabilities. So I would think that this image is a visual false-colour image of information that extends well redward of what the eye can perceive. So even the things that look blue might have a spectrum that peaks at redder wavelengths, whilst anything that looks red might actually be infrared!

However, beyond this, in order to judge what the redshifted appearance of a galaxy would be, you would need to know what the galaxy looked like with zero redshift. That is, light that is emitted in the ultraviolet could be redshifted into the visible part of the spectrum. It is entirely possible that visual colour of a redshifted galaxy would not change very much at all if the galaxy emitted lots of UV light in its frame of reference or equally, if it emitted lots of very red light that was then redshifted out of the telescope's sensitivity range.

Finally, there could be some genuine astrophysics going on. Many distant galaxies are bluer than nearby galaxies because they are undergoing intense star formation. Massive star forming regions emit copious UV light that is redshifted into the optical.

Finally, finally! Many of the galaxies in the picture will be quite close and will not be very redshifted.


There are two factors here.

First, to construct an image such as this, several optical filters are used, and then these are then merged to give an image. The colours that you see are therefore not the "true" colours (they couldn't be, the light you see is far too faint for the human eye) but are chosen to bring out details in the image. A galaxy that is blue in the image might not actually be blue.

Secondly, red shift means that all the light is shifted, If a galaxy is emitting a broad spectrum of light, the ultraviolet is shifted to the visible, as the red is shifted to the infrared, the apparent colour doesn't change much. So the apparent colour, red or blue isn't telling you about the red-shift of each galaxy.

To measure the red shift you need a detailed spectrum. A spectrum will include spectral lines, which have known frequencies. By noting the shift of these lines the redshift is measured.

The different colours you see here are probably a consequence of the type of stars in each galaxy. Galaxies with more young stars appear bluer, and this is probably emphasised in the image (but remember it isn't "true colour")


It is not a question of image coloring because there is a problem with the evolution of galaxies termed "FBG - Faint Blue Galaxies excess" that has no satisfactory explanation. There are important references not mentioned in the WP article.

A one page explanation of the problem (excess of blue galaxies and irregulars at z>2 ) and the current state of affairs can be read at GUAIX-The faint blue galaxies problem

from The Evolution of Galaxy Structure Over Cosmic Time Christopher J. Conselice (2014)

In addition to a faint blue excess, it was clear that there was also a peculiar excess, often for the faintest galaxies ..A deeper analysis showed that, using the rest-frame optical structures of galaxies, the Hubble sequence was nearly completely absent at z > 2, and only at z ∼ 1.5 did spirals and ellipticals become as common as peculiar galaxies

from FAINT BLUE GALAXIES, Richard S. Ellis, 1997

for a rapid decline in the volume-averaged star-formation rate of field galaxies since a redshift z ≈ 1. Together with the small angular sizes and modest mean redshift of the faintest detectable sources, these results can be understood in hierarchical models where the bulk of the star formation occurred at redshifts between z ≈ 1−2. The physical processes responsible for the subsequent demise of the faint blue galaxy population remain unclear ..Several puzzling features emerged that have collectively been referred to as the faint blue galaxy problem (Kron 1978). In its simplest manifestation, an apparent excess of faint blue galaxies is seen in the source counts over the number expected on the basis of local galaxy properties. A more specific version of the problem that attracted much attention followed the results of the first faint redshift surveys (Broadhurst et al 1988, Colless et al 1990). The count-redshift data from these surveys did not solve the number problem by revealing a redshift range (at either low or high redshift) where this additional population could be logically placed. Relatively complex evolutionary hypotheses were then proposed to reconcile these results, including luminosity-dependent evolution, galaxy merging, and the existence of a new population of source present at modest redshift but, mysteriously, absent locally


I used image search to find a version with more detail (Google Image Search is great for this sort of thing) about how it was taken, the RGB channels in the mage were taken with IR, Visual, and Blue filters respectively.

The main galaxy cluster in it, Abel 2744, is only at a red-shift of 0.3; so it's colors aren't that far removed from what we could see visually (with much better eyes).

Also, just looking at the picture doesn't give you any way to tell the difference between a galaxy that's relatively near but dim or much farther but bright. However I suspect a majority of the tiny blue galaxies are smaller members of Abel 2744 not extremely distant objects that would need a major blue shift to cancel the red shift from ex.


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