Why are gravitationally lensed galaxies blue in color?

Question

I have seen multiple color photographs showing distant yellow galaxies with the clear arcs of more distant galaxies beyond them. (ref. recent book "Einstein's Telescope" by Evalyn Gates). As you might expect, the light arcs from the distant objects form circumferential patterns surrounding the closer galaxies. But what is unusual to my eye is that virtually all of the arcs are blue in color, especially when compared to the closer galaxies that are actually creating the lens. This seems sort of reversed from what I would expect. I would think that the more distant galaxies would favor the red end of the spectrum, while the nearby objects would tend toward the blue, or at least the neutral green portion of the visible spectrum. I understand that it takes a whole lot of red shifting to create visible results, but this appears to be a pattern that, if anything, seems to be contradicting my expectations. Anyone understand this? Is it because the more distant galaxies represent younger stars? If so, why do all lensing galaxies appear to have younger stars than all lensed galaxies?

ReginaldQ · Accepted Answer

I am no expert in here so I would certainly put only my opinions. Most lensing galaxies are quite the massive kind. Large ellipticals are obviously massive and they have this specific star density and "well-defined edges" which makes them good lense tools. An elliptical of the same diameter as a spiral would certainly be around 4 to 10 times as massive if they are to be compared. Distance-wise, lensed objects should appear bluer since they are younger or when most galaxies are still actively forming while the lensing objects are massive now due to age and mergers as they expanded from the early universe. Still, I believe we still need to look at the greater population or sample of images. Probably, there also a lot of nearby and "bluer" galaxies out there that lense the same way, it's just that more distant lensed objects are more interesting and is generally shown to the public.

More interesting is the number of the lensed images if they don't completely surround the lenser. They rarely come in odd numbers! 

Clear skies!

CM · Answer

I'm not familiar with the phenomenon, but reading an article about the lensing process, it states that, "the galactic lens works by the gravity of the nearer galaxy bending and concentrating the light of the more distant galaxy." 

From my basic understanding, wouldn't the lensing 'focus' the light in a way, as the article states "concentrating the light," resulting in a perceived shorter wavelength? 

I'm interested in learning more about the process, if you can clarify any errors in my reasoning. 

The second link below contains an article on Microlenses - I'm not sure how useful it would be to you, but thought it might be relevant.

Anonymous · Answer

In order for a galaxy that distant to be bright enough to be visible at all, it has to be starbursting: i.e., it is in the process of rapidly forming stars (for if it were not, while it would still be a lens, it would be much dimmer and more difficult to detect). Young star clusters are dominated by the light of the exceedingly bright and massive blue giant stars that are not present in older stellar populations.

Philosopher John · Answer

Could it just be blue shift? Due to the pull of the lensing galaxies on the light, compressing the light wave. Like the Doppler Effect.

Anonymous · Answer

The closer the galaxy is to us, the bluer color it has. The farther the galaxy is, the more red color it has. Sorry, but I forgot the reason for that.

at m · Answer

Don't forget that light will bend as it passes a object with any type of gravitational field, or any type of moisture in its atmosphere. Other light sources will distort it as well.

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Why are gravitationally lensed galaxies blue in color?

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