monarchist wrote:Ann and Chris, thanks for probably answering the question of why in such images, the foreground galaxy is always (?) rather red and the lensed galaxie(s) always (?) very blue. However it would be nice to if someone could provide a really definitive explanation of why this appears to be so.
If it is just as Ann suggests a question of filters, then the images we are seeing are pretty misleading as to true color and it would be interesting to see what the "really" look like.
Is there a possibility that there is some sort of "gravitational slingshot" affecting light from the background galaxy, resulting in more energy and net blueing?
Also are we missing any nice cases of lensing because we're depending on color contrast, enhanced or not?
It is not true that the background galaxies are always very blue. Here you can see an example of a lensed background galaxy that is (mostly) red. In the picture, you can also see the massive elliptical galaxy that undoubtedly provides much of the mass needed for the lensing effect. But you can also see a spiral galaxy at lower left. This spiral looks undistorted, which means that it is probably at more or less the same distance from us as the elliptical galaxy. But the lensed red galaxy is much farther away.
So why is the lensed red galaxy red and not blue?
Why does the lensed red galaxy contain such intensely blue spots?
Why doesn't the undistorted spiral galaxy contain such bright blue spots?
The reason why the lensed red galaxy looks red overall is because it contains a large and bright population of old yellow stars. And because the lensed galaxy is more distant than the elliptical galaxy and the undistorted spiral, the light from the old yellow population of the lensed galaxy has been redshifted and "stretched" by the expansion of the universe so that it looks red. But the elliptical galaxy and the spiral are not so far away, so their light has been less redshifted and "stretched". Therefore their overall color is less red than the overall color than the lensed galaxy. (But the intrinsic
color of the elliptical galaxy may well be redder than the color of the old population of the lensed galaxy. In other words, if the elliptical galaxy and the lensed galaxy had been at the same distance from us, the elliptical galaxy would probably have looked yellower.)
The reason why the lensed red galaxy contains such bright blue spots is because it contains clusters of very hot stars. The hotter a star is, the more ultraviolet light it will emit, and the shorter the wavelengths of the ultraviolet light will be. The blue clusters in the lensed red galaxy contain very hot stars whose "ultraviolet peaks" have been shifted into the blue part of the spectrum due to the expansion of the universe.
The undistorted spiral galaxy doesn't contain brilliant blue spots for two reasons. First, it is possible that the spiral doesn't contain the same kind of clusters as the distant galaxy. In other words, there may be fewer very hot stars in the undistorted spiral than in the lensed galaxy. Second, and probably more important, because the undistorted spiral isn't as far away as the lensed galaxy, not all of its ultraviolet light has been shifted into the blue part of the spectrum. A blue filter will therefore detect most or maybe all of the ultraviolet light from the background galaxy, but it will miss much of the ultraviolet light from the more nearby spiral.
By the way, it is typical for "ultraviolet clusters" to be very concentrated and to show up as "intense spots" through an ultraviolet filter. But a "merely blue" population may be much more spread out. After all, the most intense "ultraviolet knots" will be dominated by small groups of intensely bright O- and early B-type stars, whereas a merely blue population may be made up of large numbers of widely distributed modest A- and F-type stars.
Finally, star formation has generally declined over time in the universe. Therefore, more distant galaxies will, on average, contain more star formation and more blue and ultraviolet stars than nearby galaxies. That is why lensed background galaxies are often blue, and their blue color is exaggerated by the fact that a blue filter will pick up a lot of ultraviolet light that has been redshifted into the blue part of the spectrum.
However, to have a good "lensing effect" you need a very massive foreground galaxy, or better yet, a massive foreground galaxy cluster. But the most massive galaxies are almost always ellipticals, and these galaxies are almost always yellow in the nearby or moderately nearby universe. That is why you can expect to see a massive yellow elliptical star lensing a background starforming galaxy, whose ultraviolet light has been redshifted into the blue part of the spectrum, so that the galaxy looks blue.