JohnD wrote: ↑Mon Aug 23, 2021 11:08 am
Today's blurb seems fixated on explaining gravitational lensing, and does so well, but at the expense of explaning the picture. For those who are unfamiliar with "Abell 3827", like me, it is the four bright objects near the centre. A link to, say, the Wiki entry would help:
https://en.wikipedia.org/wiki/ESO_146-5
The blurb says that Abell 3827's light is redshifted to place it 1.3 BILLION light years away. Their light in the picture appears white, but as we know, astro picture colours are artificial. There are some VERY red objects in that same frame, bottom left and top right. There are more in the larger Wiki pic, so how far away are those??
John
The picture was constructed using the following filters:
U, ultraviolet, 336 nm. NASA calls this an optical wavelength, but it really is invisible to humans. Light detected by this filter is mapped as
purple.
V, visible light, 606 nm. This wavelength corresponds to
yellow-orange light. Light detected by this filter has been mapped as
blue.
I, 814 nm, infrared light. NASA calls this wavelength optical, but it is invisible to humans. Light detected by this filter has been mapped as
green.
H, 1.6 μm, infrared light. Light detected by this filter has been mapped as
red.
As long as we understand what the colors represent, I think we can more or less trust them. So why are some very distant objects blue, if they are so redshifted? First we must remember that objects that look blue in the picture may have reached us as yellow-orange light. (But there really may be some ultraviolet mixed in with the yellow-orange.) Second, any galaxy that looks blue in the picture was originally ablaze with brilliant ultraviolet light, and this light has been redshifted to yellow-orange light, or even to another wavelength of ultraviolet light. In short: Yes, back in the days when these galaxies were young (the way we see them), they were pumping massive amounts of ultraviolet light into space from extremely hot and massive stars, or in some cases, from an extremely brilliant active galactic center.
What about the very red galaxies? I'd say that while they are distant (obviously), they are not necessarily more distant than some of the small blue galaxies in the image. Their red color is not only due to distance and redshift-reddening, but also to the fact that at least some of these galaxies only emit longwave light. Either they are devoid of star formation, or else they are so dusty that the light from the hot stars inside them is completely smothered by dust. In fact, I think that one of both of the infrared filters used for this image are extremely good at detecting dust! We could be looking at dust when we see some of these red galaxies!
Bear in mind, too, that not all galaxies are created equal. Some are really big, while others are small. Some of the very red galaxies in the picture may well be quite large, and therefore much farther away than seemingly equally-sized seemingly nearby galaxies that are much less red.
Ann
Abell 3827: Cannibal Cluster Gravitational Lens
Because they really look alike! 
