Why are lenses blue?
Assuming that the light bending around the ellipticals is coming from behind, the light of the galaxy being lensed, it seems to me, will be tend to red or orange, since it's far away, and therefore red-shifted. Yet lenses are portrayed as blue. Why would light focused around a galaxy get blue shifted?
Note, this image just happens to be what I was looking at while wondering about this. It really has nothing to do with my question.
by Tom_Collett scientist
Good question. I said this somewhere else but I'll repeat it here:
Basically blue equals hot and the color in these images is defined by the difference
in brightness at different frequencies Redshifting is broadly
irrelevant to the colors because of the shape of the source's emission spectrum. Imagine an object emits 10 units of UV
light, 8 of Blue, 6 of Red etc. We call color the difference in
brightness at different wavelengths, so the Blue-Red color would be 2,
and the pixel would be blue in the image. Now let's redshift it a bit
so that the UV becomes blue, the blue goes to red etc. The new
Blue-Red color would still be 2 (even though that's the UV-Blue color
in the rest frame). If we redshift the object even further, it will
eventually appear red, because there is a break in the emission
spectrum (Not much gets emitted beyond the far ultraviolet), but that
doesn't tend to happen to the sources likely to be found in spacewarps
because they would be super-faint due to the distance.
The picture I've presented is a little simplistic, but is qualitatively correct. Colors do change with redshift, but nothing like as much as you'd naively expect and not always in the direction you expect either. (Redshifting can make things look bluer in some cases!)
Hope that helps.
by anupreeta scientist, admin
Tom made some important points there..let me say a bit more
What appears blue to us has many reasons - here's a different angle to the question raised here:
I could make the same image with magenta-green-brown and then most lensed images might appear magenta to you. The point being the background lensed galaxies that are picked up by the telescope happen to be brighter in a certain wavelength range. We've chosen the blue color to represent that wavelength range - also because that wavelength range would look blue to your naked eyes. But this wouldn't be true if this image was taken in X-rays since our eyes can't see light at those wavelengths.
Redshifting certainly occurs but also depends on - by how much. It also depends upon what's the wavelength of light in question. Addressing the former part - for example, if the redshift is high, you might be looking at optical light stretched to look like infrared or microwave (invisible to your eyes). If the redshift is low, you might see yellowish red light turning into extremely red. The galaxies being lensed here are mostly bright in the bluer (UV-optical) wavelengths and don't get redshifted by much, so the light we measure in our telescopes also appears bluish for most cases.
There are some galaxies which don't have much blue light (ie. not many blue young stars), their lensed images would appear red. Apparently, these are not as abundant in the redshift range we are exploring with the current survey (CFHTLS). But you do expect to find a few!!
Btw, here's a famous lens with a red arc 😃 http://apod.nasa.gov/apod/ap090921.html
Wow. That was an amazing image and a HUGE galaxy cluster.
And great of explanations. Thank you both. Very much clearer now. (As usual, once it's explained I wonder, Why didn't I think of that?). 😃