Boomer12k wrote:So...um.... why are they blue again? Ann?
Ultraviolet light + sufficent redshift = blue light.
NGC 4449, a starbursting
galaxy in the nearby universe.
Photo: R. Jay GaBany.
Basically the blue arcs are blue because they are stretched and redshifted images of galaxies which are dominated by hot ultraviolet stars. These galaxies haven't had time to build up large yellow populations, and they haven't had time to create copious amounts of dust, either. Their light output peaks at ultraviolet wavelengths.
But since the galaxies are so far away and their light has traveled so far to reach us, a good deal of their ultraviolet light has been redshifted by the expansion of the universe into longer-wave visible blue and violet light.
Look at the image at right. A spectral line that was originally located exactly at 400 nm in the violet part of the visible spectrum has been redshifted to a wavelength of 530 nm in the green part of the spectrum. Please note that it isn't just the spectral line that has been redshifted, but all the violet light that was originally emitted by the galaxy has been redshifted into the green part of the spectrum. The violet light that is visible in the redshifted spectrum was originally ultraviolet light.
At left you can see three types of dwarf galaxies in the Coma Cluster. The important point is that the blue dwarf galaxies (middle row) are dominated by shortwave light, whereas the red galaxies (bottom row) are dominated by longwave light. Distant shortwave galaxies may look blue because of redshift.
The gargantuan elliptical galaxy in the foreground of today's APOD is likely made up almost exclusively of old yellow stars. This galaxy produces very little ultraviolet light in the first place, so there is precious little ultraviolet light to be redshifted into the blue part of the spectrum. This galaxy therefore looks very yellow - indeed, it is so far away that the light it emits is redshifted too, making the galaxy look a lot more orange than it would have been if it had been as nearby as the Virgo Cluster ellipticals.
I think today's APOD is gorgeous. There is so much going on in it. The background galaxies are splendid. There is such a menagerie of them! It's fun to try to classify them by visual inspection, and it's fun to guess if one or more of the background galaxies have been lensed into two or more images.
Of course the foreground elliptical looks like a real monster. I's a galaxy cluster, you say? It looks like one huge slightly (American) football-shaped monstrosity. I wonder how much mass it contains. And I wonder how massive its central black hole might be, and how massive it could become in the future. Gravitational waves out of merging black holes in Abell S1063, anyone?