Chris Peterson wrote:
Ann wrote:As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.
But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.
Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?
In the image, red is the combined light of F160W, F125W, and F098M; red thus covers 0.9-1.7 micrometers. Green is the combined light of F814W and F606W; green thus covers 460-950 nm. Blue is the light from F390W; blue thus covers 330-440 nm. However, the lensed galaxy has a redshift of z=1.7. So in terms of the emitted wavelengths, red = 630-333 nm, green = 352-170 nm, blue = 163-122 nm. So what we're seeing is UV in the green and blue channels and most of the visible spectrum mapped to red.
Thanks, Chris, really.
Make of that what you will in assessing the appearance.
NGC 4449. Photo:
My real question was, is the color mapping different in the rectangular box than in the elliptical outlines, given that the rectangular box supposedly shows a "more realistic" portrait of the galaxy?
My implied criticism is that whoever constructed the "more realistic" portrait of the galaxy perhaps wanted the galaxy to look "normal", at least in terms of color and stellar populations. My implied criticism is that this galaxy may have been quite different from most galaxies in the nearby universe, and therefore we should, perhaps, not try to give it "normal colors" or force it to appear to have "normal stellar populations".
However, it is possible that the lensed, blue-looking galaxy had a yellow population that was redshifted all the way out of the sensitivity of the filters used for this image. So yes, it is
possible that a "reconstructed" portrait of the galaxy's "real appearance" should
show it as having a yellow population.
My guess, for what it's worth, is that the lensed galaxy was relatively similar in its stellar populations to a starbursting present-day dwarf galaxy like NGC 4449. NGC 4449 certainly contains a yellow population of stars, but the yellow population is clearly not very bright and absolutely not the dominant component of of the galaxy.
By the way, and if you are interested, here is a splendid 1 MB portrait
of the central part of NHC 4449.