Let me discuss next some rich HII starforming occurrences in (dwarf) irregular galaxies
Before doing so, some more general remarks are adequate:
As mentioned above, the H-alpha spectral line that indicates the presence of starforming regions has a wavelength of 656.3 nm
. In the standard basis with visual blue, green and red filters, this wavelength corresponds to a conspicuous pink color
However, some 3 color imaging, notably from SDSS, uses instead a green, red and near infrared filter basis
mapped onto the visible blue, green, red colors.
Effectively this turns the pink 656,3 nm line into a conspicuous bright cyan blue color
!! (confusing??) . This can be discussed more mathematically, of course.
Note, color photos can vary quite a bit, even if coming from the same (Hubble) address. Besides the NGC4449 image above,
http://forum.celestialmatters.org/userp ... ions_1.jpg
here are two Hubble alternative images of the dwarf irregular galaxy NGC 4449 with conspicuous pink H II patches:
[Click on images by all means!]
Apparently, among these NGC 4449 images, the displayed differences in colors and the intensity of the H II regions are quite dramatic. Therefore finding an objective criterion of relating different color shadings is essential for a sensible visualization of colors. Color profiles are supposed to be general, flexible and quantitative tools, but due to the complexity of the matter still represent work under development.
Since we cannot compete with a Hubble type resolution, one approach for comparing with celestia.Sci
consists in blurring the photographic galaxy images to a matching degree. Along these lines, the first NGC4449 image above, http://forum.celestialmatters.org/userp ... ions_1.jpg
was rotated and blurred with a Gaussian blur of 35 pixel and then reduced in size => image on the right below):
In this qualitative comparison with NGC4449 from celestia.Sci (left) you can see a rough correspondence of the red HII patches in a statistical sense. Much depends here, however, on the amount of graphical HII enhancement that the Hubble people have implemented. See also the alternative images at the beginning of this post.
But there is more: While in the celestia.Sci image (left) I used the SDSS color profile, I rather chose the visual pink color
for the HII regions! For SDSS, based on green, red and near infrared filters, the consistent choice would have been the bright blue HII color
! Of course the HII colors may be flipped with a click. The natural pink color can be better seen, of course... Here I am still experimenting.
Finally, a couple of SDSS images with correct bright blue for HII starforming regions:
1) M51 with its starforming HII along the arms now in blue instead of pink (see Hubble image above! http://forum.celestialmatters.org/userp ... arge_1.jpg
2) NGC 2403
I hope that in this discussion the eminent importance of a general color profile framework for sensible comparisons of color in the Universe became apparent