form a large fraction of all galaxies in the Universe!
They are old, have correspondingly B-V color ~ 1, and therefore are colored in orange shades.
A good rendering of ellipticals in celestia.Sci
is therefore very important. The code is completely new in comparison with the latest Celestia 1.7.x SVN version.
I would say the rendering is almost photorealistic, at least compared to the resolution of the SDSS survey. Here is a familiar example:
Displayed are the orange E2 galaxy M 60 (NGC 4649) and superimposed the small bluish SBc spiral NGC 4647
. The image is from SDSS, and the SDSS color profile is used in the comparisons with celestia.Sci below:
In celestia.Sci this nice galaxy couple looks as follows:
You see that the rendering is pretty close to the SDSS photo (no stars, here)!
There are many examples of whole clusters of elliptical galaxies in excellent agreement with the SDSS survey, but this is for another thread...
The purpose here is to demonstrate that the light distribution of ellipticals as rendered in celestia.Sci matches perfectly the famous de Vaucouleurs 'R^(1/4)' law
as it should be.
For the less initiated reader: Here is an example of the measured
major axis surface brightness profile, mu(r), of the E1 elliptical galaxy M 105 (Ngc 3379) (solid line) compared to the de Vaucouleurs law
mu(r) = mu_e + 8.32678 [(r/r_e)^(1/4) - 1].
The parameters are: mu_e = 22.24 [mag/arcsecs^2] and r_e = 56.8" (2.7 kpc).
r is the distance from the galaxy center in arcsecs.
You can see that the fit is excellent (as usual)! The example is from deVaucouleurs' great lecture:
http://www.google.com/url?sa=t&rct=j&q= ... 2042,d.Yms
After some quite complex shader-based rendering code, it is most interesting to check the resulting brightness distribution directly
in the visualization!
This is what I did to test it: I chose a suitable E0 elliptical galaxy, blew it up, converted it to grayscale as a color average and saved it. Loaded into GIMP, I measured with the color-'pipette' the actual brightness (0..255) as function of the distance (pixels) from the galaxy center. Then I plotted the result in Maple and compared it with the theoretical result, i.e. the deVaucouleur brightness law:
Here is the plot about which I am quite happy:
The blue points correspond to my measurements of the brightness by means of the GIMP 'pipette', while the red curve displays deVaucouleurs' law. Since the display is 8bit, the brightness is clamped at 255 as you can see near the origin.
The rendering of elliptical galaxies in celestia.Sci is not just pretty to look at, but also, the brightness distribution corresponds rather precisely to the successful deVaucouleurs law!
Note that for well-known reasons, we want a logarithmic mapping
(i.e. linearly with magnitude) of a galaxy's (surface) brightness to the pixel values of the visualization! Hence I considered above log
(I_deVau(r) / I_0). This important fact I forgot to emphasize above...