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The Herschel 400 Quest

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#76 Tony Flanders

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Posted 31 August 2019 - 09:39 AM

I said:

NGC 4565 is spectacular, but when all is said and done, it's an edge-on spiral, and edge-on spirals tend to have pretty low surface brightness.

And Roger Corbett corrected me:

As far as I've observed over the years -- and as confirmed by observing reports and expert commentary here and elsewhere -- edge-on spirals have GREATER surface brightness than face-on ones do.

He is quite correct, of course. If you ignore the presence of dust, then the total amount of light that you can see from any given galaxy would be unchanged if you could somehow reach out and twist it to any angle you wanted. Since the total light is constant, and the apparent area is bigger when a galaxy is face-on than when its edge-on, that means that edge-on galaxies generally have higher surface brightness than face-ons.

However, galaxies do have dust, and when viewing NGC 4565 -- or our own Milky Way, for that matter -- the dust happens to block out most of the galaxy's core, and therefore a large fraction of the total light.

It's the absence of a bright core that makes both NGC 4565 and M33 hard to spot, especially in light-polluted skies. In the case of NGC 4565 and some other edge-ons, dust is the culprit. In the case of M33, it's because the galaxy is a bit of a freak, with an abnormally small and faint core.

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#77 Redbetter



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Posted 02 September 2019 - 01:22 AM



Both views are correct from what I have seen and I agree with you in particular about the potential blocking of the view of the bright core by dust being a complicating factor.  Edge ons run a range of surface brightness.  The "popular" ones often have high surface brightness, but there are a number that have more modest brightness.  On average the less dusty galaxies that happen to be near edge on should have higher surface brightness.  So the average is probably skewed.


There is another contrary element that I have noticed hunting for threshold galaxies with the 20" in dark skies:  aspect ratio.  For a given surface brightness I find that some of the most difficult galaxies to detect are edge on even when they should be detectable based on magnitude alone.  In images they might appear to be targets that would not be difficult compared to similarly bright ovals, but in practice this is often not the case when both are near threshold. 


My guess is that the eye can better detect a round or oval patch than what is becoming something more of a thick (or thin) line--particularly when there is little or no brightening of a core/bulge.   That is ironic because while the eye resolves bright high contrast lines (e.g. Cassini's division) , it is poor at resolving such lines when the brightness is low (threshold galaxies, resulting in far lower effective contrast.)  Perhaps another demonstration is represented by how difficult it can be to resolve photographically prominent dust lanes in edge on galaxies.  While we think of dust lanes as high contrast, the image brightness is still many orders of magnitude less than planetary. 

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