Great detail there Mark.
I'd guess the noise is from being a bit overstretched. Try resetting the black point a bit lower and using a light Gaussian Blur using colour range selection on the faint outer parts of the galaxy and background. Apply a light feather before using the Gaussian.
There's also a slight reddish cast to the background, maybe raise the red blackpoint a bit higher in levels or lower the red curve down the black end in curves.