[sjastro]

I have been able to isolate and measure the Gaussian noise contributions in the areas highlighted by ESO for investigation.

The Gaussian noise was measured in the darks and flats used in the image reduction process for the luminance image. The darks also include bias noise.
The Gaussian noise from stacking and stretching the image was also evaluated. No sharpening was performed.

Using this information the percentage of Poisson noise in the luminance image was calculated.
Poisson noise from extraneous sources such as light pollution, vignetting, hot spots and field flooding were eliminated as possibilities.
Therefore the Poisson noise was from the external halo itself.

In region 1 (the area above ngc 253 in the attachment), the percentage of Poisson noise in the total noise was 79.6%.
In region 2 (below ngc 253) the percentage was 81.8%.

As these percentages are high I believe the external halo is real.
Hopefully ESO will confirm this by their own noise evaluations on the same image.

Regards

Steven