[Placidus (Mike and Trish)]

The H-alpha represents where the great bulk of the star-forming material is. (There can of course be far more hidden within dusty areas).

The OIII represents where there is hard (high energy per photon) UV light, typically from young, super-hot OB stars formed from the dust.

The OIII will not usually be exactly co-located with the H-alpha, because gravitational collapse to form the OIII stars is unstable, running away once started, and therefore localized rather than smooth and uniform.

Because of their very high mass and therefore very high temperatures, these stars do not live long - say a few million years as opposed to ten billion for the sun. They eventually explode as a type II supernova.

The supernova explosions regurgitate highly processed heavy elements from deep within the stars - sulphur and nitrogen being the most relevant here.

(Some very large, very heavy, very hot stars called Wolf-Rayet stars can liberate heavy elements from the core during their pre-supernova lifetime. Again this process is very localized and often very assymetric - see NGC 3199 in Carina).

The supernova explosions will be even more localized, and in beautiful examples like Gabriela Mistral, or just about anything in the Magellanic clouds (where star formation is proceeding with great speed and violence due to tidal disruption), you will see extremely localized SII structures as shock fronts and super-bubble structures around the sites of recent supernova explosions. The Chalice Nebula is an excellent example.

Later, as the supernova material gets more widely dispersed, these structures will be lost, but the SII will still mark out the broad areas of past supernova activity.

One final reason for asymmetry in very large structures, such as the area around the Statue of Liberty nebula: Galactic scale compression waves that form spiral arms move through a star-forming region, initiating star formation on one side before the other. H-alpha will be the first to appear, SII the last, and as the wave moves through, that will produce very large scale asymmetry.

Just a peek in the bookshelf for some relevant textbooks that discuss parts of the process.

Phillips: The physics of stars.
Bally and Reipurth: The birth of stars and planets.

Green and Jones: An introduction to the sun and stars.
Wheeler: Cosmic Catastrophes.
Kitchin: Galaxies in Turmoil
Percy: Understanding variable stars
Jones Lambourne Serjeant: Galaxies and Cosmology.

Hope that helps explain some of the mechanisms behind asymmetry of H-alpha, OIII, and SII distribution in star forming regions.


Best,
Mike