Coalsack Loop

[matlud]

Surrounding the Coalsack is a super shell of Ha emission, known as the Coalsack Loop, which spans ~10 degrees or 20 full moons across the sky. Given its immense size there are few images of this shell. I captured this image over 2 nights from my backyard in Dunedin, NZ. I used a Nikon 200mm lens at f2 with an ASI6200mm Pro camera. This setup covers a 15.5 degree diagonal FOV with a resolution of 3.87”/pixel. Processed in Pixinsight and Photoshop, it was fun dealing with the dense star fields :). It’s a pretty interesting area, and there is a lot going on throughout the frame, so I did some investigation into some of the main features:

http://www.iceinspace.com.au/forum/attachment_browse.php?a=273945
Astrobin link: https://www.astrobin.com/4rm4wv/

OB Association Stars Powering the Shell
The Coalsack Loop was first reported by Walker & Zealey in 1998, and at that time they though this shell was most likely the expanding remains of an old supernova remnant. However the Coalsack Loop is located around the Centaurus OB association, and it is thought that this association is powering the Ha super shell. OB associations are loose star clusters that contain massive stars of spectral type O and B. In addition to these OB stars they also contain thousands of smaller mass stars. The Centaurus OB association is one of the most prominent star- forming regions of the Milky Way, and we are all familiar with some of the associations more famous members which form the Southern Cross and also NGC 4755 or the Jewel Box cluster. The Coalsack Loop shell shape is quite similar to the OB association star distribution which suggests an interaction is occurring between the OB association stellar winds and the shell material. In addition, it has been demonstrated that the OB stars contribute enough wind injection to explain the size and expansion velocity of the shell.

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=273946&stc=1&d=1618008948
Astrobin link: https://astrob.in/4rm4wv/B/

I have mapped the OB stars that are located at a similar distance to the Coalsack Loop Ha shell (labelled yellow). These stars have been reported to be significant in terms of providing the wind injection energy that powers the shell. It is interesting that these appear to be more concentrated on the Northern aspect of the loop.

OIII Emission Features
I did not expect to see a lot of OIII emission given the high extinction in this area, but there are distinct OIII shell structures in the Southwest (top right) and Northeast (bottom left) aspects of the loop. After a lot of searching, the Southwest shell appears to be an emission nebula associated with Theta Muscae, which is the second brightest known Wolf-Rayet star. There has been debate whether Theta Musca actually has an associated nebula. A WR nebula is usually defined as arcs of nebulosity centered on and ionized by the Wolf-Rayet star. As the Ha emission around Theta Muscae appears to extend into and is continuous with the larger outer Coalsack Loop emission it has been suggested that this Ha emission is not associated with Theta Muscae but is more likely an old SNR.

However, this image suggests that Theta Muscae does have an associated WR shell nebula. This image is possibly the first wide field image that shows the entire OIII emission surrounding Theta Muscae. There is a distinct OIII shell of ~1 degree diameter that appears to be largely centered on Theta Muscae, and the OIII shell morphology appears to be quite different to the adjacent Ha emission. Futhermore, there are inner OIII emission shells located closer to Theta Muscae. Overall, these OIII features appear distinct from the Coalsack Loop, and are suggestive that they are a WR nebula.

In the Northeast aspect of the Coalsack Loop there lies a further faint OIII shell. This appears to be a nebula associated with a further Wolf-Rayet star WR52, which is listed in the 7th Catalog of Galactic Wolf-Rayet stars as having a Ha bubble. In this image there also appears to be a OIII emission bubble as well, of which I can not find any previous mention of.

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=273947&stc=1&d=1618008948
Astrobin link: https://astrob.in/4rm4wv/C/

This image demonstrates the OIII emission shells that appear to be largely centred around Theta Muscae, with an outer diameter of ~1 degree and around WR52, with a diameter of ~0.5 degree. Variations in the shells may be caused by differing densities of the surrounding ISM.

Cheers,
Mat

[Robert_T]

Superb, Mat... a good example of something big hiding in plain sight...well no but you get me. Impressive structure and 3D depth pulled out of this area which I assume must be relatively faint/diffuse. And the attention to details in the write-up. Love it!

[graham.hobart]

that is splendid- well done and one for the target list whenever my house moves!

[matlud]

Thanks for the kind feedback Robert :). You are right, the outer loops are very faint and in a pretty dense star field so was a good challenge to bring them out.



Thanks Graham :), the area is definitely packed with interesting targets.

[gregbradley]

A fantastic image Mat. I first heard of the loop around the Coal Sack a few years ago but have never imaged it.

Are you using the ZWO filter wheel to be able to attach a lens?

Greg.

[Andy01]

Awesome work! A welcome and fresh view well researched & processed.
This region has been on my bucket list for some time, but now your wonderful image has really inspired me to get it on here! Thanks for posting :thumbsup:

[matlud]

Thanks heaps Greg. The imaging train required some thought given the limited back focus. I use the EFW7x2 filter wheel attached directly to the camera (with the camera tilt plate removed) and then an Astro-mechanics EF adapter attached to the filter wheel. This adapter allows AF on Canon lenses but I decided to use this as a pure mechanical adapter as it is well made and has tilt adjustment built in. I then use a novoflex EF to Nikon F adapter. I'm pretty pleased with it as it is mechanically solid and the lens mount is very snug with no play, and it allowed me to adjust tilt.



Thanks Andy, I really appreciate the feedback :). Its nice to show something not often seen.

[multiweb]

That's pretty cool. :thumbsup: A good lens is great to highlight those massive structures and dark skies are an important part as well. You're still getting a decent image scale at that FL. That lens is very sharp and well corrected.

I attempted the Eridanus loop in Ha from my backyard years ago when it was still relatively dark but it was hard yakka.

[strongmanmike]

Yeah, great work Mat, very interesting, knew it was there but we tend to forget, so, nice to see it showcased like this...cool stuff :thumbsup:

Mike

[marc4darkskies]

Now that's a wide field image!! Not often seen in all it's glory - very nicely done Mat and a very interesting read too! :thumbsup: Pleased to see it's natural palette as well! :)

[matlud]

Thanks Marc. With the small pixels the image scale is pretty good given the FOV and I have been very happy with the amount of detail the lens is resolving :)




Thanks Mike, appreciate it :thumbsup:





Thanks Marcus - I tried to get the Ha and OIII to blend in and not affect the natural colour of the dust etc so glad you liked it :)