[Weltevreden SA (Dana)]

Hi Markus & Lindon . . . Please excuse the tardy reply, I've been out of internet range in the wilds of our Karoo.


There's no relationship between the Magellanic Stream & the newly dubbed Magellan's Ghost. The former is a huge in size and in mass gas streamer torn loose over the last billion-plus years by the orbital interaction of the LMC and SMC. They loop lopsidedly around each other and have had ram-pressue & tidal stripping encounters 6, 4, & 1.3 billion years ago, plus a direct hit 300 Myr ago which ripped loose one entire arm of the LMC and decoupled the bar from the disc. The younger bluish stars of the LMC bar are now tilted at about 9° to the NW & around 1000 light years above (our side) the older reddish stars of the LMC bulge. Dual populations like this are common in barred galaxies because bars are younger features and evolve in shape and mass out of matter injected from the spiral arms, while bulge star populations are the older primordial galaxy core.



The LMC is so much larger than the SMC that guess who loses most when they tangle. (There are no winners in their marital mismatch.) The Magellanic Stream blows backwards at an angle from the SMC across such a vast expanse of sky that it reaches almost to the constellation of Andromeda. We can't see the Stream because it is atomic hydrogen. The Stream is over a billion solar masses of atomic hydrogen and will eventually settle into the Milky Way's halo, while the SMC becomes gas-starved and dead. The LMC and SMC may merge after one or perhaps two more mutual interactions. Surprisingly, they are far less disturbed by our galaxy than by each other. The LMC is presently in the lead on a passage through our outer MW disc and will cross it at about 40 kpc out. By compare, we are only a bit over 8 kpc from the core, so the Magellanics really are out there in the boonies. The Magellanics are on a hyperbolic orbit, meaning they won't be captured by the MW. However, no study I have read takes into account the far-into-the-future slide-by or sideswipe collision of the MW with M31. Starting about 2 billion years from now that will change everything so drastically no one can predict what will happen except that it sure would be fun to watch in high-speed video.



The mix-up of the LMC and SMC has also pulled two "bridges" of stars out of the SMC, an older one slightly bowed on the upper side of the line that connects them, the newer and much fainter one on the lower side. These are products of the last two interactions. Bridges like these are products of the "tidal taffy" effect of gravitation on the stars in galaxies, while ram pressure stripping affects mainly the gas (i.e., the Magellanic Stream).


None of this has anything to do with the Galactic cirrus at the heart of Magellan's Ghost. It is only by sheer chance that the Ghost is aligned such that it looks like it's a cometary tail streaming from the LMC. It's possible that the LMC does contribute a slight bit of the extra illumination that lifts the Ghost above our eyeball threshold level, but it would take some exotic telescopery and a big checkbook to prove it.



Galactic cirrus is dull stuff on its own: cold, thin, & sooty. Your wife would have fits if it was on top of the furniture. It's a different breed of cloud than the dense blotty things we see in the Dark Doodad, Coal Sack, Lupus, & etc. Those are mainly dense molecular hydrogen & dust, while cirrus is somewhat like our atmospheric kind, basically thin frozen dust with little gas. The tell-tale clue is that cirrus is filamentary or striated, while molecular clouds are blobby and dense. Most of the currus has been blown high into the disc by furious star formation shock waves in the disc. We detect it in images with very v-e-r-y long 10- to 30-hour exposures in the deep red and near IR bands. The professionals put chemical and mass data into those numbers not by measuring their emission, but by how much the cirrus absorbs light (mainly UV) from billion-plus light year quasars. That particular absorption technique is used a lot in studies of nearly invisible gas clouds that surround our galaxy.



To make this long story shorter, it's sheer luck that a swerve in our nearby spiral arm's magnetic fields have aligned vast numbers of silicate dust (not the same as sooty carbon dust) in such a way that they reflect a small amount of overall light from out Galactic disc at an angle that happens to strike our eyes. Silicate dust looks more like needles, while carbonaceous dust is round specks. Astonishingly, silaceous dust particles spin because of electromagnetic forces at rates upward of 10 million times a second. We're really looking at a huge number of tiny airplane propellers reflecting light off the blur. If we were 100 light years further up or 200 ly down below in the disc, we would miss the whole show.



Very few people have reported the Ghost, in part because we need to be so far away from any urban light domes and also in high-altitude, dry air. In suitably dark skies it looks like the LMC is a large comet head and the Ghost is its extremely tenuous tail. It passes through the vee of Mensa, parallels Chameleon 3-5° on the poleward, and crosses the northern edge of Apus. I think many don't see it even under suitable skies simply because the thing is so long and faint it's hard to believe anything that big can exist with no star clouds or glowing HII nebulae in it. It looks like a gigantic version of the beautiful Centaurus galaxy NGC 4945 in a 6-inch scope at about 120x. The one thing that won't help is optical aid. It's simply too big for binoculars. Surely it must be one of the very few things we can see only naked eye.



Hope this helped . . . =Dana in S Africa