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Old 12-05-2018, 06:02 AM
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Weltevreden SA (Dana)
Dana in SA

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Location: Nieu Bethesda, Karoo, South Africa
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Magellanic mystery solved at last

Over the years since 2005 a handful of amateur astronomers have reported a wide, elongated swath of vanishingly faint emission in the deep Southern skies. It begins near the SW corner of the LMC, brightens as it passes through Mensa, parallels the long axis of Chameleon 4° to 5° S of the chameleon’s body, touches Octans near the S celestial pole, brightens somewhat as it transects the body of Apus, and fades imperceptibly into Triangulum Australe. It is variously described as a “broad, even band of emission” “A cometary tail streaming from the LMC,” “About as bright as the the Zodiacal light as it nears the gegenschein,” and “Resembles an elongated form of the gegenschein.” Despite the legions of Australians and South Africans who search their skies as avidly as northerners look at Cygnus or Cepheus, only nine observers have seen or imaged it. One of the first was Brian Skiff of Lowell Observatory, who saw it several times from Las Campanas starting in 1989. Historically, it has been imaged only twice: once in 1955 by Gérard de Vaucouleurs and again in 2010 by Hisayoshi Kato. In 2016 after several observers requested better images, Kato went back for another imaging run and this time bagged it successfully enough that his images pretty much match the observers’ descriptions.

The very first observer to report it, Dave Riddle in 2005 (on Amastro, you might have to sign up for the link to work) guessed it to be Galactic cirrus. Timo Karhula reported it on Cloudy Nights in 2009 and IIS in 2011. All the other observers since then — Timo Karhula, Chris Beere, Dana De Zoysa, and Robin Hegenbarth — have also thought it must be cirrus. There are only two references to the emission in the historical literature, by Pannekoek in 1929 and Sergey Gaposhkin in 1958, but their descriptions are vague at best. It has been ignored by the larger professional community. There is no specific reference to it anywhere in the literature.

Hence we have a mysterious emission that has been reported by a few reliable observers and confirmed with a few images, but no explanation anywhere in the literature of what it is. Several months ago a small team of South Africans affiliated with the Astronomical Society of S Africa (ASSA) and assisted by Robin Hegenbarth in Germany, undertook to get to the bottom of it. The final report is done, and accessible here. (23 MB, be patient.) The report, “Magellanic Mystery”, names the emission “Magellan’s Ghost”, in part because that’s what it looks like, and in part because there are so many other names associated with these galaxies — Magellanic Stream, Magellanic Bridge, Leading Arm, etc., that “Ghost” is easiest to remember.

The report is 67 pages and contains 65 images. Nearly all the images will be new to northern observers. The report concludes that everyone was right: we are seeing Galactic cirrus. If Mel Bartels lived in S Africa or Australia we would have scads of his marvellous drawings of it. But we don’t have Mel, so we have to dig up the evidence the hard way: through the literature.

While there are abundant supplies of Galactic cirrus (GC) in thin lace-like clouds all over the sky, it is so cold (±15 – 20 K) that it emits very little radiation — and indeed absorbs as much as it emits though at different wavelengths. Most GC emits at about 26.5 to 28 MPSAS, while the limit of human vision is generally accepted as 25 MPSAS. A few Northern observers have reported seeing the Polaris Flare and “Angel” emission near M81–82, which have been measured at 24.5 MPSAS. (By comparison, the surface brightness of the Sculptor Dwarf galaxy in the central 15 arcmin is 24.5 MPSAS.)

Magellan’s Ghost, though, is much larger — as wide as the LMC and about 30° long. Despite its location, it is completely unassociated with the LMC. The Ghost is about 240 pc (780 lyr) away and well within our galaxy’s thin disc, while the LMC is 160,000 lyr further out. Deep images of the entire region show abundant GC clouds, yet the Magellanic Ghost manages to outshine all the rest by almost 2 magnitudes. What’s more, it lies in a nearly straight line and is longer than any other single emission in the sky except for the Milky Way itself. Why?

The answer to that is a long excursion through the arcane world of astrophysics, galactic dust, and the complexities of magnetic fields in our MW spiral arm. We can see the Ghost because by sheer chance a perturbation in the MW spiral arm in the Mensa–Chameleon–Apus region aligns the flat, elongated silicate grains of the Galactic dust in that region in such a way that they form a flat, thin, low-angle reflection surface. The reflection angle catches just enough background light from the entire spiral arm to raise the cirrus’s normal brightness from 26.5 MPSAS up to about 24.5 MPSAS.

Any ATM who has ground their own mirror knows the experience of holding the partly-ground mirror up to the light, washing off the grime, and seeing the glass surface to be curved just like it is supposed to if you’re looking across an f/8 piece of glass at a very shallow angle. Magellan’s Ghost is much the same: a low-angle forescatter reflection surface. The Ghost’s intrinsically low surface brightness and huge relative size require human eyes to be looking directly at it from the darkest possible locations on Earth. Only four places have those skies: Namibia, the Karoo region of S Africa, the Australian Outback, and the Chilean Andes. Naturally, such regions are inhospitable to long-duration human residence. You can certainly live there, ask any mole or bat. But living there with a telescope is another matter. That is the main reason for the dearth of observations. Nonetheless, observe we do, and some of us see there’s a Ghost up there, sailing along the Magellan’s clouds, the only two clouds in these skies.

As mystery stories go, Sherlock would indeed have been impressed by so circuitous a clue.

=Dana in S Africa
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Old 16-05-2018, 09:46 PM
gaseous (Patrick)
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Great story Dana, thanks for sharing.
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Old 17-05-2018, 09:20 AM
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Tinderboxsky (Steve)
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Yes, very interesting Dana. Great reward for the effort that has been put in. Thank you for posting.
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Old 17-05-2018, 10:08 AM
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astroron (Ron)
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Thanks for posting Dana,very interesting results.
Great sleuthing
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