ICEINSPACE
Most Read Articles
Moon Phase
CURRENT MOON Full Moon
99.9%
The Sun Now
Time Zones
Sydney
8:04 pm
Perth
6:04 pm
Auckland
10:04 pm
New York*
6:04 am
Paris*
12:04 pm
GMT
10:04 am




  #1  
Old 12-05-2018, 06:02 AM
Weltevreden SA's Avatar
Weltevreden SA (Dana)
Dana in SA

Weltevreden SA is offline
 
Join Date: Apr 2012
Location: Nieu Bethesda, Karoo, South Africa
Posts: 193
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 https://groups.yahoo.com/neo/groups/...messages/15545 (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
Attached Thumbnails
Click for full-size image (Magellan's Ghost, Robin Hegenbarth 2018.jpg)
92.6 KB82 views
Reply With Quote
  #2  
Old 16-05-2018, 09:46 PM
gaseous (Patrick)
Registered User

gaseous is offline
 
Join Date: May 2016
Location: Brisbane
Posts: 373
Great story Dana, thanks for sharing.
Reply With Quote
  #3  
Old 17-05-2018, 09:20 AM
Tinderboxsky's Avatar
Tinderboxsky (Steve)
I can see clearly now ...

Tinderboxsky is offline
 
Join Date: Feb 2012
Location: Tinderbox TAS
Posts: 564
Yes, very interesting Dana. Great reward for the effort that has been put in. Thank you for posting.
Reply With Quote
  #4  
Old 17-05-2018, 10:08 AM
astroron's Avatar
astroron (Ron)
Supernova Searcher

astroron is offline
 
Join Date: Mar 2005
Location: Cambroon Queensland Australia
Posts: 8,726
Thanks for posting Dana,very interesting results.
Great sleuthing
Cheers.
Reply With Quote
  #5  
Old 26-05-2018, 09:25 AM
geolindon (Lindon)
Registered User

geolindon is offline
 
Join Date: Apr 2014
Location: Carnarvon Gorge, Qld. Australia
Posts: 273
Dana,
I add my thanks for your posts on this subject and congratulations for your and other hardy Karoo inhabitants' contributions to the understanding of this feature.

After reading and digesting your article last week, i was perplexed this morning to read in American S&T: http://www.skyandtelescope.com/astro...llanic-clouds/

"It was originally thought that the Magellanic Stream was the result of tidal interactions during close encounters with the Milky Way, but precise proper motion surveys revealed that the LMC and SMC are either passing near the Milky Way for the first time or are in a long (~4-billion-year) orbit around our galaxy — so the Magellanic Stream must result from interactions between the two galaxies themselves."
"This post originally appeared on AAS Nova, which features research highlights from the journals of the American Astronomical Society." Its a summary of a paper by Dougal Mackey (ANU) et al 2018 ApJL 858 L21. doi:10.3847/2041-8213/aac175

I have not read the original paper. The findings re first pass of the Clouds is interesting but the summary assumes that the Magellanic Stream is related to the Clouds when your research shows the Magellanic Ghost is 160,000lyr closer to us than the Clouds.

I hope that my understandings are correct?
regards, L
Reply With Quote
  #6  
Old 27-05-2018, 07:00 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 686
Thanks for an interesting post. The field of astronomy moves so fast. I remember some 30 years ago hearing a talk by one of the astronomers from ?Mt Stromolo/AAT? about a gigantic trail of gas in non-visible wavelengths that arcs across the sky as evidence of a previous interaction between the LMC and SMC. Supposedly the vectors lined up. Reading your post my first thought was that maybe there is a visual component to this gas trail, but if the galactic cirrus is foreground, as you suggest, I guess that would not be the case. But then I wonder how distances to gas clouds are determined if they don't actually contain any stars?
Reply With Quote
  #7  
Old 31-05-2018, 08:54 AM
Weltevreden SA's Avatar
Weltevreden SA (Dana)
Dana in SA

Weltevreden SA is offline
 
Join Date: Apr 2012
Location: Nieu Bethesda, Karoo, South Africa
Posts: 193
Magellanic Stream -vs- Magellan's Ghost

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
Reply With Quote
  #8  
Old 31-05-2018, 05:04 PM
geolindon (Lindon)
Registered User

geolindon is offline
 
Join Date: Apr 2014
Location: Carnarvon Gorge, Qld. Australia
Posts: 273
Dana, thanks heaps for setting me straight and for such a detailed explanation. My knowledge is much increased!
regards, L
Reply With Quote
  #9  
Old 31-05-2018, 06:08 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 686
Yes, Dana, wow, I'm humbled by your expansive knowledge on the subject. Thanks so much for sharing. I feel like your post should be preserved somewhere for the good of all humanity.

Fascinating place, the universe.

Cheers

Markus
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 08:04 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
Advertisement
SkyWatcher 2018 Catalogue
Advertisement
OzScopes Authorised Dealer
Advertisement
Interest Free Finance
Advertisement
FLI Cameras and Imaging Accessories
Advertisement
NexDome Observatories
Advertisement
Atik Horizon
Advertisement
Bintel
Advertisement
Lunatico Astronomical
Advertisement
SkyWatcher WiFi Adaptor
Advertisement
Astronomy and Electronics Centre
Advertisement