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Old 26-03-2014, 03:09 AM
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Weltevreden SA (Dana)
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More about the southerly LMC halo clusters in Mensa

The LMC is a terrible place to plan for a long, lovely holiday. A simple sweep at 30x using an 80mm refractor unveils a dazzling variety of every kind of object except empty space. There are over 1800 star clusters alone. Broaden your definition of ‘interesting’ and you’ll wish you hadn’t: the Bica et al survey done in 1998 lists 7847 ‘extended objects’. Thankfully, that doesn’t include binaries.

Paddy, IIS’s resident guru about matters Magellanic, has done a magnificent job of making sense out of our Galaxy's closest neighbours. Quite by accident I came across a star chart that showed 10 globular and open clusters far to the south of Paddy’s limits. They turned out to be some of the more demanding objects I’ve ever tried, and therefore worth ever minute out there in the cold winds of the Karoo (not cold enough for the mozzies, alas) looming over the eyepiece like a vulture waiting for something to glimmer. Six hours over four nights and all I had to show for it was between three and a dozen tick marks under the various objects listed by number. I wonder if psychologists have considered studying we astronomers for latent masochistic tendencies coupled with a curious record of narcolepsy at dawn.

The 10 clusters I introduced yesterday are piffles in a puffball on the grand scale of things. Yet they have been so little studied that they may have the potential to add a new facet to globular cluster lore: the chemical history of delayed adolescence. What’s more, two of them—NGC 1777 and 2209—are open clusters, not globulars. Those two are also the toughest to get an unequivocal sighting. I’m still not entirely sure what I saw of them wasn’t wishful glimmer.

Yet it wasn’t for days after I observed them that I perused the literature using their NGC and IC numbers and discovered I had stumbled upon what we non-PhD lot regard as a sort of Ocular Holy Grail—something very few people have ever noticed, and even fewer have studied. The NED and Simbad data on them makes pretty skimpy reading. An ADS search for ‘LMC halo globular cluster’ brings up Kenneth Freeman’s name in items #3 and #4—in papers dated 1983 and 1987. As a category of study, the Magellanic halo clusters are a PhD or two awaiting suitable candidates.

Start with the difference between a globular and an open cluster. Do the Magellanic halo clusters follow different set of rules than the Milky Way’s? Let’s try age. When we think ‘globular cluster’ we automatically think of wizened decabillionarians glowing faintly into their dotage in serene loneliness. But the youngest fully accepted globular cluster is NGC 1818 in the upper LMC, at 40 million years. (This cluster has been featured 8 times on APOD since 1997.) Remoteness? Only a few globulars actually reside in the outer halo of our Galaxy, and most of them are the cores of former dwarf galaxies strewn like chicken bones on a plate after a feast by our Milky Way. (Why does the Milky Way make me think of Henry VIII?) The Milky Way bulge hosts over 20 contrarian GCs that were born before the bulge, watched their neighbourhood turn into a boxy tenement, and survived to tell the tale. Haute Province 1 (Ophiuchus) is presently the closest to the galactic core (1630 light years) and is roughly 13 billion years old. Core concentration? Nope there, too: Six Milky Way massive open clusters are far denser than Pal 3 in Sextans—we can see four of them in our scopes as NGC 3603, Westerlund 1 and 2, and R136 in the Tarantula Nebula. Some Class XII globulars such as NGC 7942 Aquarius are looser and less massive than a young hotshot like the Jewel Box. Poor NGC 7942 is falling headlong into its likely last passage through the Milky Way disc. Presently the loosest known globular, it is likely to end up a swarm of dissociated stars in roughly half a billion more years. (We could call this ‘Cluster’s Last Stand’ but that moniker has already been assigned to Palomar 13.)

As you might expect, professional astronomers have a more complicated notion of the term ‘globular cluster’ than our airy eyepiece impressions of a beautiful speckled ball. They are more impressed with the shape and contents of the cluster’s horizontal branch in the Colour Magnitude Diagram—having a blue tail is is a prize asset in the horizontal branch’s celestial kennel. Astronomers also fancy the magnitude difference between a cluster’s red giant tip and the horizontal branch red clump; this is a measure of hydrogen envelope loss as the core fuses helium into carbon and oxygen. Arrival at the horizontal red clump signifies a star’s heaviest mass loss phase is over, a data point which helps determine the age and internal dynamics of the entire cluster. Unsurprisingly, astronomers have a name for this, the d(V-I) index, and aren’t you glad you don’t have to explain it all to your fiancée’s mom and dad when your qualifications as a candidate bridegroom arise over the dinner table. Astronomers also wring their hankies over whether a cluster exhibits an anti-correlation between its oxygen and sodium abundances, an innocuous-sounding phrase that carries a big stick when it comes to defining a globular cluster’s status. The [Na/O] reciprocal arises in connection with two less-known cyclical catalytic processes that affect an ageing star’s atmospheric chemistry, the Neon-Sodium and Magnesium-Aluminium-cycles. This is an estimable topic because the richness of earthly life forms we enjoy daily is the leftover dandruff of complex cyclic nuclear reactions in the horizontal branches and AGB phases of stars long since vanished into the cosmic graveyard. (If you can’t live another moment without knowing what the professionals talk about between midnight and 5 a.m., you can read all about the importance of the horizontal branch in globular cluster studies here.)

I apologise for dumping all this into your lap at this ungodly hour, so I’ll put the horse back in front of the cart for now. All these factors and the uncountable pages of astrophysical jargon arising out of globular cluster studies have a very obvious shortcoming: what we know about the minute-by-minute pulsebeat of globular clusters comes almost entirely from Milky Way studies. MW globulars are very old and very chemically evolved compared with the youngsters in the Magellanic Clouds. Of the 10 Mensa clusters that we can see using a good scope on a good night, all are between 300 million and 3 billion years old. This age bracket is something of a gold standard for LMC halo globulars, but it is roughly 1/4 to 1/3 the age of most Milky Way globulars. Why? Surprisingly, the subject has not been studied all that well. The most detailed and yet digestible is a 2007 study of 15 of the LMC’s middle-age globular clusters. It's an appetiser. There's lots more to come on the Mensa table.

Hence, when we visually observe the 10 Mensa globulars, we really are looking at objects about which no one really knows very much at all. See any or all of them and you can put away your scope for the night pleased that finally you have looked at something that hasn’t been picked over by legions of observers before you.

Last edited by Weltevreden SA; 26-03-2014 at 05:13 PM.
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Old 26-03-2014, 10:55 AM
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As usual, you are an absolute diamond-mine of information about clusters!
Keep up that research, as you are finding some very interesting things.

As you are finding to be the case , Scratch the surface of any issue relating to nearby southern galaxies, and it is rather common to find a paper by Ken Freeman. His new work, involving Hermes spectrograph on the 4meter AAT is typical of his renewed focus on galaxies close at hand
(the GALAH survey aims to reconstruct the history of the MW, using metallicity and age and kinematic data on its stars)

Here is a grab bag full of possibly relevant info about the LMC:

(1)
In google books, I found an intriguing 1997 reference book about the clouds of Magellan, called "The Magellanic Clouds" by Bengt E. Westerlund. (it is in the "Cambridge Astrophysics Series"

Westerlund includes an entire chapter of information about the clusters of the LMC and SMC, though the information is now old.
(Google books at least enabled me to view some of the information about the clusters.)

(2)
The old stellar halo of our own spiral galaxy, is not necessarily typical. Some galaxies, even M31, have been found to have a halo which includes a significant population of intermediate-aged stars.

What is "normal" or "typical" for the halo of a low luminosity spiral galaxy like the LMC? (if there is such a thing as a "normal" halo for Magellanic Spiral)
(LMC is not a dwarf galaxy; it is what Gerard de Vaucouleurs called a "subdwarf" galaxy, a couple of magnitudes down in luminosity from a big spiral like the MW. Indeed the luminosity of LMC is comparable to that of M33. Perhaps, before its current interactions and "troubles", it is very conceivable that it may have had regular spiral structure like M33)

(3)
In relation to the possibility that stars and clusters associated with LMC have been ejected a long way from it, Jun-Hwan Choi, in his UMASS thesis "Dynamics of Satellite and Dark Matter Halo Interactions on Galaxy Formation and Evolution", suggests that if there was an interaction of the LMC with the Milky Way, there should be tidally stripped stars found a long way away from the LMC.

(4)
Here is a display of the true shape of the LMC; this is a map which plots the number density of RGB and AGB stars.
North is to the top and east is to the left.
[[ This map is from van der Marel's review paper on the structure and kinematics of the LMC;
http://www.stsci.edu/~marel/pdfdir/L...ymp03_vdm3.pdf
(this is an excellent overview of the structure of the LMC....) ]]

Click image for larger version

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Dana, it is going to be tough to explain what is going on with the objects you have observed; the gaseous distribution of the LMC is disturbed and extends to several times the radius of the distribution of old stars seen in the above image!

Here is an interesting quotation:
The Magellanic Clouds have been studied in great detail covering the entire spectral range. The interaction becomes most evident in neutral, atomic hydrogen (HI). Huge gaseous arms have been detected covering a large fraction of the southern sky. Figure 2 shows the HI column density distribution of the Magellanic Clouds and their environment and a mean velocity map obtained from Brüns et al. (2004). The LMC and the SMC are not isolated galaxies but embedded in a common HI envelope called the Magellanic Bridge (Hindman 1961). Moreover, the two galaxies possess prominent gaseous arms, the Magellanic Stream (Mathewson et al. 1974) and the Leading Arm (Putman et al. 1998), with an extension of about 180° on the sky. The Magellanic Stream shows a huge velocity gradient of ΔvLSR = 650 km/s over its extent of about 100° that is still considerable in the Galactic-standard-of-rest frame, ΔvGSR = 390 km/s, while the Leading Arm shows no clear velocity gradient.


____________

And now a word from Dana's dream girlfriend:
"Darling, I just love that way you talk about elemental abundances in stars..."

(win her heart with a heated discussion of the Sodium-Oxygen anticorrelation!)
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Last edited by madbadgalaxyman; 26-03-2014 at 12:13 PM.
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Old 26-03-2014, 03:07 PM
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I shall risk the wrath and the perpetual enmity of Cambridge University Press by including this review paper on the Star Clusters of the LMC+SMC system, from IAU Symposium S256 in 2008:

LMC & SMC__star clusters__by Basilio Santiago__from IAU Symposium S256.pdf
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Old 26-03-2014, 04:24 PM
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What wonderful gifts you two bring to this forum. Dana, I love the wealth of information and questioning in your post (some of which I can readily follow and some of which will take quite a bit of chewing) but I also love the playful poetry of it.

It is also amazing to read about the size of this system when the whole envelope is considered. I think it a bit ironic that the LMC is called a subdwarf when it is bigger than a dwarf. Wouldn't supradwarf fit better?

Thanks for the articles as well!
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Old 26-03-2014, 05:14 PM
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Hi Guys, check this thread out,
http://www.iceinspace.com.au/forum/s...35#post1068235
I think it feeds into what you are discussing.
Cheers
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Old 26-03-2014, 05:37 PM
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Weltevreden SA (Dana)
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Elemental abundances

Thanks to both Robert and Paddy for their valuable links and pointers. I'll be spending all this evening downloading those and the link-tos they turn up into PDFs to tote along on my upcoming dark site visit starting Thursday. No Internet or cell phone up there—in fact, no human presence at all besides me—so I have to load up on everything ahead of time. A lot of spaghetti and tomato sauce goes into the bin along with the PDFs—you know how it is when it comes to cooking at a dark site. And, as a newlywed (first year anniv coming up the 18th), my lovely wife rather prefers to hear me speak of the stars in her eyes vis-à-v is the stars in the skies.

Robert & Paddy, your info about the Magellanics has finally swayed my head from stellar evolution and cluster studies. Thanks for giving me a change in homework plans on this dark-skies run. I'll check back in about 10 days from now.

Cheers to all . . . =Dana
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Old 26-03-2014, 06:27 PM
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Happy travels, clear skies and happy anniversary Dana!
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Old 26-03-2014, 07:24 PM
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Paddy saith:
It is also amazing to read about the size of this system when the whole envelope is considered. I think it a bit ironic that the LMC is called a subdwarf when it is bigger than a dwarf. Wouldn't supradwarf fit better?

Bad Galaxy Man saith:
That was a typo on my part; Gerard de Vaucouleurs actually said LMC and similar galaxies are sub-giant galaxies!
Q. Is LMC a sub-giant galaxy or is it a super-dwarf galaxy?
In general, low-luminosity star-forming galaxies (this definition excludes dwarf spheroidal/elliptical galaxies), have an irregular structure, while star-forming galaxies which are similar in luminosity to the MW and M81 and M31 tend to have a reasonably regular spiral structure.
Based on the degree of regularity of its structure, I think LMC may simply be a regular Barred Spiral galaxy which has been somewhat perturbed.
((It is, in fact, possible to make a crude estimate of the luminosity of an unperturbed spiral galaxy, based on the degree of regularity of its spiral structure, which lead to the concept of a luminosity class for a spiral galaxy.))
_______________

Ron
!
Thanks very much for the pointer to the other thread on the LMC, I do appreciate it.
I should really check the literature to see if the imager in question has really and truly picked up tidal material in his image.

Someone asks in this thread....... is the LMC really a spiral galaxy?;
There is one definite spiral arm at one end of its bar, and there is a short stub of an arm at the other end of its bar.
I think that LMC is a spiral galaxy, as the spiral structure is somewhat evident in deep optical exposures, and very evident in the distribution of its cold atomic hydrogen gas (HI)

It seems plausible to me that LMC would be a lot more regular in appearance than it is, if it had not undergone one or more encounters with SMC and our own Galaxy.

LMC is often classified as the prototype of a galaxy-type called a "Magellanic Spiral". (it is not classified as an irregular galaxy.)
The pioneering paper on this galaxy morphology was in the early 70s by our own Aussie extragalactic powerhouse Kenneth C Freeman and the great extragalactic astronomer Gerard de Vaucouleurs:
they concluded that a bar structure with one arm, or a bar structure with one arm and one arm stub, is a definite sign of a perturbed galaxy.

The following set of slides explains the definition of a Magellanic Spiral Galaxy (type SBm in the Hubble system) and some of the reasons that LMC is a barred spiral galaxy:
http://www.ugr.es/~galaxybars2013/CarmeGallart.pdf
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Old 27-03-2014, 08:43 AM
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Weltevreden SA (Dana)
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Another question, Robert: Have there been secular extinction studies of the Magellanics? All the extinctions I came across referred to Galactic figures and were in the 0.35 to 0.45 range. Does anything like the Schlegel 1998 map set exist for the LMC-SMC? I wonder if we really know what we're seeing once outside the bar region. =Dana
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Old 27-03-2014, 10:39 AM
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Quote:
Originally Posted by Weltevreden SA View Post
Another question, Robert: Have there been secular extinction studies of the Magellanics? All the extinctions I came across referred to Galactic figures and were in the 0.35 to 0.45 range. Does anything like the Schlegel 1998 map set exist for the LMC-SMC? I wonder if we really know what we're seeing once outside the bar region. =Dana
You are lucky I am on holiday in the last two days, so I have time to try to answer this question!

Because of the lower mass of "sub-giant" galaxies like M33 and the LMC and NGC 300 and NGC 7793 and NGC 1313, compared to the likes of luminous spiral galaxies like MW and M31 and M51 and M83, the significantly smaller and less luminous and less massive galaxies invariably have a much lower content of heavy elements in their interstellar medium than the bigger galaxies;
the enriched "heavy element" products of gas expulsion by supernovae and OB stars and Planetary Nebulae and Red Giant stars are presumably expelled from small galaxies, as the energies in their ISM are large enough to presumably expel a lot of gas from a low mass galaxy (perhaps permanent expulsion)

So galaxies like the LMC and M33 and NGC 300 tend to look nearly dust free..... because they do in fact have little dust in their interstellar medium.

The effects of internal extinction from the host galaxy on the objects found within small galaxies are not entirely negligable, but they tend to be eclipsed by other larger sources of error in our measurements.
However, dust extinction (in this case I refer to the extinction due to the LMC itself, rather than the extinction due to the dust screen within our own Galaxy's disk component) across the face of the LMC is much better studied than in just about any other galaxy, because many of the basic "standard candles" (Cepheids, absolute magn. of the tip of the red giant branch, planetary nebulae luminosity function) for the extragalactic distance ladder are calibrated using these objects in the LMC.

I wouldn't be too obsessed about extinction in the outermost parts of galaxies, if I were you;
it gets markedly less with progressively increasing galactocentric radius in a galaxy;
indeed, the outer disks of even very massive and metal-rich spirals can be virtually transparent to light in the visual regime (but this is not the case for the inner disks of these galaxies)

[[ I have massive numbers of papers on extinction within galaxies at home on my computer, but I am away from base at present; it is something I have often studied, in the practical sense of calculating luminosities and distances of extragalactic objects]]

When I googled on the search terms "extinction + LMC", I came up with plenty of papers and even calculators to estimate extinction. However, I do believe that a lot of the papers give the total extinction (that from our Galaxy plus that from the LMC.)

This is a recent study of extinction over the face of the LMC:
http://iopscience.iop.org/0004-637X/...1004.text.html

The mean internal extinction for the LMC in the V-band was found to be only 0.3 to 0.4 magnitudes, and if LMC is like any other galaxy, then it will be less at large radii.

All of which is to say that the appearance of the LMC at visual wavelengths is not substantially falsified by the effects of dust within it.

Last edited by madbadgalaxyman; 27-03-2014 at 11:19 AM.
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Old 05-04-2014, 09:39 AM
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Weltevreden SA (Dana)
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LMC extinction paper from 2007 has good details

Hi again all. Back from the dark site after a week of heavenly bliss. You all know the feeling.

Thanks to Robert for his informative post. I sniffed around and came up with the following paper: Extinction in the Large Magellanic Cloud, Nia Imara and Leo Blitz; Astrophysical Journal, 662:969-979, 20 June 2007. Cut to the chase by scrolling down to section 3.2 & following.

Now for some Zzzz. You all know that one, too!
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