Old 24-03-2014, 10:21 AM
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Weltevreden SA (Dana)
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The 10 Mensa LMC halo globulars

Hi all . . .

The span between the western (leading) fringe of the LMC and the shallow triangle of Mensa’s base—basically the two-degree band between 74° and 76° South—contains a boggling 35 faint globulars and open clusters. None are marked on my 1988 edition of Uranometria (charts 462 & 463). None of them appear on the Michael Vlasov charts that I use in the field. The only downloadable charts that give any clue of the profusion of faint LMC globulars in Mensa are the 10 shown on Chart 217 of José Torrés B set (DSOs down to visual mag 14), and Chart 558 of Torrés’s chart set C (DSOs to mag 15.5). I’ve attached a clip from the Torres C set.

The last dark cycle here in SA saw several cold fronts move through, yielding spectacularly clear, still skies afterward. I managed to spot all 10 of the Torres C chart GCs in a 180/1800 Mak using 20mm and 9mm eyepieces. These clusters take a lot of patience. They are all in the mag 12.3 to 13.8 range—about on par with the five Fornax Dwarf GCs. Even with mag 7.3 and sub-arcsec skies, I had to look steadily at the spot where the Torres chart located the cluster to be. Typically I would see only a handful of faint glimpses wavering in and out at the object’s position across fifteen or twenty minutes. By way of comparison, E3 (ESO 37-01) in Chamaeleon recently reported here on IIS was visible steadily though faintly on the same evenings. IC 4499 in Apus was as easy as a mainstream globular despite its low surface brightness, and I could pick out 10 to 12 of its stars at any given moment. I repeated the Mensa GC set 4 different nights during the dark cycle, so they are sufficiently reliable observations to log.

If you spot any of these, go back and scan that section of the sky from -74° to -76° S at high enlargement using WikiSky. There are an amazing number of fuzzy dots ranging from obvious globulars to inconspicuous smudges with a telltale fuzzy edge. I got tired of counting at 35 globular-like concentrations. This is the same number of globulars as in the entire constellation of Sagittarius!

NED and Simbad have very little on these objects besides basic position and luminosity. A peruse of the arXiv and ADS papers don't come up with much on the LMC GC set south into Mensa. There's very little LMC extinction or starform activity in this area, so I'm wondering where these GCs were during the 10 or so million year period during which the LMC crossed the MW disc. The LMC leading edge halo gas must have been tidally stripped almost completely, leaving the GCs in the clear.

I'd love to have Robert and Paddy chime in on these objects. I don't think I've seen a report on them elsewhere, pretty much ignored in the professional papers, and a fun track-down for you & me. I'll be after them again this coming cycle, this time with a 200mm Mak.

Cheers from Dana in S Africa
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Old 24-03-2014, 11:21 PM
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FlashDrive (Col)
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Sounds like a real challenge ..... need a dark site for this.....and good night vision.....and patience.

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Old 25-03-2014, 12:34 AM
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Well done Dana
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Old 25-03-2014, 06:36 PM
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madbadgalaxyman (Robert)
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Hello there, Dana,

Have you seen the very-deep LMC mosaic by our brilliant IIS astrophotographer Marco Lorenzi?

We should search for similar images..... but with an even wider field!

I don't think I am going to make a serious assault on the problem of why these clusters are where they are, but here are some facts about LMC and SMC......
(I probably know less about the LMC than most people around here.....indeed, I tend to avoid it, for some reason. The literature on LMC is very specialized and remarkably extensive)

However, one thing I do know is that O and B stars can sometimes form, or exist, in out-of-the way places which are a long way from the optically-evident body of a galaxy. For instance, the M81 field is full of tiny knots of stars that are not in any way obviously associated with M81. As another example, faint extensions of the very distended HI (neutral atomic hydrogen) that is associated with M83 have formed stars at enormous radial distances from M83.

There is plenty of extended HI gas associated with the LMC. Indeed, in observations of the two-dimensional (apparent)(as observed from our line-of-sight) distribution of this cold atomic hydrogen gas, the LMC looks like a normally rotating spiral galaxy with fairly well-behaved spiral arms:

Click image for larger version

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Just how very extensive and extended the gas distribution is in the vicinity of the LMC and the SMC can be seen in this map of the HI gas distribution over the sky in the vicinity of these two galaxies:

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The hyper-extended gas distribution in the Magellanic Stream is usually held to be caused by one or both of these mechanisms:

(1) Tidal interactions of the LMC and the SMC with the Milky Way

(2) Stripping of the interstellar medium from the SMC and LMC by the effects of a hypothetical hot & distended gaseous halo of the Milky Way.
(arxiv 1211.0758 discusses the prospects for a hot (million degree) gas halo around our own Galaxy)

From these facts, I think we can safely conclude that the raw materials for ongoing star formation exist in some very unusual locations that are a long way from the most obvious parts of the Magellanic Clouds.
Also, it does seem possible that in the complex interactions that have taken place between LMC and SMC and the Milky Way, stars and clusters have been ripped out of the Magellanic clouds and have ended up a long way from them.
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Old 25-03-2014, 10:30 PM
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madbadgalaxyman (Robert)
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Possible role for galaxy harassment in moving member objects away from LMC?

One mechanism that can remove stars and other objects from galaxies is a very suggestively named physical process called "galaxy harassment", which is defined as a process in which multiple high-speed encounters take place between galaxies. This process is particularly common in rich clusters of galaxies; I note that some clusters of galaxies end up with 5 or 10 percent of their stars being in between the member galaxies rather than inside the galaxies!!

I know little about the process of galaxy harrassment (and I really really hate to admit that I don't know something!).

So I offer this quotation from Penn State University's Astronomy 801 course materials, in an attempt to shed some light on this issue:

The relative velocity between galaxies plays a very important role in galaxy interactions. If two galaxies are moving too fast with respect to each other, the strength of the dynamical friction between them will not be large enough to slow them down and cause them to merge. The question is: Do these galaxies remain completely unchanged, or do they experience some changes as they have a “near miss” collision? Inside of galaxy clusters, spiral galaxies should experience multiple weak encounters as they pass by many of the other galaxies in the cluster with large relative velocities. Although we do not expect each of these weak encounters to produce dramatic tidal tails, bursts of star formation, and eventual mergers, the galaxies are not unaffected. Each minor encounter is likely to alter the shape of the spiral galaxy and to strip off some of its outer, weakly bound stars. This process is called Galaxy Harassment, and the effect that it has on the galaxies is also known as “tidal truncation” because the galaxies should slowly shrink as they lose more and more stars.
Remember that clusters like the Virgo cluster are known to contain a much higher percentage of elliptical galaxies than we find in poor groups. We think that the reason for this is that the spiral galaxies are altered by processes inside the cluster. It is likely that some combination of major mergers in the outskirts of the cluster and galaxy harassment in the inner regions causes the spirals to become elliptical galaxies. The Galaxy Harassment model also predicts that there should be a population of stars floating around inside of the cluster, unattached to any particular galaxy. Recently, astronomers have identified both red giant stars and planetary nebulae in the Virgo Cluster that are not associated with any one galaxy. These “intracluster stars” appear to be those predicted by the models of Galaxy Harassment.
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Old 25-03-2014, 10:45 PM
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Paddy (Patrick)
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Hi Dana - thanks for starting fantastic thread and a opening another opportunity for Robert to bring some very interesting stuff forward.

It's not an area of the LMC that I've done a lot of observing in as when I was making the charts I had no idea that it came down this far into Mensa. The best way I've found of identifying clusters in the LMC is to match coordinates in wikisky with the catalogue in Arcinal & Hynes' "Star Clusters". Hope to get a chance to sit down and do this soon and let you know what I come up with. From my limited capacity to understand much of what I read on the LMC, there seem to be only 13 of the GC like objects that are classed as GCs in the more recent stuff I've found. There used to be more classed as GCs but their much younger age and smaller masses compared to Milky Way GCs led to then being classed mostly as open clusters. The terms "blue globular" and "young populous cluster" seem to have been ditched.

And it may be that the stripping of material from the Magellanic Clouds might not be by the Milky Way or solely by the Milky Way. From my layman's understanding of this paper they may be on their first pass, or have a very wide orbit. The two clouds might be harassing each othe if this modelling is on the money. Again, major caveats for my ignorance.
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