02/01/2016:
To answer Paddy’s question, the largest, most current (2015) catalog of the LMC clusters is Palma et al,
here.
The non-paywall version of the paper Gary cites is on
astro-ph. It looks to be a direct reprint of the MNRAS article, which is unusual for Nature, which butters its bread with pay-per-views. That alone says a lot about the Li paper’s importance, because authors usually post first-draft versions on arxiv.orh
http://arxiv.org before tidying them up for the professional journals.
There are pluses and minuses to this Chengyuan Li et al paper. On the plus side it is a straightforward observation and analysis paper reminiscent of the long series of GC studies by Ortolani, Bica, Barbuy, et al between 1996 and 2004. The Li raw data is photometric, mainly in the B – I bands which highlight the BVR luminosity. From these the authors plot isochrone-corrected CMDs in the classical mode of a photometry paper. There are selection issues here in that the Magellanics have a very large number of compact, bound stellar groupings which blur the distinctions between globulars and massive young clusters (YNCs) like no other locale in the near-field universe.
The traditional definition of a true globular embraces RR lyrae stars; a red clump of helium core burners w/solar masses 0.7 to 2.1; a HB blue tail, metallicities [Fe/O] less than 1 (the Sun is the metallicity baseline); and most important a [Na/O] anticorrelation. All these point to a 4 < z < 8 (12.2 to 13.4 billion years ago) first-gen population that seeded the 2nd gen population we detect today. This is called the bimodal GC population model and has been documented time and again in papers going back 60 years. In the MW or Galactic GCs, there are only two exceptions: Ruprecht 106 in Centaurus (a nice target for a 15 cm scope in dark skies) which has a single main sequence; it is considered to be the only known remnant from an unknown primordial dwarf galaxy captured many Gyr ago. NGC 2808 has a triple main sequence but the causes of the 3rd generation are not fully clear because the 3rd gen stars don’t exhibit metallicities representing two generations of prior AGB ejecta, i.e. the [Eu/Ba] ratio which proportionates the contributions from core-collapse SN and late-stage AGB.
The other kinetically bound Magellanic objects are manqué globulars (young massive clusters) which have the core concentration, kinetic dispersion, and half-mass -vs- half-light properties of globulars, but also exhibit non-globular CMDs indicating metallicities from nuclear processes in stars aged less than 400 Myr. This is the category of cluster the Li et al paper deals with. It is a solidly documented study which seems to have intentionally limited its conclusions to exclude causal factors. The paper does not inquire how these clusters became such oddities, and are these youthful GC morphologies limited to the Magellanics only?
The Magellanics are the Wild West of the Local Group. The usual astrophysical conventions are stretched to the breaking point and all sorts of social oddities happen. The LMC – SMC duo are orbitally bound to each other; the “Magellanic Bridge” of stars and gas between them is evidence they are edging apart from a brush-by interaction >300 Myr ago. They also had a <300 Myr swing-by with the MW somewhat beyond the outer disc of the MW but still within the MW gas halo and DM halo. The Big Picture of these events is well presented in
Besla et al 2007,
Noël 2009, and
Besla et al 2012. The Noël paper substantiates the starform history of the SMC and provides tangential evidence for the Li paper’s analysis of Magellanic stellar generations in NGC 411. The 2012 Besla paper has some the most informative graphics we’ve seen demonstrating the paths of the LMC – SMC relationship with the MW.
Let’s take it for granted that the Li paper is correct in its analysis of the Magellanic GC young-cluster populations. That said, it does not address two important issues: the MDF or metallicity distribution function of the globulars under study, and their mass-luminosity relationship. MDFs correlate the varying proportions of key chemical elements such as He, C, O, Na, Mg, Ti, Si, Fe, Ni, Tc, Ce, Ba, Eu, and a handful of others. The relative proportions of different combinations of these tell us what kind of stars exist in an object like a cluster, how old they are, what generation they came from, which stellar residues from previous generations went into the current stars, and so on. These are plotted into an age-metallicity relationship (AMR) chart which sets the studied object into a frame of reference compared with others like it in the region.
The other important issue is the mass-luminosity ratio (MLR), which indicates the effect dark matter has on the object. Globulars themselves have little dark matter, but in the Magellanic galaxies they are surrounded by it. DM reacts only gravitationally, so the DM halo around the LMC and SMC will have been warped by their close encounter. That means a torque stress (technically a shear stress tensor) has been introduced which will gradually change the structure of both galaxies. That in turn affects the rotational speeds of the stars whizzing around the cluster, and also the rotational velocity of the cluster on its own axis. If the Magellanics are as tumultuous as they appear, then the polar alignments of the globulars will tell us much about the changing mass balance of the two galaxies with respect to their DM halos. The main question here is whether the Magellanics’ self-interaction induced a spin component in the LMC’s gas halo, which will show up as a common axis in the globulars' polar spin alignments. In many IIS astro-images of the LMC (especially if there’s lots of Ha in their mix) we see hints of spirality. So far those are only hints. If a spin component was introduced during the LMC – SMC interaction, we may be watching the rare treat of an irregular barred galaxy turning into a spiral barred galaxy. (Don’t quit your day job hoping for the fame of catching it in the act.)
What’ else?
By any Local Group and even Local Sheet standards, the Magellanics are an exotic case. Two large dwarf galaxies wrangle with each other whilst waltzing a ponderous gravitational pirouette around a monster galaxy that casually eats dwarfs for snacks. This time the MW responded too sedately and all it has to show for its gravitational grab is the 200°-long Magellanic Stream. The LMC shows the severest effects. The northern side (our earthly “north” on the Dorado side) shows tidal truncation and ram pressure stripping at its most resplendent. A visual pan even in binoculars shows a tumultuous spatter of star clouds, hot H2 regions, embedded clusters, and globular-like objects such as the three in the Li paper. It is a royal mess. The south, or Mensa side, is much the opposite: an unruffled region with little hot gas nebulosity and a surprising number of “clean” globular-appearing globular-like objects. The Mensa globulars (as I call them) are the most interesting region in the LMC. Why did this region escape largely unscathed while the opposite side of the galaxy is an unholy mess? Why are those globulars so evenly spread throughout the southern “empty quarter” of the LMC?
Attached below is the picture that tells it all. The LMC is clearly defined in the centre. There is a dome-shaped bright region to its right. Further right the dark is immense, palpable; it’s intergalactic space. The dome is a shock front cause by the LMC’s motion through the MW’s outer gas halo as the LMC departs the region. The shock front is like a ship’s bow wake. The bow wake marks the region where supersonic flow of the LMC’s approach is reduced into subsonic flow. The mass of the galaxy’s stars turn the inflowing subsonic gas into turbulent shock fronts. These compress as they collide with other turbulent pockets, and the end result is massive star clusters and inflamed H2 nebula. That shock front is the ultimate source of the curious very young stellar component in the Li paper’s globulars.
The physics of what happens behind the LMC’s bow wake is as complex as any that astrophysicists can dream up in their celebratory beers after a conference. You can bet that the Li paper, and others like it, are generating stacks of grant applications and observing time requests as the Magellanic devotees around the world hatch plots and write simulation programmes.
Hmm. Clear night tonight here in S Africa. Dunno about you, but I’m planning a long LMC look tonight.
=Dana in S Africa
