03/31/2013:
Thanks to Robert for posting the beautiful HST image of 3603. To my taste, it’s the best of all of them all associated with this fascinating object, Maybe it has to do with the spider diffraction spikes that show up on so many of the HST’s ACS/WFC imager. With our long histories at the library poring over books of images from the likes of Mt. Wilson up to the 8m giants, and as many hours at the eyepiece peering past Newt spider spikes, maybe we’ve just become accustomed to them and think of them as sort of beauty marks. As Larsen puts it in his 2004 paper, ‘It is a human habit to characterise those things with which we are most familiar as normal.’
Well, ‘normal’ or not, I am always excited at diffraction spikes in an astro image, distracting though they may be (the details of the HST’s NGC 6741 white dwarf images are negatively affected by so many spike crossings).
Anyway, before getting on with the topic at hand—the properties of cluster formation as described in the papers Robert cites—I have a question about the many so-called ‘star lines’ in the HST 3603 and so many other astro photography images I see. (Maybe it should be thread all its own in the ‘Observational and Visual Astronomy’ forum.) Counting very quickly, I see around 20 apparent lineations of 4 or more stars of perhaps 2 mag difference with respect to each other, marching in all directions across the 3603 image. Giant globulars are even worse. I know they are my mind’s eye trying to enforce order in what I know to be chaos, and that they can’t possibly be genuinely linear bound-objects. Not in clusters whose real-time orbitals mimic the boggling chaos of an N-body simulation. I don’t see this effect at the eyepiece to nearly the same degree, though ‘threads and filaments’ are frequent phrases in my logs of globulars. (Check out
NGC 5460—an OC that at the eyepiece looks like a snake charmer did it.) At one time I thought ‘star lines’ might by MHD artifacts—stars lining up with magnetic field lines. But that’s grasping at straws, because field line strengths in galactic arms are on the order of 3 to 5 microteslas, which would barely bestir an iron filing here on Earth, much less a 0.1 – 10 Msol string of stars in a massive cluster. Anyone care to weigh in on this illusion?
Of Robert's citations, the Bastian & Adamo M83 paper was the most informative and cogent. It’s going to get another read, yellow high-lighter at the ready. (And, of course, at the eyepiece if and when the current jet-stream drench above me heads over towards you folks.)
Larsen’s papers are very good. Thorough within their self-defined limits and do not either muddle with too much detail or overstretch their conclusions. He also makes the best use of the Harris Property which categorises cluster luminosity divided by host galaxy luminosity x 100. It’s a handy shorthand when describing objects so diverse as star cluster masses.
OTOH, I had a problem with the Adamo, Ostlin et al 2011 paper’s management of cluster diversity. Table 1 reveals a selectivity problem: they quote mostly their own research. That’s understandable, because they chose three unusual, atypical objects, Blue Compact Galaxies. There’s not exactly been a rush to the observatories to chase down these three; they are certainly not as sexy as NGC 3603 or dwarf irregulars. Table 1 appears in § 3, ‘Relations Between Cluster and Star Formation Rate’. The discussion that follows is more like an analysis of the vexatious diversity of clusters as a broad category. I kept thinking as I read the ifs-ands-&-buts, ‘Why don’t they just say that massive clusters in compact galaxies get that way because there’s nothing to stop them?’ No high-velocity infall, no dark matter halo, no territorial disputes with their flatmates (there aren’t any), no apogalacticons with anything, no rotational instability. A lot of gas, a large gravity well, and no competition—what more could a massive young cluster carving out territory ask for? I longed for a word about dark matter potential, but they didn’t address it—probably because no one has asked for a radio study of BCGs. Maybe I’m being superficial and obtuse about the paper, but I came away looking for the kind of picture that I find so readily in papers about dwarf galaxies and their environs. To be fair, though, the trio does state their case early: ‘. . . shear in rotationally supported galaxies (i.e., spirals) acts on the collapse of the giant molecular clouds (GMCs), causing fragmentation and favouring the formation of the less clustered OB associations and low mass clusters . . . The lack of rotation in dwarf galaxies and high external pressures in merging systems favour the collapse of massive and gravitationally bound cluster.’ I just wish they’d had an editor with an iron red pencil.
Clusters are like herding cats. I followed up on the subject using The Usual Suspects (arXiv, IOP, Scholarpedia) and came away longing for a couple of hours with my cat snoozing on my lap. And me with it.
So thanks again to Robert for all the leads.
=Dana