Old 26-05-2017, 06:24 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: 216
First Light, Intes-Alter MN84, #2

Easy: NGC 5824 Lupus

A couple of weeks ago Amastro posted a paper about the globular NGC 5824 in Lupus. I’d never heard of it. That’s quite a feat of oversight given that N5824 is 104,000 light years above the other side of our galaxy and one of the remotest bright globulars in the sky. It is a tiny thing, remarkably bright for its remoteness due to its high Class IV core concentration. Also its location is 33° above the Galactic plane where there is little dust in the way. The Landolt V-band extinction is 0.54 mag. In piquant irony this 7th brightest globular was discovered by the grand master of dark objects, E. E. Barnard in Tennessee using a 6-inch scope. It was rediscovered half a decade later by William Finlay at the Cape of Good Hope, also using a 6-inch refractor. It would have been unobservable to William & Caroline Herschel due to low latitude. In the southern hemisphere where it is near zenith May through September, John Herschel and James Dunlop claimed having spotted it, but the documentation in their records is unclear given the vagueness of their wording, the presence of NGC 5694 Hydra nearby, and the scanty star populace in lower Libra. The cluster is also listed as NGC 5834, a fact better addressed by Steve Gottlieb, the Top Dog on matters of duck pond nomenclature.

N5824 was the first previously-unobserved object I chased down during the MN84’s shakedown cruise. It is surprisingly bright given its remoteness, seeming more like mag 7 apparent than 9.1. The listed diameter of 7.4 arcmin is misleading; that that refers to its mag 25 arcsec2 diameter, an unrealistic prospect with a cluster this bright. The listed 2.5 arcmin diameter which defines the mag 22 arcsec2 diameter is closer to real-see. To my eye about 60% of the GC’s light was in the core, with 20% on each side rapidly tapering off into a seeming bluish halo. The bluish cast also appears on the WikiSky image. Perhaps we are so accustomed to globulars being reddened that our eyes over interpret an reddened GC as blue. N5824 especially sparkles because of the barrenness of the field. No hope of resolution in an 8-inch scope. One suspects that if it was at the same 14,700 lyr distance of 47 Tucanae, N5824 would much resemble it.

While N5824 may be a bit under the radar for many of us, there’s been a steady succession of professional papers about this or that feature of this cluster over the last 15 years. The latest scoop is here.

Basically, N5824 is one of the Milky Way’s earliest objects to shrink out of the retinue of dark matter overdensities comprising our galaxy’s infancy (see first 30 seconds of this ERIS sim). Like M2, M22, and NGC 5286 Centaurus, the baby N5824’s first massive stars went SN in 3–10 million years. Because of the relatively high mass of the still-unused formation gas, the ejected SN gases did not achieve escape velocity (i.e., reach the virial radius), and so fell back to form the second stellar population. The time scale for a typical 2-gen globular genesis is usually given as 250 to 350 million years. Typically the cores of adolescent globulars are not as dense with stars as they are with leftover gas, so when the re-collapsing gas comes in it achieves high mass densities quickly and without the disruptive effects of magnetic fields and supersonic turbulence. (In astrophysics “turbulence” almost always means supersonic; Mach values can exceed 100 during early star formation. It’s only when gas goes subsonic that stars can form.) When N5824’s 2nd generation SN exploded, there was no longer enough gas in its halo to restrain the ejecta, so it escaped into the Galactic outer halo. Much of it probably still resides there. [Here’s a good 15-min U of Santa Cruz lecture on this.]

N5824’s brief genesis was followed by 12.2 to 13.3 billion years of near-solitude. N5824 is one of the fortunate few born so far out in the primordial dark matter halo that it has seldom interacted with the disc plane—evidenced, again, in that exceptionally concentrated core. GCs on the opposite end of the compaction scale, very airy Class XI or XII globulars (see Pal 6 observation in the next post) have endured many disc encounters and in time will be slowly stripped to bits. Pal 5 in Ophiuchus is a bare remnant embedded a long stellar stream that will eventually dissipate completely into the halo. The “most difficult NGC GC to observe” NGC 6749 Aquila may be on its last mix-up with the Galactic disc as we toil away down here simply trying to see it. (I'll have an observing report about this cluster’s fate coming up later this year.)

About 55% to 75% of the Milky Way’s globulars are sabra objects like N5824; the other 25% to 45% were accreted from dwarf galaxies. The MW got to be a big fish the good old fashioned way—eating the little fish. That’s great for us, we’ve got pretty things all over the sky to look at all night long. But for the little fish: Urp.

You can read the whole story here. And here’s an informative visualization of star cluster formation showing core collapse and halo evaporation.
Reply With Quote
Old 31-05-2017, 02:23 PM
Tinderboxsky's Avatar
Tinderboxsky (Steve)
I can see clearly now ...

Tinderboxsky is offline
Join Date: Feb 2012
Location: Kingston TAS
Posts: 1,040
Another very interesting and informative read. Thank you.
Reply With Quote


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 01:51 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.