tracing MWG Mass via Halo GCs vs. Disk MCs ?

[Widdekind]

(1) I found a plot, of the (non-circular, eccentric) absolute space velocities (in the [math]\Pi, W, \Theta[/math] directions), of a sizeable sample, of Globular Clusters (GCs), orbiting our Milky Way Galaxy (MWG) (page 9)

 

(2) I found a plot, of the (circular) space velocities, of a sizeable sample, of Molecular Clouds (MCs), rotating within the disk, of the MWG (figure 11)

 

(1) + (2) I over-plotted all of the above, "like a stack of transparencies". Visually, the velocities of halo GCs, and disk MCs, match, quite closely (~200 km/s), beyond the galacto-centric orbital radius, of our star system (~8 kpc). However, the "inner disk" (r < R_sun) MCs, still revolving around our galaxy's core at ~200 km/s, are moving much faster (~2x), than "inner halo" GCs, at comparable galacto-centric radii.

 

QUESTION: Why would disk material "behave differently", from halo material ?

 

 

 

 

ADDENDUM:

 

According to CalTech,

 

In contrast to the halo objects, about 20% of the globular clusters in our Galaxy are less metal poor; and, are found within about 1 or 2 kpc of the galactic plane (compared to most Population I stars lying within 0.4 kpc of the plane); and, thus, belong to intermediate Population II, often called the thick-disk population. These clusters are also in orbit around the galactic center, of course, but their orbits are more nearly circular and are oriented near the galactic plane. These clusters are moving in the same direction as galactic rotation, though at a rate that is slightly slower than the rotation of the thin disk of Population I stars.

 

Nearly all of the thick-disk globulars lie closer to the galactic center than does the Sun (i.e., within about 8 kpc of the center), but halo clusters are found out to much larger distances, though also with a strong concentration toward the center. Globular clusters of both types are thus found in large numbers in the general direction of the galactic center, but only halo clusters are found well away from the center. An individual globular can usually be assigned unambiguously to one population or the other on the basis of its chemical composition or velocity.

 

All of these differences presumably reflect changes that happened in the Galaxy very early in its history, as stars and star clusters formed first from a large, nearly spherical proto-galactic cloud of gas. This cloud would have contracted under its own gravitational pull and its small initial rotation would have sped up as the cloud contracted and caused the contraction to end in a disk rather than a small sphere. According to a commonly accepted picture, stars and clusters that formed a little later would have been formed in a thick disk and would have a higher metal abundance, as the chemical composition of the gas from which stars could form was enriched by the products of nuclear reactions in the most massive of the earliest stars.

 

A major issue in current research on globular clusters is the question of whether there are significant age differences from cluster to cluster, and especially between the two populations of globulars... It presently appears that the difference in ages is no more than a few billion years

Thus, halo-GCs may have formed z~5 (at the start of the cosmic Quasar-AGN era); then, (thick-)disk-GCs may have fored z~2 (at the end of the cosmic Quasar-AGN era). If so, then Quasars-AGNs were active (i.e., actively accreting material) during that "disking-down" period (5 > z > 2). And also, then, (1) the end of the Quasar-AGN era coincides with the (2) the end of the (thick-disk-)GC-formation era -- i.e., once our galactic disk fully formed (z~2), whatever had been conducive to GC formation, and galactic-core-BH accretion, in earlier eras (z>2), no longer existed.

 

Today, surviving galactic GCs are ~100 ly across (Inglis. Observer's Guide to Stellar Evolution). However, GCs may "evaporate" from their peripheries, whilst contracting towards their cores (Wikipedia). So, if proto-GCs are ~0.1 kpc across, then perhaps a thick-disk environment >10x larger in extent (1-2 kpc) is required, for the formation, of such stellar systems ?? That is, the modern thin-disk, being only a few times thicker, than a proto-GC (0.4 kpc : 0.1 kpc), is not big enough, to support the assembly, of such stellar systems ??

 

In any event, there seems to be a close correlation, between (1) fully formed, stable, thin, disk; (2) end of core BH accretion & AGN action; (3) end of GC formation.

Edited September 10, 2011 by Widdekind