The Dreadful Details
Cheerful warning: The following section is heavy-slog astrophysics. If you're not in the mood for it, skip on to Part III where we get real again.)
(Caption to Fig. 6 below)
BH176 and ESO 92-SC05 locations on disc plane. On the b-axis vertical scale both clusters are well removed from the star-forming thin disc. Instead they lie in the upper regions of the thick disc, at b = 4650 ly for ESO 92-SC05 and b = 4340 ly for BH176.
BH176 and ESO 92-SC05 are astrophysical correlates (as is ESO 92-18, not discussed here). Both clusters are a very old >5 Gyr. They are improbably large spatially and in stellar mass given their location in the upper Galactic thin disc. They differ kinematically with their surroundings—NH176’s proper motion relative to its surroundings is 24 km s-1; ESO 92-SC05’s is 15 km s-1 (both prograde). If they were formed in the thin disc like Galactic open clusters, they shouldn’t be where they are. Nor would they travel as fast as they are. Indeed, they shouldn’t be at all. Yet there they are. Why?
It has been shown by many authors that very old open clusters in the thin disc tend to migrate slowly outward along the Galactic b-axis into the thick disc due to disc heating from HVC infall and GMC star formation. However, it is doubtful that disc heating alone can move >1,000 M☉ clusters >5° above or below a Galactic disc. Outer disc heating exterior to the solar circle can come from cold High Velocity Clouds (HVCs) infalling into the disc and shock-compressing to star-forming densities. The cluster Blanco 1 in Sculptor began as a rapid HVC that penetrated into the disc nearly vertically from below, quickly formed a loose star cluster, which kept right on going to its present position 750 lyr above the disc plane and pulling away. Heating in the inner disc regions comes from a very different source: Giant Molecular Clouds (GMCs) entering a spiral density wave at high velocity but shallow entry angle, breaking up, and forming multiple massive clusters. The M8–M16–M17–M20 collect-and-collapse cluster complex in Aquila and Sagittarius formed from such an event. The differential heating generated by these distinct processes sets up pressure gradients in the thin disc which transmit to the thick disc as disc heating. One consequence would be vertical waves in the thick disc analogous to vertical waves in the Earth’s atmosphere. Atmospheric vertical waves manifest visibly in herringbone clouds and lenticular mountain waves. Now they have been found to exist in our Galaxy’s disc.
Bonatto et al. 2006 showed that Galactic open clusters older than 1 Gyr can reach heights up to 350 pc (1140 ly) outside the thin disc due to disc heating. Disc heating could be efficient for thickening the disc in terms of field stars, but it is not established how old open clusters might acquire the vertical velocity to venture into such heights. Yet manage it they do, and handily. There are many well known examples of >5 Gyr clusters lying well into the thick disc. Northern observers can pay a call on NGC 188 and M76 as examples (though NGC 6791 Lyra preferred to stay closer to home). Southern observers have Collinder 261 (Musca), ESO 96-SC04 (also Musca and
cataloged as Andews-Lindsay 1).
ESO 92-SC05 is 1.4 kpc (4560 lyr) above the disc plane.
Ortolani et al 2008 showed that this cluster has a total of 1800 ±400 M☉—a surprisingly massive cluster given its age. M67, by compare, at 4 Gyr and a known Galactic disc cluster, has a mass of just 724 M☉. ESO 92-SC05 also has a very low metallicity of
Z=0.005 [M/H] = –0.7. These and many other bits of evidence suggested to several authors that large open clusters in the outskirts of disc galaxies were not formed within their present habitat but rather acquired from other galaxies disrupted by tidal stress by the Milky Way. For example,
Janes & Phelps 1994 sampled 72 old open clusters; their analysis revealed that a Galactic disc is subject to be continuous disturbance by shocks from infalling material.
Frinchaboy et al. 2004 suggested that some outer-disc open clusters were accreted from disrupted dwarf galaxies. Rocha-Pinto et al. 2006* interpreted evidence from anomalously located open clusters and red giants as deriving from an accreted dwarf galaxy which they called called Argo.
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* RevMexAA (Serie de Conferencias), 26, 84{85 (2006)}
Let’s look at these clusters in more detail.
BH176 is one of just thirteen >6 Gyr metal-rich clusters known in the MW, [Fe/H] ≥ –0.24 (0.58% M☉). It is 15.2 ± 0.2 kpc (49,550 lyr) from MW core, 1.5 times further out than the Galactic bar tip in Norma where the bar abruptly swerves leftward (CCW) into the Perseus Arm. BH176 is 7.0 ±0.5 Gyr based on α-element abundance of red clump stars [α/Fe] ~ 0.25 dex (dex as used here is the mean of [Mg/Fe], and [Ca/Fe]). It is spatially and kinematically consistent with Monoceros Ring with proper motion Vh = +15 km s-1 wrt (with respect to) the Galactic disc rotational velocity.
Sharina et al 2014 aver that BH176 may have originated as a massive star cluster after either (a) the high velocity infall of a massive gas cloud into the thin disk, or (b) as a satellite dwarf galaxy now completely absorbed into the Galactic disc. This would imply a shallow entry angle, which is consistent with BH176’s velocity nearly parallel with the thick disc rotation vector (see Janes & Phelps
Summary, “. . . the repeated accretion of low angular momentum material onto the disk from the halo or beyond would also explain the observed radial composition gradient and the lack of a correlation between open cluster metallicity and age”).
ESO 92-SC05 is also >6 Gyr old and orbiting in the outer thick disc. Its metallicity [M/H] is −0.7 ±0.2, which makes it one of the metal-poorest clusters in the Galaxy.
Chen et al. 2003 state that open clusters with [Fe/H] < −0.5 are extremely rare. At a distance of 11.4 ±1.0 kpc (37,160 lyr) from the Galactic centre (10.9 kpc or 35,530 lyr from us), ESO 92-SC05 is too massive and far out to have originated as a Galactic disc cluster. It thus must be an accreted open cluster from a now disrupted dwarf galaxy. The problem is, which one? Or rather, which stream belonging to which dwarf? The metals/mass DNA is solid for some streams, like the Sagg Dwarf Elliptical which “donated” (that’s the word the astronomers use) M54 to the eyes of we amateur astronomer looking back in time via the scenic route. For other streams the evidence is less conclusive. What are our options?
In
Option A, the red giants of ESO 92-SC05 are chemically similar to the field-drift red giants of the Orphan Stream (see ¶ 2 in Part I); they also fall within the range of the Orphan Stream’s velocity distribution as its former dwarf galaxy stars orbit around their original disc plane.
In
Option B, ESO 92-SC05 shows characteristics similar to the ancient open clusters Berkeley 29 (Gemini), Saurer 1 (Canis Minor), and BH176, which are thought to have been accreted from GASS, the Galactic Anticenter Stellar Structure (
Frinchaboy et al. 2006). Unfortunately, ESO 92-SC05’s location at Galactic coordinates l = 286.2°, b = −7.5° is too far from the direction to GASS (l = 240°, b = −8°) and GASS’s accreted dwarf galaxy Canis Major. There is evidence that the Canis Major dwarf is not part of an accretion stream at all but rather part of the Galactic warp.
In
Option C, ESO 92-SC05 Carina location and its 11.4 ±1.0 kpc distance from the Galactic centre match the Rocha-Pinto et al. 2006 proposed Argo Structure, which they state in their Abstract as, “While we are able to confirm various previously reported features of the Monoceros tidal stream, a major finding of our work is that the apparent core of the Monoceros system is not in Canis Major, as other surveys have suggested, but more likely at a larger Galactic longitude in the region of the ancient constellation Argo (present-day Puppis, Vela, and Carina).”
Sounds like we need to let the experts noodle this one out. As for us, we can safely say that ESO 92-SC05 is a thoroughly engaging challenge for we amateurs, but a thoroughly vexing one for the professionals.
