The clear detection of Ca II H & K absorption at +100 km/s in USNO-A0600-15865535, coincident with HI emission from Complex WD, indicates that the complex is closer than the star, at a distance 4.4 kpc. The column of Ca II measured is comparable between the cloud and the disk, even though they have wildly different HI columns. This is consistent with the weak \cite{Wakker_2000} or non-existent \cite{Bekhti_2012} correlation between HI and Ca II column density, which also explains the non-detection of the small intermediate velocity cloud along the line of sight at 60 km/s. Unfortunately, this lack of correlation makes it impossible to infer anything about the metallicity of the cloud from these metal absorption lines. Future metallicity measurements, perhaps toward USNO-A0600-15865535, will be critical in determining the origin of Complex WD, as gas of extragalactic origin typically has lower metallicities.
The kinematics and location of Complex WD do give us some clues as to its origin. Originally, \citet{1991A&A...250..509W} suggested that because complex WD is at positive velocity it is likely part of the structure of the Galaxy itself co-rotating with the disk. We call this the “far” scenario in Figure \ref{fig:contour}, and it is emphatically ruled out by our detection of absorption. The appeal of the scenario is quite clear from the Figure – a cloud at 20 kpc could easily be nicely corotating with the disk. We now know that Complex WD is mostly inside the solar circle toward the fourth quadrant. Along the line of sight to USNO-A0600-15865535, Complex WD sits above a portion of the disk moving at \(-\)30 km/s LSR if we assume that it is at the maximal distance of 4.4 kpc, decreasing to 0 km/s LSR as we assume a closer distance. Complex WD is therefore strongly not in corotation with the disk. This is in rather stark contrast with other HVCs; a simplified model of HVCs with known distances found that they rotated with the disk at 77 km/s – slower than Galactic rotation, but with the same sense \citep{Putman_2012}.
A number of scenarios could account for an overall difference in velocity between the cloud and the disk. An accreting cloud could easily have a much lower accretion velocity than the rotation speed of the disk, and the positive velocity observed could be an artifact of the solar motion. Similarly, it is possible that Complex WD is material ejected from star-forming regions closer to Galactic center \citep[e.g.][]{Ford_2010}, and thus the high positive velocity is an effect of the lower specific angular momemtum of that material. Both of these scenarios suffer from the fine tuning required to meet the very small LSR velocity gradient found in the Complex. A flux-weighted first-order polynomial fit to the velocity gradient in the Wakker & van Woerden 1991 catalog of WD clouds find \(-0.072 \pm 0.146\) km/s per degree of Galactic longitude. The reflex velocity of the solar motion represents 100 km/s across 40 degrees of Complex WD – unless the Cloud is conspiring to thwart our detection of a velocity gradient, we should see some effect of the solar motion. While we cannot fully rule out the “intermediate” scenario, where the bulk of the cloud is at \(\sim 5\) kpc, this velocity structure puts very tight constraints on any future model.
Finally we examine a “near” scenario, where Complex WD is only 1-2 kpc away. In this scenario the cloud originated from an area near the sun, and thus has inherited the overall solar motion, largely solving the fine-tuning of the LSR velocity. In the “near” scenario the cloud is ejected from the disk by some kind of impulsive event, perhaps connected to star formation in the Gould Belt or Saggitarus Arm, imparting an overall 100 km/s bulk velocity. A cloud this far away would only be about 0.5 kpc above the disk, which is quite low for most known HVCs, and would make it distinct from all other known HVCs in its origin. The shearing effect of differential rotation is much weaker close to the sun, which makes this impulsive scenario more credible for a closer cloud. The somewhat symmetric Complex WE, with a similar velocity and location, but at negative Galactic latitude, could conceivably have been generated by the same event.