Vignettes in astrophysical data representation

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Take a look at the top plot.  It shows how many stars can be found in hypothetical boxes of space at positions z above or below the sun, and at longitudinal angles l within the Galactic disk.  Do you see anything amiss?

If you stare very closely, you may find that the distribution is not symmetric.  There is an overabundance of stars (hot spots) just below the sun, and an under-abundance (cool spots) above it.  

Yanny & Gardner took a better approach than staring - they compared the number density measurements to a symmetric model and found a wave-like pattern in the residuals.  The bottom plot literally shows this ripple in the distribution of stars in the Milky Way.

But what causes this wave-shaped asymmetry?  Perhaps it is the gravitational signature of an encounter our galaxy had long ago with a disruptive satellite galaxy.