Quantifying the anisotropic density structure of the Central Molecular
  Zone -- a 2D correlation function approach
  The Central Molecular Zone (CMZ) is a ring-like structure sitting at the
center of the Milky Way. Using the 870 $\mu$m continuum map from the APEX
Telescope Large Area Survey of the Galaxy (ATLASGAL), we study the anisotropy
of the density structure of the gas in the Central Molecular Zone (CMZ) using
the 2D correlation function. To quantify the spatial anisotropy, we define the
critical angle $\theta_{\rm half}$, as well as the anisotropy parameter
$A\equiv \frac{\pi}{4\theta_{\rm half}}-1$. We find that the density structure
is strongly anisotropic at the large scale (~ 100 pc), and the degree of
spatial anisotropy decreases with the decreasing scale. At the scale of ~ 10
pc, the structure is still mildly anisotropic. In our analyses, we provide a
quantitative description of the anisotropic density structure of gas in the
CMZ, and the formalism can be applied to different regions to study their
differences.
