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.