A statistical study of giant molecular clouds traced by $^{13}$CO,
  C$^{18}$O, CS, and CH$_3$OH in the disk of NGC 1068 based on ALMA
  observations
  We present $1''.4$ (98 pc) resolution ALMA observations of
$^{13}$CO($J$=1-0), C$^{18}$O($J$=1-0), CS($J$=2-1) and
CH$_3$OH($J_K$=$2_K$-$1_K$) molecular lines in the central $1'$ (4.2 kpc)
region of NGC 1068 to study the physical and chemical properties of giant
molecular clouds (GMCs) and to test whether these GMC-scale properties are
linked to the larger-scale galactic environment. Using the derived $^{13}$CO
cube, we have identified 187 high-significance ($> 8\sigma$) GMCs by employing
the CLUMPFIND algorithm. The molecular gas masses of GMCs $M_{\rm ^{13}CO}$,
derived from the $^{13}$CO data, range from $1.8 \times 10^4 ~ M_\odot$ to $4.2
\times 10^7 ~M_\odot$.
  A mass function of GMCs in NGC 1068 has been obtained for the first time at
$\sim$100 pc resolution. We find the slope of the mass function $\gamma = -1.25
\pm 0.07$ for a mass range of $M_{\rm ^{13}CO} \geq 10^5M_\odot$. This is
shallower than the GMCs in the disk regions of the Milky Way, M51 and NGC 300.
Further, we find that the high mass cut-off of the GMC mass function occurs at
$M_{\rm ^{13}CO} \sim 6 \times 10^7 M_\odot$, which is an order of magnitude
larger than that in the nuclear bar region of M51, indicating that the more
massive clouds dominate the mass budget in NGC 1068. The observed
C$^{18}$O)/$^{13}$CO intensity ratios are found to be fairly uniform (0.27
$\pm$ 0.05) among the identified GMCs. In contrast, the CH$_3$OH/$^{13}$CO
ratios exhibit striking spatial variation across the disk, with the smallest
values around the bar-end ($<$ 0.03), and larger ratios along the spiral arms
($\sim$ 0.1-0.2). We find that GMCs with detectable methanol emission tend to
have systematically larger velocity widths than those without methanol
emission, suggesting that (relatively weak) shocks are responsible for the
enhancement of the CH$_3$OH/$^{13}$CO ratios of GMCs in the disk of NGC 1068.
