A statistical study of giant molecular clouds traced by 13CO, C18O, CS, and CH3OH in the disk of NGC 1068 based on ALMA observations

Abstract We present 1 ${^{\prime\prime}_{.}}$ 4 (98 pc) resolution ALMA observations of 13CO(J = 1–0), C18O(J = 1–0), CS(J = 2–1), and CH3OH(J K = 2 K –1 K ) molecular rotational lines in the central 1΄ (4.2 kpc) diameter 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 13CO cube, we have identified 187 high-significance (>8 σ) GMCs by employing the CLUMPFIND algorithm. The molecular gas masses of GMCs ( $M_{\rm ^{13}CO}$ ), derived from the 13CO data, range from 1.8 × 104 M ⊙ to 4.2 × 107 M ⊙. A mass function of GMCs in NGC 1068 has been obtained for the first time at ∼100 pc resolution. We find the slope of the mass function γ = −1.25 ± 0.07 for a mass range of $M_{\rm ^{13}CO} \ge 10^{5}$ M ⊙. This is shallower than the GMCs in the disk regions of the Milky Way, M 51, 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 ⊙, which is an order of magnitude larger than that in the nuclear bar region of M 51, indicating that the more massive clouds dominate the mass budget in NGC 1068. The observed C18O(J = 1–0)/13CO(J = 1–0) intensity ratios are found to be fairly uniform (0.27 ± 0.05) among the identified GMCs. In contrast, the CH3OH(J K = 2 K –1 K )/13CO(J = 1–0) ratios exhibit striking spatial variation across the disk, with the smallest values around the bar-end (