We present 1 ^ { \prime \prime } .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 rotational lines in the central 1 ^ { \prime } ( 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 ^ { 13 } CO cube , we have identified 187 high-significance ( > 8 \sigma ) GMCs by employing the CLUMPFIND algorithm .
The molecular gas masses of GMCs M _ { { } ^ { 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 _ { { } ^ { 13 } CO } \geq 10 ^ { 5 } M _ { \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 _ { { } ^ { 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 ( J =1–0 ) / ^ { 13 } CO ( J =1–0 ) intensity ratios are found to be fairly uniform ( 0.27 \pm 0.05 ) among the identified GMCs .
In contrast , the CH _ { 3 } OH ( J _ { K } = 2 _ { K } – 1 _ { K } ) / ^ { 13 } CO ( J =1–0 ) 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 .