We present a CO ( 3–2 ) survey of selected regions in the M31 disc as part of the JCMT large programme , HARP and SCUBA-2 High-Resolution Terahertz Andromeda Galaxy Survey ( HASHTAG ) .
The 12 CO ( 3–2 ) fields in this survey cover a total area of 60 square arcminutes , spanning a deprojected radial range of 2 – 14 kpc across the M31 disc .
Combining these observations with existing IRAM 30m CO ( 1–0 ) observations and JCMT CO ( 3–2 ) maps of the nuclear region of M31 , as well as dust temperature and star formation rate surface density maps , we are able to explore the radial distribution of the CO ( 3–2 ) /CO ( 1–0 ) integrated intensity ratio ( R _ { 31 } ) and its relationship with dust temperature and star formation .
We find that the value of R _ { 31 } between 2 – 9 kpc galactocentric radius is 0.14 , significantly lower than what is seen in the nuclear ring at 1 kpc ( R _ { 31 } \sim 0.8 ) , only to rise again to 0.27 for the fields centred on the 10 kpc star forming ring .
We also found that R _ { 31 } is positively correlated with dust temperature , with Spearman ’ s rank correlation coefficient \rho = 0.55 .
The correlation between star formation rate surface density and CO ( 3–2 ) intensity is much stronger than with CO ( 1–0 ) , with \rho = 0.54 compared to –0.05 , suggesting that the CO ( 3–2 ) line traces warmer and denser star forming gas better .
We also find that R _ { 31 } correlates well with star formation rate surface density , with \rho = 0.69 .