The HASHTAG project I. A Survey of CO(3-2) Emission from the Star
  Forming Disc of M31
  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 (R31) and its relationship with dust temperature and star
formation. We find that the value of R31 between 2 - 9 kpc galactocentric
radius is 0.14, significantly lower than what is seen in the nuclear ring at ~1
kpc (R31 ~ 0.8), only to rise again to 0.27 for the fields centred on the 10
kpc star forming ring. We also found that R31 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 R31 correlates well with star formation rate
surface density, with $\rho$ = 0.69.
