PAMS: The Perseus Arm Molecular Survey -- I. Survey description and first results

The external environments surrounding molecular clouds vary widely across galaxies such as the Milky Way, and statistical samples of clouds from surveys are required to understand them. We present the Perseus Arm Molecular Survey (PAMS), a James Clerk Maxwell Telescope (JCMT) survey of ¹³CO and C¹⁸O (J=3$-$2) of several molecular cloud complexes including W5 and NGC 7538 in the outer Perseus spiral arm situated at ≈ 110∘ and ≈ 135∘, with a total survey area of ~6 deg². The PAMS data have an effective resolution of 17.2 arcsec, and rms sensitivity of Tₘb = 0.7 K in 0.3 km/s channels. We present a first look at the data, and compare the PAMS regions in the Outer Galaxy with Inner Galaxy regions from the CO Heterodyne Inner Milky Way Plane Survey (CHIMPS), incorporating archival ¹²CO (3$-$2) data. By comparing the various CO data with maps of H₂ column density from Herschel, we find that the CO-to-H₂ column density X-factors do not vary significantly between Galactocentric radii of 4 and 10 kpc, and present representative values of X_¹²CO 3-2 and X_¹³CO 3-2. We find that the emission profiles, size-linewidth and mass-radius relationships of ¹³CO-traced structures are similar between the Inner and Outer Galaxy. Although PAMS sources are more massive than their Inner Galaxy counterparts for a given size scale, the discrepancy can be accounted for by the Galactic gradient in gas-to-dust mass ratio, uncertainties in the X-factors, and selection biases. We have made the PAMS data publicly available, complementing other CO surveys targeting different regions of the Galaxy in different isotopologues and transitions.