2025MNRAS.538..198R

The external environments surrounding molecular clouds vary widely across galaxies such as the Milky Way, and statistical samples of clouds are required to understand them. We present the Perseus Arm Molecular Survey (PAMS), a James Clerk Maxwell Telescope (JCMT) survey combining new and archival data of molecular-cloud complexes in the outer Perseus spiral arm in $^{12}$CO, $^{13}$CO, and C$^{18}$O (J = 3-2). With a survey area of $\sim$8 deg$^2$, PAMS covers well-known complexes such as W3, W5, and NGC 7538 with two fields at $\ell \approx 110^{\circ }$ and $\ell \approx 135^{\circ }$. PAMS has an effective resolution of 17 arcsec, and rms sensitivity of $T_\mathrm{mb}= 0.7$-1.0 K in 0.3 km s$^{-1}$ channels. Here 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). By comparing the various CO data with maps of H$_2$ column density from Herschel, we calculate representative values for the CO-to-H$_2$ column-density X-factors, which are $X_\mathrm{^{12}CO\, (3-2)}$$\, =4.0\times 10^{20}$ and $X_\mathrm{^{13}CO\, (3-2)}$$\, =4.0\times 10^{21}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ with a factor of 1.5 uncertainty. We find that the emission profiles, size-linewidth, and mass-radius relationships of $^{13}$CO-traced structures are similar between the Inner and Outer Galaxy. Although PAMS sources are slightly 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.