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

TL;DR

The PAMS survey provides detailed molecular cloud data in the Perseus arm, revealing similarities with inner Galaxy clouds and offering new insights into molecular cloud properties across different galactic environments.

Contribution

This study presents the first large-scale CO survey of the Perseus arm, including new data, analysis of cloud properties, and comparison with inner Galaxy regions, expanding understanding of molecular cloud environments.

Findings

CO-to-H2 conversion factors determined for the Perseus arm.

Similar size-linewidth and mass-radius relationships in inner and outer Galaxy.

Outer Galaxy clouds are slightly more massive, explained by galactic gradients.

Abstract

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 $X_{^{12}\mathrm{CO (3-2)}}=4.0\times10^{20}$ and $X_{^{13}\mathrm{CO (3-2)}}=4.0\times10^{21}$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.