Resolved Measurements of Xco in NGC 6946

TL;DR

This study provides a detailed analysis of giant molecular clouds in NGC 6946, measuring their properties and the CO-to-H2 conversion factor with high-resolution observations, revealing similarities and differences with Milky Way clouds.

Contribution

First high-resolution mapping of GMCs in NGC 6946 combining single dish and interferometric data to accurately measure Xco and cloud properties.

Findings

Xco in NGC 6946 is consistent with Galactic values.

Most GMCs follow Milky Way trends, except near the galaxy center.

Clouds near the center show larger velocity dispersions.

Abstract

We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the 12CO (J=1-0) transition. By combining observations from the Nobeyama Radio Observatory 45-meter single dish telescope and the Combined Array for Research in Millimeter Astronomy (CARMA) interferometer, we are able to obtain high image fidelity and accurate measurements of LCO compared with previous purely interferometric studies. We resolve individual giant molecular clouds (GMCs), measure their luminosities and virial masses, and derive Xco - the conversion factor from CO measurements to H2 masses - within individual clouds. On average, we find that Xco = 1.2 \times 10^20 cm-2 / (K km s-1), which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within \sim400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.