The CO-to-H₂ Conversion Factor in the Barred Spiral Galaxy M83

Abstract

We analyze the CO-to-H₂ conversion factor (α _CO) in the nearby barred spiral galaxy M83. We present new H I observations from the VLA and single-dish GBT in the disk of the galaxy, and combine them with maps of CO(1-0) integrated intensity and dust surface density from the literature. α _CO and the gas-to-dust ratio (δ _GDR) are simultaneously derived in annuli of 2 kpc width from R = 1–7 kpc. We find that α _CO and δ _GDR both increase radially, by a factor of ∼2–3 from the center to the outskirts of the disk. The luminosity-weighted averages over the disk are α _CO = 3.14 (2.06, 4.96) M⊙pc‑2[Kkms‑1]‑1 and δ _GDR = 137 (111, 182) at the 68% (1σ) confidence level. These are consistent with the α _CO and δ _GDR values measured in the Milky Way. In addition to possible variations of α _CO due to the radial metallicity gradient, we test the possibility of variations in α _CO due to changes in the underlying cloud populations, as a function of galactic radius. Using a truncated power-law molecular cloud CO luminosity function and an empirical power-law relation for cloud mass and luminosity, we show that the changes in the underlying cloud population may account for a factor of ∼1.5–2.0 radial change in α _CO.