Cloud-scale elemental abundance variations and the CO-to-dust-mass conversion factor in M31

TL;DR

This study investigates elemental abundance variations in M31, revealing radial gradients, stochastic scatter, and the relationship between CO-to-dust-mass conversion and oxygen abundance, combining spectroscopic and molecular cloud data.

Contribution

It provides the first detailed analysis of elemental abundance gradients and their impact on CO-to-dust-mass conversion in M31, integrating spectroscopic and molecular cloud observations.

Findings

Oxygen gradient is relatively flat across M31.

Nitrogen gradient is significantly steeper, indicating higher N/O ratio in the inner regions.

No trend of CO-to-dust-mass conversion factor with Oxygen abundance.

Abstract

From a spectroscopic survey of candidate H II regions in the Andromeda galaxy (M31) with MMT/Hectospec, we have identified 294 H II regions using emission line ratios and calculated elemental abundances from strong-line diagnostics (values ranging from sub-solar to super-solar) producing both Oxygen and Nitrogen radial abundance gradients. The Oxygen gradient is relatively flat, while the Nitrogen gradient is significantly steeper, indicating a higher N/O ratio in M31's inner regions, consistent with recent simulations of galaxy chemical evolution. No strong evidence was found of systematic galaxy-scale trends beyond the radial gradient. After subtracting the radial gradient from abundance values, we find an apparently stochastic and statistically significant scatter of standard deviation 0.06 dex, which exceeds measurement uncertainties. One explanation includes a possible collision with M32 200 - 800 Myrs ago. Using the two-point correlation function of the Oxygen abundance, we find that, similar to other spiral galaxies, M31 is well-mixed on sub-kpc scales but less so on larger (kpc) scales, which could be a result of an exponential decrease in mixing speed with spatial scale, and the aforementioned recent merger. Finally, the MMT spectroscopy is complemented by a dust continuum and CO survey of individual Giant Molecular Clouds, conducted with the Submillimeter Array. By combining the MMT and SMA observations, we obtain a unique direct test of the Oxygen abundance dependence of the $\alpha^{\prime}(^{12}\mathrm{CO})$ factor which is crucial to convert CO emission to dust mass. Our results suggest that within our sample there is no trend of the $\alpha^{\prime}(^{12}\mathrm{CO})$ with Oxygen abundance.