Azimuthal Metallicity Structure in the Milky Way Disk

TL;DR

This study investigates the azimuthal metallicity variation in the Milky Way disk by measuring electron temperatures in HII regions, revealing significant azimuthal differences and potential influence of the Galactic Bar on chemical mixing.

Contribution

It provides new measurements of electron temperatures in previously unexplored azimuthal regions, expanding understanding of the Milky Way's chemical structure and azimuthal metallicity variations.

Findings

Radial metallicity gradient is steeper at Az = 90-130° than at Az = 0-60°.

Azimuthal metallicity structure is not statistically significant due to distance uncertainties.

Radial mixing from the Galactic Bar may influence metallicity distribution.

Abstract

Elemental abundance patterns in the Galactic disk constrain theories of the formation and evolution of the Milky Way. HII region abundances are the result of billions of years of chemical evolution. We made radio recombination line and continuum measurements of 21 HII regions located between Galactic azimuth Az = 90-130 degree, a previously unexplored region. We derive the plasma electron temperatures using the line-to-continuum ratios and use them as proxies for the nebular [O/H] abundances, because in thermal equilibrium the abundance of the coolants (O, N, and other heavy elements) in the ionized gas sets the electron temperature, with high abundances producing low temperatures. Combining these data with our previous work produces a sample of 90 HII regions with high quality electron temperature determinations. We derive kinematic distances in a self-consistent way for the entire sample. The radial gradient in [O/H] is -0.082 +/- 0.014 dex/kpc for Az = 90-130 degree, about a factor of two higher than the average value between Az = 0-60 degree. Monte Carlo simulations show that the azimuthal structure we reported for Az = 0-60 degree is not significant because kinematic distance uncertainties can be as high as 50% in this region. Nonetheless, the flatter radial gradients between Az = 0-60 degree compared with Az = 90-130 degree, are significant within the uncertainty. We suggest that this may be due to radial mixing from the Galactic Bar whose major axis is aligned toward Az ~30 degree.