Metallicity Structure in the Milky Way Disk Revealed by Galactic H ii Regions

Abstract

The metallicity structure of the Milky Way disk stems from the chemodynamical evolutionary history of the Galaxy. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to observe ∼8−10 GHz hydrogen radio recombination line and radio continuum emission toward 82 Galactic H ii regions. We use these data to derive the electron temperatures and metallicities for these nebulae. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the dominant cooling mechanism in H ii regions, the nebular metallicity can be inferred from the electron temperature. Including previous single dish studies, there are now 167 nebulae with radio-determined electron temperature and either parallax or kinematic distance determinations. The interferometric electron temperatures are systematically 10% larger than those found in previous single dish studies, likely due to incorrect data analysis strategies, optical depth effects, and/or the observation of different gas by the interferometer. By combining the interferometer and single dish samples, we find an oxygen abundance gradient across the Milky Way disk with a slope of −0.052 ± 0.004 dex kpc −1 . We also find significant azimuthal structure in the metallicity distribution. The slope of the oxygen gradient varies by a factor of ∼2 when Galactocentric azimuths near ∼30 • are compared with those near ∼100 • . This azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.

3D gas-phase elemental abundances across the formation histories of Milky Way-mass galaxies in the FIRE simulations: initial conditions for chemical tagging

“…At z = 0 and R = 8 kpc (near the solar circle) the typical azimuthal scatter across the gas disk is 0.053 dex for (Wenger et al 2019) and observations of external galaxies (Sakhibov et al 2018;Kreckel et al 2019;Kreckel et al 2020), though we emphasize that we are not measuring azimuthal scatter in the same way: those observations typically measure differences in abundances between arm and inter-arm regions or measure abundance variations between HII regions within an aperture of a given size.…”

“…Mollá et al 2019a, and references therein). Furthermore, evidence persists for azimuthal variations in this radial gradient, based on HII regions (Balser et al 2011(Balser et al , 2015Wenger et al 2019).…”

“…We show uncertainties for all samples. The red error bars for Balser et al (2011);Wenger et al (2019) show the impact of measuring the radial gradient along different galactic azimuths. Balser et al ( 2011) finds gradients ranging from −0.03 to Mollá et al (2019a, M19), shown in grey, is the gradient derived from a compilation of the data from R06, B11, E13, E17, and F17.…”

“…Regarding the latter criterion, observations of the MW and external galaxies show radial and azimuthal variations in abundances across the disk (e.g. Sánchez-Menguiano et al 2016;Mollá et al 2019b;Wenger et al 2019;Kreckel et al 2020), although more work is needed to understand these spatial variations in the context of chemical tagging. Weak chemical tagging is subject to the same assumptions but applied to larger regions of gas across the disk (or in accreting galaxies).…”

Migration and Mixing in the Galactic Disc from Encounters between Sagittarius and the Milky Way

“…From Wheeler et al (2020), these gradients in the Milky Way look to be on the order of ∼0.1, using LAMOST data, in some elements (see Figure 14 in Wheeler et al (2020)). There are also observed azimuthal gradients in the HII gas on the Milky Way on the same order, ∼ 0.1 dex (Wenger et al 2019). Azimuthal variations appear to be a common feature of disc galaxies, and on the order of 0.02-0.03 dex (Kreckel et al 2020).…”