Electron temperature and emission measure of H ii regions in the central molecular zone (CMZ) from H40α recombination line and continuum emissions by ALMA CMZ Exploration Survey (ACES)

This is the authors' abstract. We don't add key points for this paper.

Star formation activity in the central molecular zone (CMZ) directly manifests itself as radio continuum free–free emission (Bremsstrahlung) and radio recombination line emission from H ii regions surrounding newly formed massive stars. We derive the overall distribution of the H ii regions and their fundamental properties: electron temperature (Tₑ) and emission measure (EM), and hence electron density in the form of two-dimensional distribution maps over the CMZ by analyzing the ACES (ALMA CMZ Exploration Survey) H40α (99.02 GHz) recombination line and 99.6 GHz continuum emission data with synthesized beam widths of 2.^ 45 (0.097 pc at 8.2 kpc) and 2.^ 14, respectively. We apply the “TeEM” method (Tₑ–EM mapping), which creates Tₑ and EM maps from input 2D maps of the continuum and integrated line intensity. The analysis covers the entire ACES field from l~ -0.∘6 to +0.\!\!∘8 and from b~ -0.\!\!∘ 2 to +0.\!\!∘ 1. The area analyzed is complete and includes previously known H ii regions such as Sgr B2, Sgr B1, the Sickle, the Pistol, thermal filaments (Bridges), Sgr A H ii regions, the Minispiral, and many other known H ii regions. Sgr C is not included in the analysis due to the insufficient signal-to-noise ratio in the recombination line map. The mean electron temperature over the CMZ is determined to be Tₑ CMZ= 5872 ± 78 ~ (SE) ~± 3682~ (SD) K (SE: standard error of the mean, SD: pixel-to-pixel standard deviation). Some H ii regions, such as Sgr B2 Main and the Minispiral, exhibit large scatter and an internal Tₑ gradient of several thousand K per parsec. The EM distribution is more diverse, varying by orders of magnitude from ~ 10⁵ to ~ 3× 10⁸ pc cm⁻⁶ within the CMZ, as well as within individual H ii regions.