Abstract

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 (T-e) 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) H40 alpha ( 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 similar to-0.degrees 6 to +0 .degrees 8 and from b similar to-0 .degrees 2 to +0( .)degrees 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 (T-e)CMZ = 5872 f 78 (SE) f 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 similar to 10(5) to similar to 3 x 10(8) pc cm(-6) within the CMZ, as well as within individual H II regions.