2024ApJ...974...34G

We employ observations of the 205 μm [N II] fine structure (FS) line and radio recombination line (RRL) emission to derive the electron density in 10 well-known H II regions. The combination of these two spectral lines (the RRL-FS line method) provides a sensitive probe of electron density in regions with n(e) ≥ 30 cm^–3 without requiring knowledge of the size of the ionized region. By using H54α data from the Green Bank Telescope and 205 μm data from the SOFIA Airborne Observatory, we have almost identical 18″ beamwidths, removing a significant source of error for observations of H II regions due to nonuniform density across the sources observed. The electron densities vary widely among the sources observed, from 2600 to 36,000 cm^–3, with two low-density outliers at 94 and 520 cm^–3. On average, these densities are a factor of 4 greater than the highest-resolution single-antenna data and a factor of almost 13 greater than the 182″ angular resolution single-antenna data having more sources in common. The total 1σ fractional uncertainties in n(e) are in the range 0.15-0.29. In the RRL-FS line method, the observationally determined quantity is proportional to ∫n²(z)dz / ∫n(z)dz. For a Gaussian density distribution much more extended than its 1/e radius, this is equal to ${n}_{0}/\sqrt{2}$, where n₀ is the peak electron density. The high values of electron density found are plausibly the result of the RRL-FS line technique sampling the peak of a centrally condensed density distribution.