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We employ observations of the 205 μ m [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 H54 α data from the Green Bank Telescope and 205 μ m 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 1 σ 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 ∫ n ^2 ( 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 n _0 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.

Molecular lines are powerful diagnostics of the physical and chemical properties of the interstellar medium (ISM). These ISM properties, which affect future star formation, are expected to differ in starburst galaxies from those of more quiescent galaxies. We investigate the ISM properties in the central molecular zone of the nearby starburst galaxy NGC 253 using the ultrawide millimeter spectral scan survey from the Atacama Large Millimeter/submillimeter Array Large Program ALCHEMI. We present an atlas of velocity-integrated images at a 1.″6 resolution of 148 unblended transitions from 44 species, including the first extragalactic detection of HCNH ^+ and the first interferometric images of C _3 H ^+ , NO, and HCS ^+ . We conduct a principal component analysis (PCA) on these images to extract correlated chemical species and to identify key groups of diagnostic transitions. To the best of our knowledge, our data set is currently the largest astronomical set of molecular lines to which PCA has been applied. The PCA can categorize transitions coming from different physical components in NGC 253 such as (i) young starburst tracers characterized by high-excitation transitions of HC _3 N and complex organic molecules versus tracers of on-going star formation (radio recombination lines) and high-excitation transitions of CCH and CN tracing photodissociation regions, (ii) tracers of cloud-collision-induced shocks (low-excitation transitions of CH _3 OH, HNCO, HOCO ^+ , and OCS) versus shocks from star formation-induced outflows (high-excitation transitions of SiO), as well as (iii) outflows showing emission from HOC ^+ , CCH, H _3 O ^+ , CO isotopologues, HCN, HCO ^+ , CS, and CN. Our findings show these intensities vary with galactic dynamics, star formation activities, and stellar feedback.

In the lower atmosphere of Earth, oxygen contains a higher fraction of the heavy ^{18}O isotope than ocean water does (Dole effect). This isotopic enrichment is a signature of biological activity, set by the equilibrium between oxygenic photosynthesis and respiratory metabolisms in terrestrial and oceanic ecosystems. While the mixing between stratospheric and tropospheric oxygen leads to a slow isotopic homogenization, little is known about the isotopic oxygen enrichment in the mesosphere and thermosphere of Earth. In situ measurements from rocket-borne air samplers are limited to altitudes below the mesopause, while higher layers have only been accessible through the analysis of the oxidation of ancient cosmic spherules. Here we report the detection of the far-infrared fine-structure lines (^{3}P_{1}←^{3}P_{2} and ^{3}P_{0}←^{3}P_{1}) of ^{18}O in absorption against the Moon, and determine the ^{16}O/^{18}O ratio in atomic oxygen from the mesosphere and lower thermosphere in absorption. After correcting for isotopic exchange between atomic and molecular oxygen, our values for the bulk ^{16}O/^{18}O ratio of 468 and 382 in February and November 2021, respectively, fall significantly below that found in solar wind samples (530±2), and encompass, within uncertainties, the corresponding ratios pertaining to the Dole effect in the troposphere (487), and those found in stratospheric ozone (429 to 466). We show that with existing technology, future, more sensitive measurements will allow us to monitor deviations from isotopic homogeneity in the mesosphere and lower thermosphere of Earth by remote sensing. We demonstrate that the collisional excitation of the fine-structure levels of the ^{3}P ground-state triplet of ^{18}O may compete with isotopic exchange reactions, implying a deviation from the Boltzmann distribution that would be established under local thermodynamic equilibrium.

We present analysis of [O i ] 63 μ m and CO J = 5 − 4 and 8 − 7 multiposition data in the W3A region and use it to develop a model for the extended low-density foreground gas that produces absorption features in the [O i ] and J = 5 − 4 CO lines. We employ the extinction to the exciting stars of the background H ii region to constrain the total column density of the foreground gas. We have used the Meudon photodissociation region code to model the physical conditions and chemistry in the region employing a two-component model with a high-density layer near the H ii region responsible for the fine-structure line emission and an extended low-density foreground layer. The best-fitting total proton density, constrained largely by the CO lines, is n (H) = 250 cm ^−3 in the foreground gas and 5 × 10 ^5 cm ^−3 in the material near the H ii region. The absorption is distributed over the region mapped in W3A and is not restricted to the foreground of either the embedded exciting stars of the H ii region or the protostar W3 IRS5. The low-density material associated with regions of massive-star formation, based on an earlier study by Goldsmith et al., is quite common, and we now see that it is extended over a significant portion of W3A. It thus should be included in modeling of fine-structure line emission, including interpreting low-velocity-resolution observations made with incoherent spectrometer systems, in order to use these lines as accurate tracers of massive-star formation.