The dense warm ionized medium in the inner Galaxy

Abstract

Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N II] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N II] 122μm data, and spectrally resolved 12CO. Methods. We observed eight lines of sight (LOS) in the 20° < l < 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N II] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N II] and RRL linewidths. Results. The regions with [N II] 205 μm emission are characterized by electron densities, n(e) ~ 10−35 cm−3, temperatures range from 3400 to 8500 K, and nitrogen column densities N(N+) ~ 7 × 1016 to 3 × 1017 cm−2. The ionized hydrogen column densities range from 6 × 1020 to 1.7 × 1021 cm−2 and the fractional nitrogen ion abundance x(N+) ~ 1.1 × 10−4 to 3.0 × 10−4, implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N II] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C II] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N+ resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium.

The origin of the [CII]-deficit in a simulated dwarf galaxies starburst

“…t < 200 Myr), this sudden increase in L CII is due to the newly formed Hii regions which, in their early evolutionary stages, are dense and very bright in [Cii]. Notably, such a high contribution has been recently observed in ionized regions of the inner Galaxy (Langer et al 2021). At later times after the main encounter (e.g.…”

Filamentary structures of ionized gas in Cygnus X

“…In the Cygnus X region, ELD ionized gas may be filled and replenished by photoevaporating regions (similar in properties to [N ii] gas of (Goldsmith et al 2015;Pineda et al 2019;Langer et al 2021)) eroding neutral clouds over the lifetime of the massive stars. As we calculated in Section 6.2.1, a characteristic filamentary structure in the region photoevaporates 1.2 × 10 4 M of cloud mass over the lifetime of the massive stars.…”

“…As shown in Figure 9, the column densities of ionized gas found in the [N ii] survey are consistent with the column densities of filamentary ionized gas in the Cygnus X region. Langer et al (2021) used a combination of [N ii] 122 µm and 205 µm, RRLs, and 12 CO to determine electron temperature and density of ionized gas and assess its proximity to star-forming regions. They concluded that this dense warm ionized gas is located in or near star-forming regions.…”

“…Thus the volume filling factor of gas with these densities is unclear. Highresolution pinhole surveys of FIR fine structure lines (Goldsmith et al 2015;Pineda et al 2019;Langer et al 2021) trace somewhat denser gas (with mean values of n e ≈ 30 − 40 cm −3 ) and could plausibly be tied with these three possibilities.…”

Filamentary structures of ionized gas in Cygnus X

“…Thus, the volume filling factor of gas with these densities is unclear. High-resolution pinhole surveys of FIR fine structure lines (Goldsmith et al 2015;Pineda et al 2019;Langer et al 2021) trace somewhat denser gas (with mean values of n e ≈ 30−40 cm −3 ) and could plausibly be tied with these three possibilities.…”