Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [N ii] Fine-structure and Hydrogen Recombination Lines

Pineda, J. L.; Horiuchi, S.; Luisi, Matteo; Langer, W. D.; Goldsmith, Paul F.; Kuiper, T. B. H.; Bryden, G.; Soriano, Melissa; Lazio, T. Joseph W.

doi:10.3847/1538-4357/ab46c2

Cited by 13 publications

(31 citation statements)

References 56 publications

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“…205 µm lines with Herschel PACS at low spectral resolution (Goldsmith et al 2015). We also observed hydrogen Radio Recombination Lines along all lines of sight observed in [N ii] using the Green Bank Telescope as discussed below and described in Pineda et al (2019). These lines of sight are marked on Figure 1 superimposed on a portion of the Green Bank Telescope (GBT) hydrogen radio recombination line, Hnα, Diffuse Ionized Gas Survey (GDIGS) in the Galactic midplane (Anderson et al 2021).…”

Section: Datamentioning

confidence: 88%

“…The fourth column gives the nominal V LS R used for [N ii] observations (these are not necessarily the velocities of the peak in emission). The observing details for [N ii] are discussed below, while that for the RRL lines are discussed in Pineda et al (2019) and the [C ii] in Pineda et al (2013). The GOT C+ archival data sets are available as a Herschel User Provider Data Product under KPOT_wlanger_1.…”

Section: Datamentioning

confidence: 99%

“…In their derivation of n(e) and N(N + ) Goldsmith et al (2015) adopted a fixed canonical temperature, 8000K, characteristic of warm ionized gas. Whereas Pineda et al (2019) used a fit to the temperature as a function of galactocentric distance derived from the ratio of the hydrogen RRL line-to-continuum emission measurements in H ii regions (e.g., Balser et al 2015), which is independent of n(e) and proportional to T −1.15 e . Balser et al (2015) derived electron temperatures for a large sample of H ii regions across the Galaxy and find that they range from ∼4500K to ∼13,000K with a fit, T e = 4446 + 467R gal (kpc).…”

Section: Kinetic Temperature and Velocity Dispersionmentioning

confidence: 99%

“…In the Herschel Galactic [N ii] sparse survey, ten lines of sight observed with PACS were also observed with spectrally resolved 205 µm emission with Herschel's Heterodyne Instrument in the Far-Infrared (HIFI) and found to have ∼ 20 distinct velocity components (Langer et al 2016). Pineda et al (2019) analyzed the electron abundance and column density of N + in these 20 spectral components along with one in the Scutum arm l ∼30 • that had been observed in [N ii] 205 µm with SOFIA GREAT (Langer et al 2017). To derive the electron density, in the absence of spectrally resolved 122 µm [N ii] emission, Pineda et al (2019) developed a technique combining spectrally resolved Hnα radio recombination lines (RRLs) with the spectrally resolved [N ii] 205 µm line.…”

Section: Introductionmentioning

confidence: 99%

“…Pineda et al (2019) analyzed the electron abundance and column density of N + in these 20 spectral components along with one in the Scutum arm l ∼30 • that had been observed in [N ii] 205 µm with SOFIA GREAT (Langer et al 2017). To derive the electron density, in the absence of spectrally resolved 122 µm [N ii] emission, Pineda et al (2019) developed a technique combining spectrally resolved Hnα radio recombination lines (RRLs) with the spectrally resolved [N ii] 205 µm line. They found that the line of sight averaged electron abundances were consistent within a factor of two with the densities derived from the PACS spectrally unresolved 205 and 122 µm emission, thus validating this approach to determining n(e) from the ratio of the [N ii] to the Hnα RRL intensities.…”

Section: Introductionmentioning

confidence: 99%

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The Dense Warm Ionized Medium in the Inner Galaxy

Langer,

Pineda,

Goldsmith

et al. 2021

Preprint

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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 warm ionized medium 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 12 CO. 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 warm ionized medium. 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 to 35 cm −3 , temperatures range from 3400 to 8500 K, and nitrogen column densities N(N + ) ∼ 7×10 16 to 3×10 17 cm −2 . The ionized hydrogen column densities range from 6×10 20 to 1.7×10 21 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.

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Section: Datamentioning

confidence: 88%

Section: Datamentioning

confidence: 99%

Section: Kinetic Temperature and Velocity Dispersionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Dense Warm Ionized Medium in the Inner Galaxy

Langer,

Pineda,

Goldsmith

et al. 2021

Preprint

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The dense warm ionized medium in the inner Galaxy

Langer

Pineda²,

Goldsmith³

et al. 2021

A&A

Self Cite

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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.

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Filamentary structures of ionized gas in Cygnus X

Emig

White

Salas³

et al. 2022

A&A

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Context. Ionized gas probes the influence of massive stars on their environment. The Cygnus X region (d ~ 1.5 kpc) is one of the most massive star-forming complexes in our Galaxy, within which the Cyg OB2 association (age of 3–5 Myr and stellar mass 2 × 104 M⊙) has a dominant influence. Aims. We observe the Cygnus X region at 148 MHz using the Low Frequency Array (LOFAR) and take short-spacing information into account during image deconvolution into account. Together with data from the Canadian Galactic Plane Survey, we investigate the morphology, distribution, and physical conditions of low-density ionized gas in a 4° × 4° (~100 pc × 100 pc) region at a resolution of 2′ (0.9 pc). Methods. The Galactic radio emission in the region analyzed is almost entirely thermal (free-free) at 148 MHz, with emission measures (EM) of 103 < EM [pc cm−6] < 106. As filamentary structure is a prominent feature of the emission, we use DisPerSE and Fil ChaP to identify filamentary ridges and characterize their radial (EM) profiles. Results. The distribution of radial profiles has a characteristic width of 4.3 pc and a power-law distribution (β = −1.8 ± 0.1) in peak EM down to our completeness limit of 4200 pc cm−6. The electron densities of the filamentary structure range between 10 ≲ ne [cm−3] ≲ 400 with a median value of 35 cm−3, remarkably similar to [N II] surveys of ionized gas. Conclusions. Cyg OB2 may ionize at most two-thirds of the total ionized gas and the ionized gas in filaments. More than half of the filamentary structures are likely photoevaporating surfaces flowing into a surrounding diffuse (~5 cm−3) medium. However, this is likely not the case for all ionized gas ridges. A characteristic width in the distribution of ionized gas indicates that the stellar winds of Cyg OB2 create a fraction of the ionized filaments through swept-up ionized gas or dissipated turbulence.

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Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [N ii] Fine-structure and Hydrogen Recombination Lines

Cited by 13 publications

References 56 publications

The Dense Warm Ionized Medium in the Inner Galaxy

The Dense Warm Ionized Medium in the Inner Galaxy

The dense warm ionized medium in the inner Galaxy

Filamentary structures of ionized gas in Cygnus X

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