Herschel Galactic plane survey of [NII] fine structure emission

Abstract

We present the first large scale high angular resolution survey of ionized nitrogen in the Galactic Plane through emission of its two fine structure transitions ([NII]) at 122 $\mu$m and 205 $\mu$m. The observations were largely obtained with the PACS instrument onboard the Herschel Space Observatory. The lines-of-sight were in the Galactic plane, following those of the Herschel OTKP project GOT C+. Both lines are reliably detected at the 10${}^{-8}$ - 10${}^{-7}$ $W$m${}^{-2}$sr${}^{-1}$ level over the range -60${}^o$ $\le$ $l$ $\le$ 60${}^o$. The $rms$ of the intensity among the 25 PACS spaxels of a given pointing is typically less than one third of the mean intensity, showing that the emission is extended. [NII] is produced in gas in which hydrogen is ionized, and collisional excitation is by electrons. The ratio of the two fine structure transitions provides a direct measurement of the electron density, yielding $n(e)$ largely in the range 10 to 50 cm${}^{-3}$ with an average value of 29 cm${}^{-3}$ and N${}^{+}$ column densities 10${}^{16}$ to 10${}^{17}$ cm${}^{-2}$. [NII] emission is highly correlated with that of [CII], and we calculate that between 1/3 and 1/2 of the [CII] emission is associated with the ionized gas. The relatively high electron densities indicate that the source of the [NII] emission is not the Warm Ionized Medium (WIM), which has electron densities more than 100 times smaller. Possible origins of the observed [NII] include the ionized surfaces of dense atomic and molecular clouds, the extended low density envelopes of HII regions, and low-filling factor high-density fluctuations of the WIM.

51 pages, 24 figures, accepted by The Astrophysical Journal

References in corpus (8)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• The Electron Temperature Gradient in the Galactic Disk
• Detection of the 205 um [NII] Line from the Carina Nebula
• The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
• A Herschel [CII] Galactic plane survey III: [CII] as a tracer of star formation
• The H+ Region Contribution to [C II] 158 Micron Emission
• Origin and z-distribution of Galactic diffuse [CII] emission
• Ionized gas at the edge of the Central Molecular Zone

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