Ionized gas in the Scutum spiral arm as traced in [N ii] and [C ii]

Abstract

Context. The warm ionized medium (WIM) occupies a significant fraction of the Galactic disk. Determining the WIM properties at the leading edge of spiral arms is important for understanding its dynamics and cloud formation. Aims. To derive the properties of the WIM at the inner edge of the Scutum arm tangency, which is a unique location in which to disentangle the WIM from other components, using the ionized gas tracers C + and N + . Methods. We use high spectral resolution [C ii] 158 µm and [N ii] 205 µm fine structure line observations taken with the upGREAT and GREAT instruments, respectively, on SOFIA, along with auxiliary H i and 13 CO observations. The observations consist of samples in and out of the Galactic plane along 18 lines of sight (LOS) between longitude 30• and 32• . Results. We detect strong [N ii] emission throughout the Scutum tangency. At V LS R = 110 to 125 km s −1 where there is little, if any, 13 CO, we are able to disentangle the [N ii] and [C ii] emission that arises from the WIM at the arm's inner edge. We find an average electron density ∼0.9 cm −3 in the plane, and ∼0.4 cm −3 just above the plane. The [N ii] emission decreases exponentially with latitude with a scale height ∼55 pc. For V LS R <110 km s −1 there is [N ii] emission tracing highly ionized gas throughout the arm's molecular layer. This ionized gas has a high density, n(e) ∼30 cm −3 , and a few percent filling factor. We also find evidence for [C ii] absorption by foreground gas. Conclusions. [N ii] and [C ii]observations at the Scutum arm tangency reveal a highly ionized gas with average electron density about 10 to 20 times those of the interarm WIM, and is best explained by a model in which the interarm WIM is compressed as it falls into the potential well of the arm. The widespread distribution of [N ii] in the molecular layers shows that high density ionized gas is distributed throughout the Scutum arm. The electron densities derived from [N ii] for these molecular cloud regions are ∼30 cm −3 , and probably arise in the ionized boundary layers of clouds. The [N ii] detected in the molecular portion of the spiral arm arises from several cloud components with a combined total depth ∼8 pc. This [N ii] emission most likely arises from ionized boundary layers, probably the result of the shock compression of the WIM as it impacts the arm's neutral gas, as well as from extended H ii regions.

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

“…Hence, the spiral arm detection by means of RM observations in our Milky Way model is mostly a consequence of the increase in magnetic field strength within the arms. We note that this particular aspect of our MHD model may not accurately represent the Milky Way where the density of free electrons n el is also found to be higher within the arms (see, e.g., Cordes & Lazio 2002;Langer et al 2017). Thus, the RM signal in the Milky Way may be much more influenced by electron fractions than in our synthetic observations suggest.…”

The nature of molecular cloud boundary layers from SOFIA [O I] observations

“…The presence of dense highly ionized gas, as derived from [N II], is an indication that the edge of the neutral gas does not transition rapidly to the low density WIM. Langer et al (2017) have argued, based on [N II] and [C II] observations of the Scutum arm tangency, that the dense ionized gas is a result of the compression of the WIM as it falls into the gravitational well of the spiral arm. The widespread distribution of dense highly ionized gas in the Goldsmith et al (2015)…”

“…In addition, Goldsmith et al (2015) and Langer et al (2016) found that a significant fraction of the [C II], of order 20%-30%, comes from highly ionized dense gas. A related study of the Scutum spiral arm tangency found that [N II] 205 µm emission arose from two components, a low density, n(e) ∼ 0.9 cm −3 , compressed WIM at the arm-interarm interface and from high density, n(e) ∼ 30 cm −3 , ionized gas associated with the molecular clouds deep in the arm (Langer et al 2017).…”

The Dense Warm Ionized Medium in the Inner Galaxy

“…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.…”