II Poster No. A-5 National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of Technology Pasadena, California www.nasa.gov Copyright 2015. All rights reserved. National Aeronautics and Space Administration Herschel Galactic plane survey of ionized gas traced by [NII] [ ] plays a significant role in star formation as it is produced only in ionized regions; in HII regions as well as diffuse ionized gas. The ionization potential of nitrogen (14.5 eV) is greater than that of hydrogen (13.6 eV), and carbon (11.3 eV), therefore the ionized nitrogen [NII] lines reflect the effects of massive stars, with possible enhancement from X-ray and shock heating from the surroundings. Motivation Results [NII] has two fine structure transitions at 122 and 205 µm. The two lines can be used to calculate the electron density and column density of N+, if one assumes that the emission from both transitions originate in the same gas and under roughly uniform conditions. [NII] detections are mainly toward the Galactic center. [NII] electron densities are found to be between 10 to 50 cm-3 (Fig. 2) and N+ column densities range between 1016 to 1017 cm-2. High densities indicate that the source of the emission is not the WIM (Warm Ionized Medium), which has densities 10-100 times smaller. Far infrared and sub-/millimeter atomic & ionic fine structure and molecular rotational lines are powerful tracers of star formation. Although CO lines trace cool to moderately warm molecular gas, ionized carbon [CII] produces the strongest lines, which arise from almost all warm (T >35 K) parts of the ISM. However,[CII] alone cannot distinguish fully ionized gas from weakly ionized gas. Methods Two far-infrared 122 µm and 205 µm [NII] fine structure spectral lines are observed via Photodetector Array Camera and Spectrometer (PACS) onboard Herschel Space Observatory. The sample consists of 149 lines-of- sight (LoS) positions in the Galactic plane, following those of the [CII] 158 µm observations obtained with the GOT C+ survey. PACS data have 5x5 spatial pixels therefore all 25 spectra are averaged to improve S/N, and that led to increase the number of detections in the averaged spectra. [CII] is the most important coolant of atomic hydrogen clouds and the photon dominated regions (PDRs) of molecular clouds and is widely used as a tracer of star formation. We found that [NII] measured with HIFI as well as PACS are linearly correlated with the [CII] emission. Fig 3: Position vs. intensities of [NII] and [CII] are shown through the Galactic plane. Fig 4: [NII] vs [CII] intensities shows tight correlation. Fig 2: Electron densities are shown as function of the position in the Galactic Plane. Galactic longitude (l) log I (W m-2 sr-1) # of LoS Int Ratio n(e) = 100 cm-3 n(e) = 50 cm-3 n(e) = 10 cm-3 n(e) = 1 cm-3 [NII] Fig 1: Histogram of intensities [NII] emission is highly correlated with that of [CII], implying that between 1/3 and 1/2 of the [CII] emission arises in the ionized gas. Possible origin 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. The pixel to pixel variation of the emission within a single Herschel pointing is relatively small, which is interpreted as the [NII] emission comes from an extended gas. N [NII] detections are mainly toward the Galactic center (intensity distribution shown in Fig. 1), but the opposite side of the Galactic center gives upper limits of ~4x10-10 W/m2/sr-1 for the 122 µm line and ~2x10-9 W/m2/sr-1 for the 205 µm line (Fig. 3). Principal Investigator: Umut Yildiz (3266) Paul Goldsmith (3266), William Langer (3200), Jorge Pineda (3266) It is important to quantify what fraction of [CII] emission arises in the ionized gas, to resolve the different parts of the ISM, leading to determine the total mass of the ISM.