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The [NII] Line Ratio

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Detection of the 205  m [NII] Line from the Carina Nebula. G. J. Stacey 1 , T. E. Oberst 1 , A. Loehr 2 , S. C. Parshley 1 , T. Nikola 1 , N. F. H. Tothill 2 , J. I. Harnett 2 , A. P. Lane 2 , A. A. Stark 2 , and C. R. Tucker 3 1 Department of Astronomy, Cornell University

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Detection of the 205 m [NII] Line from the Carina Nebula

G. J. Stacey1, T. E. Oberst1, A. Loehr2, S. C. Parshley1, T. Nikola1, N. F. H. Tothill2, J. I. Harnett2, A. P. Lane2, A. A. Stark2, and C. R. Tucker3

1Department of Astronomy, Cornell University

2Harvard-Smithsonian Center for Astrophysics

3Cardiff University


We report the first detection of the [NII] 205 m (1.46 THz) fine structure line from the ground using a direct detection spectrometer. The line was detected and mapped in emission from the Carina star formation region in the Galaxy using the South Pole Imaging Fabry-Perot Interferometer (SPIFI) on the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at the South Pole. The [NII] line is an important coolant for the diffuse ionized ISM, and (together with the 122 m [NII] line) is an excellent probe of gas density. The [NII] 205 m line emission peaks at the Carina II HII region, and its strength indicates low density (n ~ 30 cm-3) low ionization state gas envelopes the HII region. When compared with the ISO [CII] observations of this region, we find that a surprising large fraction (~ 50%) of the [CII] line emission arises from this low density ionized gas, a result with implications for [CII] studies of photodissociation regions (PDRs). The detection of this line demonstrates the utility of Antarctic sites for THz spectroscopy.

The Carina Nebula

  • The Carina Nebula is a large star formation complex in the southern Galactic plane subtending nearly 4  4 or ~ 150  150 pc at d = 2.7 kpc
  • The optical nebula is dominated by the Carina I and Carina II HII regions which together form an arrow -head shaped nebulosity (Figure 1)
  • The Carina I and Carina II HII regions appear to be excited by the Trumpler 14 and Trumpler 16 star clusters – although the star clusters are offset by ~ 2 to 4 pc with respect to the free-free continuum peaks
  • The Trumpler 16 and 14 star clusters contain ~ 36 early type stars, including the peculiar IR bright star  Carina, a member of Trumpler 16 cluster

The [NII] maps entailed ~ 20 pointings, or > 400 spectra covering the two regions:

  • Carina I: G287.4-0.6
  • Carina II: G287.6-0.6

Figure 1. 5  5 pixel SPIFI footprint (red square) and regions mapped (blue outlines) superposed on Digital Sky Survey image (from Mizutani et al. 2002).

The [NII] Lines

  • Far-IR [NII] fine-structure lines arise from ground state term (Figure 2)
  • Levels are collisionally excited (by electrons), energetically accessible, and the lines are typically optically thin so that [NII] lines are important coolants for the ionized gas
    • [NII] 205 m is an important coolant on Galactic scales: ~ 3rd brightest far-IR/submm line observed by FIRAS on COBE – after [CII] and [NII] 122 m (Wright et al. 1991)
    • In ionization-bounded HII regions [NII] 205 m can be a proxy for the Lyman continuum flux
    • The [NII] 122/205 m line ratio yields cloud density
    • The [NII]/[CII] 158 m line ratio constrains fraction of [CII] from ionized gas
  • While the optical fine structure lines are probes of electron temperature, the far-IR lines are extinction-free probes of gas density

[NII] contours: fraction of peak (0.51 K)











5755 Å

3063 Å


6583 Å

122 m

205 m

6548 Å



[CII]/[NII] 205

Figure 3. SPIFI [NII] 205 m contours (blue & red) superposed on (black) 21 cm continuum contours (Retallack, 1983)

 Carinae

Figure 2. N+ Energy level diagram.

[NII] 205 n

[NII]: 122/205

Carina II



Figure 5. Theoretical, and observed [NII] 122/205 m line ratio (blue line and dot) and [CII] 158 m/ [NII] 205 m line ratio (red line and dot) as a functions of ionized gas density.

Figure 4. [NII] 205 m line detected from G287.57-0.59 (Carina II).

Aurora over the South Pole August 22, 2005 (photo by Stephen Parshley)

The SPIFI instrument mounted on the Nasmyth arm of the AST/RO telescope.


Prior ISO Results

  • ISO/LWS mapped much of the same regions in the [OIII] and [NII] 122 m lines (Mizutani, Onaka, and Shibai, 2002)
  • O++ takes 35 eV to form so it traces gas exposed to UV light from high mass stars. From the [OIII] lines, they find the highly ionized gas has two components:
    • Relatively dense (ne ~ 100 to 350 cm-3) component associated with the Carina I and II HII regions
    • Relatively diffuse (ne < 100 cm-3) “halo” component encompassing the entire complex with a diameter of ~ 30 pc

The [NII] Line Ratio

  • The ISO 122 m [NII] to SPIFI 205 m line ratio yields the density of the low ionization gas (Figure 5, blue line and circle)
    • The ratio is ~ 1.2  ne ~ 30 cm-3
    • Even with a 50% calibration uncertainty, it is clear that the [NII] lines (tracing low ionization gas) arise from a very low density medium: 10 < ne < 100 cm-3
  • Therefore, the “halo” of emission seen in the [OII] lines also contains gas in lower ionization states
  • This low ionization, diffuse gas is similar to the warm ionized medium in the Galaxy as a whole

[NII] and [CII]

  • The [CII] line was mapped with ISO LWS and the KAO, and is both bright and widespread over the Carina Nebula (Mizutani, Onaka, and Shibai, 2004, Brooks et al. 2003)
  • In galaxies the [CII] line emission predominantly arises from the warm dense photodissociation regions (PDRs) on the far-UV exposed surfaces of molecular clouds (cf. Stacey et al. 1991)

-- However, [CII] can also arise from low density ionized gas

  • With an ionization potential of 14.5 eV, the 205 m [NII] line only arises from ionized gas regions
  • Since the [NII] 205 m line has a nearly identical critical density for thermalization as [CII] in ionized regions (~ 50 cm-3), the [CII]/[NII] 205 m line intensity ratio yields the fraction of the observed [CII] that arises from the ionized medium – subject to relative C+ to N+ abundance estimates
  • Figure 5 displays the predicted [CII]/[NII] line intensity ratio (red line) for ionized gas as a function of gas density
  • The observed value (red dot) of 8 means that 4/8 ~ 50% of the observed [CII] line emission arises from the ionized medium
  • This is a much larger fraction than the 10 to 25 % commonly assumed in neutral gas studies of external galaxies, and has implications for the modeling of photodissociation regions including gas density and temperature, and the strength of stellar radiation fields in these studies


  • The Carina Nebula was observed in the 205.178 m [NII] line with SPIFI (Bradford et al. 2002) on the 1.7 m AST/RO telescope (Stark et al. 2001) at the South Pole. The data presented here were obtained on 23 to 25 August 2005, near the middle of our 30 day run. We mapped the region outlined in red in Figure 1 in weather that was unusually poor for the South Pole site, with zenith transmission ~ 7.6%.
  • SPIFI-AST/RO Parameters
  • Beam size: ~ 54” (FWHM)
  • Field of view: ~ 325”  325”
  • Resolving Power: R  /~ 5000, or v ~ 60 km/sec
  • Sensitivity: NEP ~ 3.4  10-15 W/Hz1/2 (front end)

Trec(DSB) ~ 150 K


  • The map of Figure 3 contains only about 20% of our data
  • The [NII] line appears in emission over most of the region
  • The observed line emission largely follows the free-free radio contours (Retallack, 1983)
  • The line peaks at ~ 0.51 K on the position of the Carina II radio continuum peak (G287.57-0.59, Figure 4)
  • The line velocity centroid well matches that of the radio recombination lines (Brooks, Storey, & Whiteoak, 2001)


This work was supported by NSF grants OPP-0094605, OPP-0338149, and NASA grant NNG05GK70H. We are indebted to the Cardiff group under P. A. R. Ade for their excellent filters, to the GSFC group (S.H. Moseley, D. J. Benford, & J. G. Staguhn) for their excellent bolometers, and to J.W. Kooi who set up LO’s for our frequency calibration at the pole. We also thank the many people who have contributed to the success of SPIFI both at AST/RO and the JCMT including, and especially C. M. Bradford, A. D. Bolatto, J.A. Davidson, & M. L. Savage


Bradford et al. Ap. Opt. 41, 2561 (2002)

Brooks, K.J. et al. A&A, 412, 751 (2003)

Brooks, K.J., Storey, J.W.V., & Whiteoak, J.B. MNRAS 327, 46 (2001)

Mizutani, Onaka, & Shibai A&A 382, 610 (2002)

Mizutani, Onaka, & Shibai A&A 423, 579 (2004)

Retallack, D.S. MNRAS 204, 669 (1983)

Stacey et al. ApJ 373, 423 (1991)

Stark, A.A. et al., PASP, 115, 567 (2001)

Wright, E.L. et al. ApJ 381, 200 (1991)