The [NII] Line Ratio

1
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.

• Discussion
• 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 lt 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 lt ne lt 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

• Observations
• 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
• Results
• 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)

Acknowledgements 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 LOs 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

• References
• Bradford et al. Ap. Opt. 41, 2561 (2002)
• Brooks, K.J. et al. AA, 412, 751 (2003)
• Brooks, K.J., Storey, J.W.V., Whiteoak, J.B.
  MNRAS 327, 46 (2001)
• Mizutani, Onaka, Shibai AA 382, 610 (2002)
• Mizutani, Onaka, Shibai AA 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)