Notes on Cygnus A coronal lines

RAEF & CNT, Aug/Sep 2002
RAEF & CNT, May 2004

Observational data

• Thornton et al. (1999): Keck optical (PA -42 deg) and IR spectroscopy
• Tadhunter et al.: WHT optical (PA 105 deg, slit 1".3) and Keck IR (PA 105, 180 deg, slit 0"57) spectroscopy
• Ogle et al. (1997): Keck optical (PA 101 deg) spectrpolarimetry
• Young et al. (2002): Chandra X-ray - nuclear region

Spatial distributions

• [OIII] - X-ray
• R-band - X-ray
• [SVI] - X-ray
• [FeXI], [Ar III], [SXI] - X-ray
• 6cm - X-ray
• IR lines and ctm. (0.178 arcsec/pixel), Ogle et al. apertures as coloured bands.
• The E and W apertures compared from Ogle et al. The apertures are: 1" by East 1".3; Nucleus 1".1; West 0".8 - see the overlay on the Keck B-band polarimetry map and HST continuum colour image (neg).

Detected coronal lines (> [FeVII])

Discussion

The coronal line problem in Cygnus A:
links with the extended soft X-ray emission in the cones?

Deductions from the Chandra X-ray observations of the nuclear region (Young et al. 2002)

• It is striking that extended soft X-rays in the cones have a similar SE-NW asymmetry to that seen in the coronal lines visible in optical and IR long-slit spectra -- the soft X-rays and coronal lines are significantly stronger on the SE side of the nucleus.
• If the extended soft X-rays flux is due to the scattering of the nuclear X-ray flux in the cones, the scattering medium is required to have a mean density of ~6 cm-3, assuming complete filling of the cones.
• But if the temperature in the scattering medium is T_e = 2x10^4 K (as suggested by Young et al.) the gas at a density of ~6 cm-3 will be *under-pressured* with respect to the hot ISM of the Cygnus A cluster by a factor ~5-10. There are two possible ways of explaining this discrepancy: (a) the gas has T_e = 2x10^4 K but a higher density (~60 cm-3) and a lower covering factor (~0.1); (b) the density of the gas is ~6 cm-3 but the gas temperature is higher (> 10^5 K). In the latter case, the gas would probably appear as a highly ionized coronal line emitting region (not a conventional 'warm' emission line component).
• In order for the ionization of the scattering plasma to be large enough to avoid significant soft X-ray absorption, an ionization parameter of U > 5x10^-2 is required (Young et al. 2002).
• Also, in order to get significant emission from the highly ionized Neon lines tentatively detected at soft X-ray wavelengths by Young et al., the ionization parameter is required be in the range (1 < U < 15: Young et al. 2002).

Optical-IR Coronal lines (I. van Bemmel, PhD thesis + Keck/NIRSPEC spectra)

• Optical/IR coronal lines are detected from [FeXIV], [FeXI], [SXII], [SXI] and, more tentatively, from [FeX] and [ArXI] (I think the [ArXI] is pretty clear in Thornton et al. and in the Ogle et al. spectra - Bob).
• [FeVII] and [SiVI] appear to be associated with the warm emission line gas that emits the [OIII] etc. (they have similar spatial distributions to the warm gas emission lines, and don't show such a marked spatial asymmetry as coronal lines and soft X-ray emission).
• There is some evidence that the coronal lines measured in the SE cone are blushifted relative to the galaxy rest frame, but this not this is not entirely convincing (e.g. [ArIII], which has a similar wavelength and similar strength to [SXII] and [FeXI], also shows a blueshift)
• The photoionzation models that produce significant coronal line emission require large ionization parameters (0.1 < U < 10); Kraemer and Crenshaw (2000) and Ferguson et al. (1997) have shown that the temperatures for the coronal line plasma can exceed 10^5K if U > 1; in addition, Ferguson et al. (1997) show that the highest ionization line of Fe and S are emitted most efficiently from highly ionized regions with T > 10^5K.
• The problems with detailed comparisons between observations and model results for the coronal lines are: (i) the atomic data for the lines are poor; and (ii) the issue of whether Fe and other elements are significantly depleted onto dust grains in the coronal line region.

Other constraints on the ionization parameter and density

• For Ho=75, the 1-2 keV luminosity of Cygnus A falls in the range: 0.4x10^44 < L_X < 2.1x10^44 erg s-1.
• Using SEDs of radio-loud quasars from Elvis et al. (1994), this translates into ionizing photon luminosities of 1x10^55 < Q_ion < 1.4x10^56 photons s-1.
• For n_e = 6 cm-3 the corresponding ionization parameters are as follows (for radial distances of 0.5, 1.0, 1.5 kpc from the nucleus):

  r= 0.5kpc: 1.85 < U < 26
     1.0kpc: 0.46 < U < 6
     1.5kpc: 0.20 < U < 3
  
  * For n_e = 60 cm-3 we have:
  
  r= 0.5kpc: 0.18 < U < 2.6
     1.0kpc: 5x10^-2 < U < 6x10^-1
     1.5kpc: 2x10^-2 < U < 3x10^-2
  

• Therefore, it is likely that n_e = 60 cm-3 gas can only produce significant coronal line radiation if U, L_X, Q_ion are at the extreme high end of the allowed range (But of course, we can't rule out the idea that the nucleus was significantly brighter sometime in the recent past!)
• Further constraints come from the requirement that the coronal line gas is most likely to be optically thin to the ionizing radiation. If the coronal line region fills the cones, then this implies that the Stroemgren depth must be substantially greater than 1kpc. For U = 0.1, n_e = 6 cm-3, T_e = 1x10^5K the Stroemgren depth is ~10kpc, so there is no problem with the gas being optically thin. On the other hand, for U=0.1, n_e = 60 cm-3, T_e = 2x10^4 the Stroemgren depth is only ~0.2 kpc; and for U=0.1, n_e = 60 cm-3, T_e = 1x10^5 it is ~1kpc.
• Overall, a low density (n_e ~ 6 cm-3) gas that fills the cones fits in better with the requirement that the ionization parameter in the coronal line region is high and that the gas is optically thin to the ionizing continuum.

Conclusion

It appears that a consistent model might be made in which the coronal lines are produced in a photoionized low density plasma (n_e ~ 6 cm-3), that has a high ionization parameter and electron temperature (U > 1, T_e > 10^5K), that fills the cones out to a radius of ~1kpc, and is responsible for producing the extended soft X-ray emission via electron scattering of the nuclear soft X-ray emission.

The model in which the scattering clouds are warm and have n_e ~ 60 cm-3, an electron temperature of T_e = 2x10^4K, and a covering factor within the cones of ~10%, seems less consistent with all the observations.

Outstanding questions and uncertainties

• Can the low density, filled cone model produce coronal line strengths and detailed line ratios that are in agreement with the optical/IR observations? (see below)
• Given that the low density coronal line region has a large covering factor, would it produce a high ionization X-ray absorption edge if Cygnus A was observed from the direction of the radio axis? (the electron column depth would be ~3.5x10^22 cm-2, with U > 1) Do any radio-loud quasars show evidence for such absorption edges?
• Can the optical and near-IR coronal line spatial distributions be reconciled? Does the dust lane have a large effect on the distribution of the optical coronal lines close to the nucleus (particularly on the SE side)?

Line luminosity estimate

References

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