NGC 7027Near infrared laser radiation from molecular hydrogen was detected in planetary nebula NGC 7027. The central star is the bright point in the center of the image. Note the series of faint concentric shells far from the central star.
(Hubble Space Telescope Photo)
The nonthermal nature of the excitation of molecular hydrogen in photodissociation regions becomes most prominent in the planetary nebula NGC 7027. In 1989, Hasegawa, et al. (1989) discovered an extraordinarily strong emission of the H_2 (molecular hydrogen) v=3-2, S(2) emission at 2.2864 microns in NGC 7027. This line was originally found in the Fourier spectrum taken by Treffers et al. (1976) and remained unidentified since then. The population distribution of H_2 analyzed by Hasegawa, Tanaka, and Brand exhibits a highly nonthermal nature with v=3, J=4, 5, and 6 levels and v=2, J=4 level significantly overpopulated. The v=3-2, S(2) transition is inverted, and the contribution of induced emission, B I / c, is comparable to that of spontaneous emission, A, where A and B are Einstein coefficients. These are the characteristics of 'Laser' emission.
Detection of the unidentified line at 2.2864 microns in other sources are found in the literature. Isaacman (1984) reports detection of this line in two planetary nebulae IC 5117 and NGC 6572. In addition, Thompson et al. (1978) reports detection of this line in the nuclear region of the Seyfert galaxy NGC 1068. Although measurements at higher spectral resolution are required to assign these reported features observed at relatively low resolution to the v=3-2, S(2) line of molecular hydrogen, these may suggest that the laser emission of this line is a relatively common phenomenon.
Spectra of NGC 7027Logarithmic energy distribution spectra of NGC 7027 and several other comparison objects as a function of wavelength in meters. Although quasars and similar objects called 'Seyfert galaxies' are assumed to be extragalactic, their spectral similarities to planetary nebula NGC 7027 leads one to question this assumption. The theory proposed by Varshni (1979) offers a simpler alternative, all these objects are laser stars, a class of star related to the central stars of planetary nebula and high excitation shell stars.
(plot from Weymann, 1969)
NGC 3923The visual appearance of the shells in NGC 3923 resembles those of NGC 7027. Cosmologists are having a hard time coming up with models to explain these concentric spherical shell, especially their sharp outer edges. The currently popular theoretical 'patch' to explain these stunning results is the galaxy collision merger model.
(Courtesy David Malin, copyright Anglo-Australian Telescope Board)
However a recent search for secondary nuclei in 'shell galaxies' (Forbes et al., 1994) has revealed very few candidates, any or all of which could be due chance coincidence with foreground stars. Also Schombert and Wallin (1987) found shells which don't appear bluer that the core, contradicting the primary assumption of the merger scenario. The galaxy collision and merger model is becoming increasingly untenable, exceedingly complex and obscure. Rather than admit that this recent evidence is a strong indication that these objects could be within our galaxy ...
As opposed to regular spontaneous emission, amplified spontaneous emission increases exponentially as a function of distance travelled through the laser gain medium. This exponential factor can lead to extremely large discrepancies in the spatial variations between regular spontaneous emission and the laser emissions. Therefore we conclude that there need not be spatial correlation between the laser line and spontaneous emission lines. On the contrary, we expect certain irregularities in the spatial distribution of laser emission lines due to the slightly longer line-of-sight paths or stronger laser gains in areas where the plasma deviates strongly from thermodynamic equilibrium.
Wether the strong line at 2.2864 microns turns out to originate from H_2 or not, there need not be spatial correlation between the 2.2864 micron H_2 laser line and other spontaneous emission lines from the same species.