Properties of the gas and stellar content of the superluminous galaxy NGC 6240

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Abstract

We present H and K band spectra at resolving powers between 160 and 625 of the superluminous galaxy NGC 6240. Our data show a rich spectrum of molecular hydrogen emission lines and [Fe II]. CO first and second overtone absorption bands characteristic of cool stars are observed, indicating that the near-infrared light from the galaxy is not substantially diluted with nonthermal emission from the active nuclei. Weak Brγ is detected in the source, and the inference that the ratio of ionizing radiation-to-total luminosity in NGC 6240 is very much lower than in star-forming regions is confirmed, as is the relatively small extinction to the near-infrared emitting region. With the exception of one line, the molecular hydrogen spectrum is well fitted by shock models, with kinetic temperatures in the gas ranging from 1300 to 2800 K. About 1% of the molecular gas in the galaxy appears to be shock heated. Using computed shock models, the strength of the 1.64 micron [Fe II] suggests that much of this ionized hydrogen may be due to the shocks themselves, rather than OB stars. The velocity dispersion of the molecular hydrogen emitting gas is identical to that of the shock-ionized gas that is seen in the optical part of the spectrum, suggesting that they are coextensive. Both continuum and 1-0 S(1) line emission of H_2_ are spatially resolved at the kiloparsec level. The physical size and continuum luminosity are consistent with a nuclear bulge population in two merged galaxies that are close to the bright end of the luminosity function for normal late-type spirals. The extent of the molecular hydrogen emission is also comparable to the size scale of molecular gas in more nearby spirals. These facts, combined with the known presence of tidal tails and a double nucleus support a picture in which two spiral galaxies have collided, their bulge populations are now seen overlapped, if not merged, and their molecular disks have impacted in a strongly dissipative manner. The strongest remaining evidence for widespread star formation is the depth of the CO bands in the near-infrared spectrum. These bands suggest that a young population of supergiants dominates the near-infrared continuum from this object, though an extremely late-type and perhaps metal-rich giant population could be the cause. There is some evidence that the H_2_ emission is spatially more extended than the continuum, however, so that the mechanism for H_2_ line emission may be uncoupled from the region in which stars have been formed.

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