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Stellar velocity dispersion in ARP 220 and NGC 6240: Elliptical galaxies in formation
We present high-resolution (R is approximately 7000) spectra of the 2.3micron CO absorption bandhead of two luminous merging galaxies, NGC 6240and Arp 220, obtained with CGS4 on the United Kingdom InfraredTelescope. We have also secured subarcsecond images of NGC 6240 at 2.2micron using the Montreal Infrared Camera (MONICA) on theCanada-France-Hawaii Telescope (CFHT). The velocity dispersion(sigmaobs = FWHM/2.354) inferred from the broadening of theCO bandhead in the central 2 sec of the brightest nucleus of NGC 6240 is359 +/- 21 km/s, which is among the highest values ever found in thecenter of a galaxy. Arp 220 shows a velocity dispersion of 150 +/- 21km/s. The infrared light profile of NGC 6240 beyond a radius of 0.7 secis well fitted by an r1/4 law, but the overall brightnessdistribution is best represented by a shallower profile such as a King(1966) profile with a core radius of 0.20 kpc. Both Arp 220 and NGC 6240have mass densities, velocity dispersions, and central surfacebrightnesses consistent with the fundamental parameter plane ofelliptical galaxies.
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