Stellar velocity dispersion in ARP 220 and NGC 6240: Elliptical galaxies in formation

Abstract

We present high-resolution (R ~ 7000) spectra of the 2.3 micron CO absorption bandhead of two luminous merging galaxies, NGC 6240 and Arp 220, obtained with CGS4 on the United Kingdom Infrared Telescope. We have also secured subarcsecond images of NGC 6240 at 2.2 microns using the Montreal Infrared Camera (MONICA) on the Canada-France-Hawaii Telescope (CFHT). The velocity dispersion (σ_obs- = FWHM/2.354) inferred from the broadening of the CO bandhead in the central 2" of the brightest nucleus of NGC 6240 is 359 +/- 21 km s^-1^, which is among the highest values ever found in the center of a galaxy. Arp 220 shows a velocity dispersion of 150 +/- 21 km s^-1^. The infrared light profile of NGC 6240 beyond a radius of 0.7" is well fitted by an r^1/4^ law, but the overall brightness distribution 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 6240 have mass densities, velocity dispersions, and central surface brightnesses consistent with the fundamental parameter plane of elliptical galaxies.