Stellar Velocity Dispersion in NGC 6240 and Arp 220

Summary

ABSTRACT

We present high resolution spectroscopic measurements of the 2.3 μm CO band of the luminous IRAS galaxies NGC 6240 and Arp 220. A convolution analysis yields velocity dispersions σ (FWHM/2.354) of 355 and 150 km s^-1 for NGC 6240 and Arp 220, respectively. The velocity dispersion found for NGC 6240 is amongst the highest ever found in a galaxy and it is probably due to violent relaxation associated with the merging of two galaxies. The stellar velocity dispersion of Arp 220 is much smaller than that inferred from the Brα measurement of DePoy et al (1987). Our result implies that there is no dynamical evidence for an AGN in this galaxy. From the dynamical mass derived from our measurements, we infer an infrared mass-to-light ratio M/L_K of 0.3 and 0.1 M⊙/L⊙ for NGC 6240 and Arp 220, respectively. For comparison, the ratio typically found in bulges of normal spiral galaxies is 0.6. These observations suggest that the bulk of the 2.2 μim luminosity emitted from the nucleus of NGC 6240 is associated mostly with normal giant stars while the 2.2 μm continuum of Arp 220 seem to have a significant contribution from young red supergiants.

INTRODUCTION

The velocity dispersion is a fundamental quantity for studying the kinematics and stellar populations of galaxies. In normal galaxies, the velocity dispersion is easily obtained by measuring optical emission or absorption lines. Unfortunately, the situation is more complicated for luminous IRAS galaxies.