Superwind-driven intense H2 emission in NGC 6240

Abstract

We have performed a long-slit K-band spectroscopic observation of the luminous infrared galaxy NGC 6240. Spatially extended H2 emission is detected over 3.3 kpc around the two nuclei. The peak position of the H2 v =1-0 S(1) emission is located at ∼ 0″.3-0″.4 north of the southern nucleus. Based on line-ratio analyses, we suggest that the excitation mechanism of H2 is pure thermal at most positions. We find the following three velocity components in the H2 emission around the southern nucleus and its south region: a blue-shifted component (≈ -250 km s-1 with respect to Vsys), which is recognized as a distinct C-shape distortion in the velocity field around the southern nucleus; a high-velocity blue-shifted "wing" component (∼ -1000 km s-1 with respect to Vsys); and a component indicating possible line splitting of ∼ 500 km s-1. We show that these kinematic properties can be reproduced by expanding motion of a shell-like structure around the southern nucleus. The offset peak position of the H2 emission can be understood if we assume that the shell expanding to the north interacts with the extragalactic molecular gas which has been transferred during the merging of the two nuclei. At the interface the cloud-crushing mechanism may work efficiently, and shock-excited intense H2 emission is thus expected there. With this mechanism, the H2 luminosity can be explained without global shock driven by the collision of two nuclei. All of these findings lead us to propose a model in which most of the H2 emission is attributed to shock excitation driven by the superwind activity of the southern nucleus.