2016ApJ...820...48Y

We revealed the detailed structure of a vastly extended Hα-emitting nebula (''Hα nebula'') surrounding the starburst/merging galaxy NGC 6240 by deep narrow-band imaging observations with the Subaru Suprime-Cam. The extent of the nebula is ∼90/kpc in diameter and the total Hα luminosity amounts to L_Hα≈ 1.6x10⁴² erg/s. The volume filling factor and the mass of the warm ionized gas are ∼10^–4-10^–5 and ∼5x10⁸ M_☉, respectively. The nebula has a complicated structure, which includes numerous filaments, loops, bubbles, and knots. We found that there is a tight spatial correlation between the Hα nebula and the extended soft-X-ray-emitting gas, both in large and small scales. The overall morphology of the nebula is dominated by filamentary structures radially extending from the center of the galaxy. A large-scale bipolar bubble extends along the minor axis of the main stellar disk. The morphology strongly suggests that the nebula was formed by intense outflows–superwinds–driven by starbursts. We also found three bright knots embedded in a looped filament of ionized gas that show head-tail morphologies in both emission-line and continuum, suggesting close interactions between the outflows and star-forming regions. Based on the morphology and surface brightness distribution of the Hα nebula, we propose the scenario that three major episodes of starburst/superwind activities, which were initiated ∼10² Myr ago, formed the extended ionized gas nebula of NGC 6240.