We revealed the detailed structure of a vastly extended H \alpha -emitting nebula ( “ H \alpha 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 \sim 90 kpc in diameter and the total H \alpha luminosity amounts to L _ { H \alpha } \approx 1.6 \times 10 ^ { 42 } erg s ^ { -1 } .
The volume filling factor and the mass of the warm ionized gas are \sim 10 ^ { -4 } –10 ^ { -5 } and \sim 5 \times 10 ^ { 8 } M _ { \odot } , 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 H \alpha 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 bi-polar 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 H \alpha nebula , we propose the scenario that three major episodes of starburst/superwind activities which were initiated \sim 10 ^ { 2 } Myr ago formed the extended ionized gas nebula of NGC 6240 .