Giant H$\alpha$ nebula surrounding the starburst merger NGC 6240

TL;DR

This study reveals a vast, complex Hα-emitting nebula around NGC 6240, shaped by starburst-driven superwinds, with detailed structure, correlations with X-ray gas, and evidence of multiple outflow episodes over the past 100 million years.

Contribution

First detailed imaging of the extended Hα nebula around NGC 6240, linking its morphology to starburst-driven superwinds and multiple outflow episodes.

Findings

Nebula extends ~90 kpc with Hα luminosity ~1.6×10^{42} erg/s.

The nebula contains filaments, loops, bubbles, knots, and correlates with soft X-ray emission.

Evidence of multiple starburst episodes over ~100 Myr shaping the nebula.

Abstract

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 $\approx 1.6 \times 10^{42}$ 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.