Ionization Source of a Minor-axis Cloud in the Outer Halo of M82

TL;DR

This study reveals that the ionization of a gas cloud in M82's outer halo is primarily caused by shocks from a fast wind, challenging previous photoionization hypotheses and providing insights into superwind evolution.

Contribution

The paper demonstrates that shock ionization, rather than photoionization, explains the ionization of the M82 cap, based on high-resolution optical observations and line ratio analysis.

Findings

Shock velocities of 40-80 km/s explain the emission line ratios.

Momentum from the starburst can drive the observed shocks.

Photoionization requires unrealistically high UV escape fractions.

Abstract

The M82 `cap' is a gas cloud at a projected radius of 11.6 kpc along the minor axis of this well known superwind source. The cap has been detected in optical line emission and X-ray emission and therefore provides an important probe of the wind energetics. In order to investigate the ionization source of the cap, we observed it with the Kyoto3DII Fabry-Perot instrument mounted on the Subaru Telescope. Deep continuum, Ha, [NII]6583/Ha, and [SII]6716,6731/Ha maps were obtained with sub-arcsecond resolution. The superior spatial resolution compared to earlier studies reveals a number of bright Ha emitting clouds within the cap. The emission line widths (< 100 km s^-1 FWHM) and line ratios in the newly identified knots are most reasonably explained by slow to moderate shocks velocities (v_shock = 40--80 km s^-1) driven by a fast wind into dense clouds. The momentum input from the M82 nuclear starburst region is enough to produce the observed shock. Consequently, earlier claims of photoionization by the central starburst are ruled out because they cannot explain the observed fluxes of the densest knots unless the UV escape fraction is very high (f_esc > 60%), i.e., an order of magnitude higher than observed in dwarf galaxies to date. Using these results, we discuss the evolutionary history of the M82 superwind. Future UV/X-ray surveys are expected to confirm that the temperature of the gas is consistent with our moderate shock model.