UV Cooling via O VI Emission in the Superwind of M82 Observed with the Far Ultraviolet Spectroscopic Explorer (FUSE)

TL;DR

This study uses FUSE data to detect O VI emission lines in M82's outflow, revealing cooling of warm-hot gas and differing velocities across gas phases, highlighting complex outflow dynamics.

Contribution

It provides the first detection of O VI emission in M82's outflow, indicating radiative cooling of warm-hot gas and revealing velocity differences across gas phases.

Findings

Detected O VI emission lines in M82's outflow.

Measured outflow velocity of ~420 km/s from O VI lines.

Indicated multi-phase gas with varying temperatures and kinematics.

Abstract

We examined archival Far Ultraviolet Spectroscopic Explorer data to search for far-ultraviolet emission lines in the starburst galaxy M82. The observations were made in an outflow region that extends beyond the galactic disk. We found the O VI $\lambda\lambda$ 1032, 1038 emission lines from the galaxy's southern outflow region. The O VI lines suggest that the outflowing warm-hot gas is undergoing radiative cooling. We measured a radial velocity of $\sim$420 km s$^{-1}$ from the O VI lines, which is faster than the velocity seen in H$\alpha$ observations. The O VI $\lambda$1038 emission line seems to be blended with the C II $\lambda$1037 emission line, which has a radial velocity of $\sim$300 km s$^{-1}$, similar to what is observed in H$\alpha$ observations. The outflow medium of M82 appears to be composed of gas in multiple phases with varying temperatures and kinematics. Future spectroscopic observations in high energy regimes covering a wider spatial area are necessary to understand better the properties of the warm-hot gas medium in the outflow.