Charge Exchange X-ray Emission of Nearby Star-forming Galaxies

TL;DR

This study investigates the contribution of charge exchange X-ray emission (CXE) to the soft X-ray line emission in nearby star-forming galaxies, revealing that CXE is likely a common and significant process influencing galaxy outflow diagnostics.

Contribution

It provides the first systematic analysis of OVII triplet line ratios in multiple galaxies to identify CXE as a major source of X-ray emission, challenging the assumption of purely thermal origins.

Findings

Forbidden lines are often as strong or stronger than resonance lines.

Line ratios in several galaxies are consistent with CXE predictions.

Multiple origins of X-ray emission are indicated in some galaxies.

Abstract

Properties of hot gas outflows from galaxies are generally measured from associated X-ray line emission assuming that it represents atomic transitions in thermally excited hot gas. X-ray line emission, however, can also arise from the charge exchange between highly ionised ions and neutral species. The K\alpha\ triplet of He-like ions can be used as a powerful diagnostic, because the charge exchange X-ray emission (CXE) favours the inter-combination and forbidden lines, while the thermal emission favours the resonance line. We analyse the OVII triplet of a sample of nine nearby star-forming galaxies observed by the XMM-Newton reflection grating spectrometers. For most galaxies, the forbidden lines are comparable to or stronger than the resonance lines, which is in contrast to the thermal prediction. For NGC 253, M51, M83, M61, NGC 4631, and the Antennae galaxy, the observed line ratios are consistent with the ratio of the CXE; for M94 and NGC 2903, the observed ratios indicate multiple origins; for M82, different regions show different line ratios, also indicating multiple origins. We discuss other possible mechanisms that can produce a relatively strong forbidden line, such as a collisional non-equilibrium-ionization recombining/ionizing plasma, which are not favoured. These results suggest that the CXE may be a common process and contribute a significant fraction of the soft X-ray line emission for galaxies with massive star formation.