Charge-exchange X-ray emission of nearby star-forming galaxies

Jiren Liu, Q. Daniel Wang, Shude Mao

    Research output: Contribution to journalArticlepeer-review


    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 ionized ions and neutral species. The Kα triplet of He-like ions can be used as a powerful diagnostic, because the charge-exchange X-ray emission (CXE) favours the intercombination 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 (Arp 244), the observed line ratios are consistent with the ratio of 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 phenomenon and contribute a significant fraction of the soft X-ray line emission for galaxies with massive star formation. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.
    Original languageEnglish
    Pages (from-to)3389-3395
    Number of pages6
    JournalMonthly Notices of the Royal Astronomical Society
    Issue number4
    Publication statusPublished - Mar 2012


    • Atomic processes
    • Galaxies: starburst
    • ISM: jets and outflows
    • Plasmas
    • X-rays: galaxies


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