Charge-exchange emission and cold clumps in multiphase galactic outflows

Abstract

Large-scale outflows from starburst galaxies are multi-phase, multi-component fluids. Charge-exchange lines which originate from the interfacing surface between the neutral and ionised components are a useful diagnostic of the cold dense structures in the galactic outflow. From the charge-exchange lines observed in the nearby starburst galaxy M82, we conduct surface-to-volume analyses and deduce that the cold dense clumps in its galactic outflow have flattened shapes, resembling a hamburger or a pancake morphology rather than elongated shapes. The observed filamentary Hα features are therefore not prime charge-exchange line emitters. They are stripped material torn from the slow moving dense clumps by the faster moving ionised fluid which are subsequently warmed and stretched into elongated shapes. Our findings are consistent with numerical simulations which have shown that cold dense clumps in galactic outflows can be compressed by ram pressure, and also progressively ablated and stripped before complete disintegration. We have shown that some clumps could survive their passage along a galactic outflow. These are advected into the circumgalactic environment, where their remnants would seed condensation of the circumgalactic medium to form new clumps. The infall of these new clumps back into the galaxy and their subsequent re-entrainment into the galactic outflow form a loop process of galactic material recycling.

Flux Contribution and Geometry of Charge Exchange Emission in the Starburst Galaxy M82

“…Numerical (e.g., Cooper et al 2008) and hydrodynamical simulations (e.g., Melioli et al 2013;Schneider et al 2020) show that these vast Hα structures are created by swept-up gas that has been dragged by plasma outflows. Our results support the idea that the CX process occurs at the interface between the plasma and gas components within the outflows; similar conclusions were reached by Lallement (2004) and Wu et al (2020) from numerical simulations.…”

Outflows from starburst galaxies with various driving mechanisms and their X-ray properties

“…For a clump with a density ∼ 10 cm −3 (Melioli et al 2013), a minor-axis size of ∼ 10 pc and a velocity of ∼ 600 km s −1 (e.g. Strickland & Heckman 2009) (comparable to the galactic outflow velocities obtained the HD simulations shown in this work) embedded within the hot component of a flow of temperature ∼ 3 × 10 6 K, the evaporation length-scale (and, presumably, the length-scale over which such entrainment would persist) is of order 10s of kpc (see Wu et al 2020). This evaporation length would be longer if the temperature of the outflow gas is cooler.…”

“…A further confirmation of the cold clumps could be derived from the strengths of charge-exchange lines (see e.g. Zhang et al 2014;Wu et al 2020). The detection of a H-like 6.97 keV Fe K𝛼 line, due to photo-ionisation, implies the presence of a strong hard X-ray radiation field, which might be provided by a weak AGN, or by populations of ULXs in the host galaxy of an outflow.…”

“…Galactic outflows are multi-phase in nature (see Aguirre et al 2005;Konami et al 2011;Zhang et al 2014;Martizzi et al 2016;Li et al 2017;Kim & Ostriker 2018;Lopez et al 2020;Schneider et al 2020;Wu et al 2020). The charge-exchange lines observed in the X-ray spectra of outflows from starburst galaxies indicate the presence of neutral clumps (Konami et al 2011;Zhang et al 2014).…”

“…The gas temperature in some outflows may reach 10 7 K (McKeith et al 1995;Shopbell & Bland-Hawthorn 1998). There is evidence of multi-phase structure in galactic winds, with cooler dense clumps, of temperature 10 2 − 10 4 K (Strickland et al 1997;Lehnert et al 1999), entrained within the hot gas (see Heckman 2003;Zhang et al 2014;Walter et al 2017;Wu et al 2020). The presence of this multi-phase structure implies the coexistence of material with very different structural and local thermal conditions.…”