A hydrodynamical study of outflows in starburst galaxies with different driving mechanisms

Abstract

Outflows from starburst galaxies can be driven by thermal pressure, radiation and cosmic rays. We present an analytic phenomenological model that accounts for these contributions simultaneously to investigate their effects on the hydrodynamical properties of outflows. We assess the impact of energy injection, wind opacity, magnetic field strength and the mass of the host galaxy on flow velocity, temperature, density and pressure profiles. For an M82-like wind, a thermally-dominated driving mechanism is found to deliver the fastest and hottest wind. Radiation-driven winds in typical starburst-galaxy configurations are unable to attain the higher flow velocities and temperatures associated with thermal and cosmic ray-driven systems, leading to higher wind densities which would be more susceptible to cooling and fragmentation at lower altitudes. High opacity winds are more sensitive to radiative driving, but terminal flow velocities are still lower than those achieved by other driving mechanisms at realistic opacities. We demonstrate that variations in the outflow magnetic field can influence its coupling with cosmic rays, where stronger fields enable greater streaming but less driving near the base of the flow, instead with cosmic rays redirecting their driving impact to higher altitudes. The gravitational potential is less important in M82-like wind configurations, and substantial variations in the flow profiles only emerge at high altitude in massive haloes. This model offers a more generalised approach to examine the large scale hydrodynamical properties for a wide variety of starburst galaxies.

The radio continuum perspective on cosmic-ray transport in external galaxies

“…This is the basic picture by the analytic wind model of Chevalier and Clegg (1985). Including different driving agents such as radiation pressure and cosmic rays change this picture only slightly (Yu et al 2020). Heesen et al (2018a) applied such a model successfully to the dwarf irregular galaxy IC 10.…”

“…Our radio haloes may require acceleration in particular if the lateral expansion needs to be limited as the morphology of the radio haloes suggests. On the other hand, the wind models such as of Chevalier and Clegg (1985) even with the inclusion of cosmic rays (Samui et al 2010;Yu et al 2020) all predict rapid acceleration near the disc even when adopted to the flux tube geometry (Heald et al 2021). Hence, the jury is still out whether the wind velocity profiles are more in agreement with a linear acceleration across the size of the halo (∼10 kpc), possibly extending even further, as some wind models predict that do not include an extended area of mass-loading but inject all energy at z = 0 kpc Breitschwerdt et al (1991), Everett et al (2008), Recchia et al (2016).…”

“…More generally speaking, we can explore which effects are driving galactic winds, with processes related to stellar feedback and active galactic nuclei (AGNs) the main candidates (Yu et al 2020). Not only the mass-loss rates, but also the composition of the wind fluid is important for galaxy evolution as is the final fate of the gas and the relation that galaxies have with the circum-galactic medium (CGM; see Tumlinson et al 2017, for a recent review).…”

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

“…Outflows driven by thermal mechanical pressure, radiation, and cosmic rays (CRs), were investigated by Yu et al 2020 (hereafter Y20) using a phenomenological HD model (see also Chevalier & Clegg 1985;Thompson et al 2015;Sharma & Nath 2013;Ipavich 1975;Samui et al 2010). Outflows predominantly driven by thermal mechanical pressure were found to be the hottest and have the highest velocities.…”

Outflows in the presence of cosmic rays and waves with cooling

“…For instances, from the hydrodynamic perspective, cosmic rays can affect instabilities, such as, Parker instability, Jeans instability, magnetorotional instability (e.g., Parker 1969Parker , 1966Hanasz & Lesch 2000Ryu et al 2003;Kuwabara et al 2004;Kuwabara & Ko 2006;Ko & Lo 2009;Lo et al 2011;Kuwabara & Ko 2015;Heintz & Zweibel 2018;Heintz et al 2020;Kuwabara & Ko. 2020), and they can modify structures and outflows (e.g., Ko et al 1991;Yang et al 2012;Girichidis et al 2016;Dorfi & Breitschwerdt 2012;Recchia et al 2016;Ruszkowski et al 2017;Mao & Ostriker 2018;Farber et al 2018;Holguin et al 2019;Dorfi et al 2019;Yu et al 2020;Recchia 2020;Ramzan et al 2020).…”