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ABSTRACT In several models of galaxy formation feedback occurs in cycles or mainly at high redshift. At times and in regions where feedback heating is ineffective, hot gas in the galaxy halo is expected to form a cooling flow, where the gas advects inward on a cooling timescale. Cooling flow solutions can thus be used as a benchmark for observations and simulations to constrain the timing and extent of feedback heating. Using analytic calculations and idealized 3D hydrodynamic simulations, we show that for a given halo mass and cooling function, steady-state cooling flows form a single-parameter family of solutions, while initially hydrostatic gaseous haloes converge on one of these solutions within a cooling time. The solution is thus fully determined once either the mass inflow rate $${\dot{M}}$$ or the total halo gas mass are known. In the Milky Way halo, a cooling flow with $${\dot{M}}$$ equal to the star formation rate predicts a ratio of the cooling time to the free-fall time of ∼10, similar to some feedback-regulated models. This solution also correctly predicts observed $$\rm{O\,{\small VII}}$$ and $$\rm{O\,{\small VIII}}$$ absorption columns, and the gas density profile implied by $$\rm{O\,{\small VII}}$$ and $$\rm{O\,{\small VIII}}$$ emission. These results suggest ongoing heating by feedback may be negligible in the inner Milky-Way halo. Extending similar solutions out to the cooling radius however underpredicts observed $$\rm{O\,{\small VI}}$$ columns around the Milky-Way and around other low-redshift star-forming galaxies. This can be reconciled with the successes of the cooling flow model with either a mechanism which preferentially heats the $$\rm{O\,{\small VI}}$$-bearing outer halo, or alternatively if $$\rm{O\,{\small VI}}$$ traces cool photoionized gas beyond the accretion shock. We also demonstrate that the entropy profiles of some of the most relaxed clusters are reasonably well described by a cooling flow solution.
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Cosmic-ray driven galactic winds from the warm interstellar medium
ABSTRACT We study the properties of cosmic-ray (CR) driven galactic winds from the warm interstellar medium using idealized spherically symmetric time-dependent simulations. The key ingredients in the model are radiative cooling and CR-streaming-mediated heating of the gas. Cooling and CR heating balance near the base of the wind, but this equilibrium is thermally unstable, leading to a multiphase wind with large fluctuations in density and temperature. In most of our simulations, the heating eventually overwhelms cooling, leading to a rapid increase in temperature and a thermally driven wind; the exception to this is in galaxies with the shallowest potentials, which produce nearly isothermal $$T \approx 10^4\,$$ K winds driven by CR pressure. Many of the time-averaged wind solutions found here have a remarkable critical point structure, with two critical points. Scaled to real galaxies, we find mass outflow rates $$\dot{M}$$ somewhat larger than the observed star-formation rate in low-mass galaxies, and an approximately ‘energy-like’ scaling $$\dot{M} \propto v_{\rm esc}^{-2}$$. The winds accelerate slowly and reach asymptotic wind speeds of only ∼0.4vesc. The total wind power is $$\sim 1~{{\ \rm per\ cent}}$$ of the power from supernovae, suggesting inefficient preventive CR feedback for the physical conditions modelled here. We predict significant spatially extended emission and absorption lines from 104–105.5 K gas; this may correspond to extraplanar diffuse ionized gas seen in star-forming galaxies.
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- Award ID(s):
- 2107872
- PAR ID:
- 10439945
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 524
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 6374-6391
- Size(s):
- p. 6374-6391
- Sponsoring Org:
- National Science Foundation
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