TY - JOUR
T1 - An origin for multiphase gas in galactic winds and haloes
AU - Thompson, Todd A.
AU - Quataert, Eliot
AU - Zhang, Dong
AU - Weinberg, David H.
N1 - Funding Information:
TAT thanks the Kavli Institute for Theoretical Physics for support while preparing this work, and Ondrej Pejcha, Crystal Martin, and Tim Heckman for discussions. TAT also thanks Smita Mathur and Laura Lopez for help in interpreting X-ray observations of local starburst galaxies. We thank the Simons Foundation and the organizers of the workshop Galactic Winds: Beyond Phenomenology (J. Kollmeier and A. Benson) where this work germinated. We thank B. Oppenheimer for providing the cooling/heating tables used in this work. We thank the anonymous referee for providing a timely, thorough, and helpful report. EQ was supported in part by NASA ATP grant 12-APT12-0183 and a Simons Investigator award from the Simons Foundation. TAT and DZ were supported in part by NASA grant NNX10AD01G. TAT was also supported by NSF grant 1516967.
Publisher Copyright:
© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
PY - 2020/1/1
Y1 - 2020/1/1
N2 - The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 107-108 K temperature and the shape of the cooling function for T ≾ 107 K imply that outflows with hot gas mass-loss rate relative to star formation rate of β = M hot/M * ≿ 0.5 cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At rcool, the gas produces X-ray and then UV/optical line emission with a total power bounded by ∼10−2 L* if the flow is powered by steady-state star formation with luminosity L*. The wind is thermally unstable at rcool, potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.
AB - The physical origin of high-velocity cool gas seen in galactic winds remains unknown. Following work by B. Wang, we argue that radiative cooling in initially hot thermally-driven outflows can produce fast neutral atomic and photoionized cool gas. The inevitability of adiabatic cooling from the flow's initial 107-108 K temperature and the shape of the cooling function for T ≾ 107 K imply that outflows with hot gas mass-loss rate relative to star formation rate of β = M hot/M * ≿ 0.5 cool radiatively on scales ranging from the size of the energy injection region to tens of kpc. We highlight the β and star formation rate surface density dependence of the column density, emission measure, radiative efficiency, and velocity. At rcool, the gas produces X-ray and then UV/optical line emission with a total power bounded by ∼10−2 L* if the flow is powered by steady-state star formation with luminosity L*. The wind is thermally unstable at rcool, potentially leading to a multiphase medium. Cooled winds decelerate significantly in the extended gravitational potential of galaxies. The cool gas precipitated from hot outflows may explain its prevalence in galactic haloes. We forward a picture of winds whereby cool clouds are initially accelerated by the ram pressure of the hot flow, but are rapidly shredded by hydrodynamical instabilities, thereby increasing β, seeding radiative and thermal instability, and cool gas rebirth. If the cooled wind shocks as it sweeps up the circumgalactic medium, its cooling time is short, thus depositing cool gas far out into the halo. Finally, conduction can dominate energy transport in low-β hot winds, leading to flatter temperature profiles than otherwise expected, potentially consistent with X-ray observations of some starbursts.
KW - Galaxies: evolution
KW - Galaxies: formation
KW - Galaxies: star clusters: general
KW - Galaxies: starburst
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U2 - 10.1093/mnras/stv2428
DO - 10.1093/mnras/stv2428
M3 - Article
AN - SCOPUS:85088536250
SN - 1745-3925
VL - 455
SP - 1830
EP - 1844
JO - Monthly Notices of the Royal Astronomical Society: Letters
JF - Monthly Notices of the Royal Astronomical Society: Letters
IS - 2
ER -