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Abstract We present an analytic model for the cool,T∼ 104K, circumgalactic medium (CGM), describing the gas distribution, and thermal and ionization states. Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation, and allows for nonthermal pressure support, parameterized by the ratio of thermal pressures,η=Phot,th/Pcool,th. We apply the model to the COS-Halos measurements and find that a nominal model withη= 3, gas distribution out tor≈ 0.6Rvir, andMcool= 3 × 109M⊙, corresponding to a volume filling fraction offV,cool≈ 1%, reproduces the Hiand low/intermediate metal ions (Cii, Ciii, Siii, Siiii, and Mgii) mean column densities. Variation of ±0.5 dex inηorMcoolencompasses ∼2/3 of the scatter between objects. Our nominal model underproduces the measured Civand Siivcolumns, and these can be reproduced with (i) a cool phase withMcool∼ 1010M⊙andη≈ 5, or (ii) cooling or mixing gas at intermediate temperatures, withM∼ 1.5 × 1010M⊙and occupying ∼1/2 of the total CGM volume. For cool gas withfV,cool≈ 1%, we estimate an upper limit on the cloud sizes,Rcl≲ 0.5 kpc. Our results suggest that for the average galaxy CGM, the mass and nonthermal support in the cool phase are lower than previously estimated, and extreme scenarios are not necessary. We estimate the rates of cool gas depletion and replenishment, and find accretion onto the galaxy can be offset, allowing over long timescales.
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A Simple Model for Mixing and Cooling in Cloud–Wind Interactions
Abstract We introduce a simple entropy-based formalism to characterize the role of mixing in pressure-balanced multiphase clouds and demonstrate example applications usingenzo-e(magneto)hydrodynamic simulations. Under this formalism, the high-dimensional description of the system’s state at a given time is simplified to the joint distribution of mass over pressure (P) and entropy (K=Pρ−γ). As a result, this approach provides a way to (empirically and analytically) quantify the impact of different initial conditions and sets of physics on the system evolution. We find that mixing predominantly alters the distribution along theKdirection and illustrate how the formalism can be used to model mixing and cooling for fluid elements originating in the cloud. We further confirm and generalize a previously suggested criterion for cloud growth in the presence of radiative cooling and demonstrate that the shape of the cooling curve, particularly at the low-temperature end, can play an important role in controlling condensation. Moreover, we discuss the capacity of our approach to generalize such a criterion to apply to additional sets of physics and to build intuition for the impact of subtle higher-order effects not directly addressed by the criterion.
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- Award ID(s):
- 1835509
- PAR ID:
- 10362258
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 925
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 199
- Size(s):
- Article No. 199
- Sponsoring Org:
- National Science Foundation
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