Multiphase gas in the circumgalactic medium: relative role of $t_{rm cool}/t_{rm ff}$ and density fluctuations. (arXiv:1901.02903v1 [astro-ph.GA])

Multiphase gas in the circumgalactic medium: relative role of $t_{rm cool}/t_{rm ff}$ and density fluctuations. (arXiv:1901.02903v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Choudhury_P/0/1/0/all/0/1">Prakriti Pal Choudhury</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sharma_P/0/1/0/all/0/1">Prateek Sharma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Quataert_E/0/1/0/all/0/1">Eliot Quataert</a>

We perform a suite of simulations with realistic gravity and thermal balance
in shells to quantify the role of the ratio of cooling time to the free-fall
time ($t_{rm cool}/t_{rm ff}$) and the amplitude of density perturbations
($delta rho/rho$) in the production of multiphase gas in the circumgalactic
medium (CGM). Previous idealized simulations, focussing on small amplitude
perturbations in the intracluster medium (ICM), found that cold gas condenses
out of the hot ICM in global thermal balance when the background $t_{rm
cool}/t_{rm ff} lesssim 10$. Sources such as galaxy wakes and dense
cosmological filaments can seed large density contrast in the medium and give
rise to cold gas in halos with $(t_{rm cool}/t_{rm ff}) > 10$, consistent
with the recent observations. From our simulations, we introduce a condensation
curve in the $(delta rho/rho)$ – min$(t_{rm cool}/t_{rm ff})$ space, that
defines the threshold for condensation of multiphase gas in the CGM. We show
that this condensation curve corresponds to ${(t_{rm cool}/t_{rm ff})}_{rm
blob} lesssim 10$ applied to the overdense blob instead of the background for
which $t_{rm cool}/t_{rm ff}$ can be higher. We also study the modification
of the condensation curve under different conditions like entropy
stratification, jet injection and the introduction of buoyantly rising bubbles.
Steeper (positive) entropy gradients and bubbles shift the condensation curve
to higher amplitudes of perturbations (i.e., make condensation difficult) while
gentle outflows shift it to lower amplitudes of perturbations, making
condensation easier. A constant entropy core, applicable to the CGM in smaller
halos, shows condensation over a larger range of radii as compared to the
steeper entropy profiles in the ICM.

We perform a suite of simulations with realistic gravity and thermal balance
in shells to quantify the role of the ratio of cooling time to the free-fall
time ($t_{rm cool}/t_{rm ff}$) and the amplitude of density perturbations
($delta rho/rho$) in the production of multiphase gas in the circumgalactic
medium (CGM). Previous idealized simulations, focussing on small amplitude
perturbations in the intracluster medium (ICM), found that cold gas condenses
out of the hot ICM in global thermal balance when the background $t_{rm
cool}/t_{rm ff} lesssim 10$. Sources such as galaxy wakes and dense
cosmological filaments can seed large density contrast in the medium and give
rise to cold gas in halos with $(t_{rm cool}/t_{rm ff}) > 10$, consistent
with the recent observations. From our simulations, we introduce a condensation
curve in the $(delta rho/rho)$ – min$(t_{rm cool}/t_{rm ff})$ space, that
defines the threshold for condensation of multiphase gas in the CGM. We show
that this condensation curve corresponds to ${(t_{rm cool}/t_{rm ff})}_{rm
blob} lesssim 10$ applied to the overdense blob instead of the background for
which $t_{rm cool}/t_{rm ff}$ can be higher. We also study the modification
of the condensation curve under different conditions like entropy
stratification, jet injection and the introduction of buoyantly rising bubbles.
Steeper (positive) entropy gradients and bubbles shift the condensation curve
to higher amplitudes of perturbations (i.e., make condensation difficult) while
gentle outflows shift it to lower amplitudes of perturbations, making
condensation easier. A constant entropy core, applicable to the CGM in smaller
halos, shows condensation over a larger range of radii as compared to the
steeper entropy profiles in the ICM.

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