4,702 research outputs found
Galaxy Evolution: Modeling the Role of Non-thermal Pressure in the Interstellar medium
Galaxy evolution depends strongly on the physics of the interstellar medium
(ISM). Motivated by the need to incorporate the properties of the ISM in
cosmological simulations we construct a simple method to include the
contribution of non-thermal components in the calculation of pressure of
interstellar gas. In our method we treat three non-thermal components -
turbulence, magnetic fields and cosmic rays - and effectively parametrize their
amplitude. We assume that the three components settle into a quasi-steady-state
that is governed by the star formation rate, and calibrate their magnitude and
density dependence by the observed Radio-FIR correlation, relating synchrotron
radiation to star formation rates of galaxies. We implement our model in single
cell numerical simulation of a parcel of gas with constant pressure boundary
conditions and demonstrate its effect and potential. Then, the non-thermal
pressure model is incorporated into RAMSES and hydrodynamic simulations of
isolated galaxies with and without the non-thermal pressure model are presented
and studied. Specifically, we demonstrate that the inclusion of realistic
non-thermal pressure reduces the star formation rate by an order of magnitude
and increases the gas depletion time by as much. We conclude that the
non-thermal pressure can prolong the star formation epoch and achieve
consistency with observations without invoking artificially strong stellar
feedback.Comment: 18 pages, 14 figures, accepted to MNRAS. Updated to match final
versio
Instability of Supersonic Cold Streams Feeding Galaxies II. Nonlinear Evolution of Surface and Body Modes of Kelvin-Helmholtz Instability
As part of our long-term campaign to understand how cold streams feed massive
galaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a
supersonic, cold, dense gas stream as it penetrates through a hot, dilute
circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for
realistic conditions, KHI may produce nonlinear perturbations to the stream
during infall. Therefore, we proceed here to study the nonlinear stage of KHI,
still limited to a two-dimensional slab with no radiative cooling or gravity.
Using analytic models and numerical simulations, we examine stream breakup,
deceleration and heating via surface modes and body modes. The relevant
parameters are the density contrast between stream and CGM (), the Mach
number of the stream velocity with respect to the CGM () and the
stream radius relative to the halo virial radius (). We
find that sufficiently thin streams disintegrate prior to reaching the central
galaxy. The condition for breakup ranges from for
to for
. However, due to the large stream
inertia, KHI has only a small effect on the stream inflow rate and a small
contribution to heating and subsequent Lyman- cooling emission.Comment: The main astrophysical results are Figure 22 and Figure 23. Final 7
pages are appendices. Accepted to MNRA
Gravitational Quenching by Clumpy Accretion in Cool Core Clusters: Convective Dynamical Response to Overheating
Many galaxy clusters pose a "cooling-flow problem", where the observed X-ray
emission from their cores is not accompanied by enough cold gas or star
formation. A continuous energy source is required to balance the cooling rate
over the whole core volume. We address the feasibility of a gravitational
heating mechanism, utilizing the gravitational energy released by the gas that
streams into the potential well of the cluster dark-matter halo. We focus here
on a specific form of gravitational heating in which the energy is transferred
to the medium thorough the drag exerted on inflowing gas clumps. Using
spheri-symmetric hydro simulations with a subgrid representation of these
clumps, we confirm our earlier estimates that in haloes >=10^13 solar masses
the gravitational heating is more efficient than the cooling everywhere. The
worry was that this could overheat the core and generate an instability that
might push it away from equilibrium. However, we find that the overheating does
not change the global halo properties, and that convection can stabilize the
cluster by carrying energy away from the overheated core. In a typical rich
cluster of 10^{14-15}solar masses, with ~5% of the accreted baryons in gas
clumps of ~10^8 solar masses, we derive upper and lower limits for the
temperature and entropy profiles and show that they are consistent with those
observed in cool-core clusters. We predict the density and mass of cold gas and
the level of turbulence driven by the clump accretion. We conclude that
gravitational heating is a feasible mechanism for preventing cooling flows in
clusters.Comment: 16 pages, 7 figures, accepted by MNRA
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