145 research outputs found
The density distribution of accreting cosmic filaments as shaped by Kelvin-Helmholtz instability
Cosmic filaments play a crucial role in galaxy evolution transporting gas
from the intergalactic medium into galaxies. However, little is known about the
efficiency of this process and whether the gas is accreted in a homogenous or
clumpy way. Recent observations suggest the presence of broad gas density
distributions in the circumgalactic medium which could be related to the
accretion of filaments. By means of high-resolution hydrodynamical simulations,
we explore here the evolution of cold accreting filaments flowing through the
hot circumgalactic medium (CGM) of high-z galaxies. In particular, we examine
the nonlinear effects of Kelvin-Helmholtz instability (KHI) on the development
of broad gas density distributions and on the formation of cold, dense clumps.
We explore a large parameter space in filament and perturbation properties,
such as, filament Mach number, initial perturbation wavelength, and thickness
of the interface between the filament and the halo. We find that the time
averaged density distribution of the cold gas is qualitatively consistent with
a skewed log-normal probability distribution function (PDF) plus an additional
component in form of a high density tail for high Mach-numbers. Our results
suggest a tight correlation between the accreting velocity and the maximum
densities developing in the filament which is consistent with the variance-Mach
number relation for turbulence. Therefore, cosmological accretion could be a
viable mechanism to produce turbulence and broad gas density distributions
within the CGM.Comment: 12 pages, 14 figures, submitted to MNRAS on April 3rd 201
A high baryon fraction in massive haloes at z~3
We investigate the baryon content of the circumgalactic medium (CGM) within
the virial radius of haloes at z ~ 3, by
modelling the surface brightness profile of the giant Ly nebulae
recently discovered by MUSE around bright QSOs at this redshift. We initially
assume fluorescent emission from cold photo-ionized gas confined by the
pressure of a hot halo. Acceptable CGM baryon fractions (equal or smaller than
the cosmological value) require that the cold gas occupies 1% of the
volume, but is about as massive as the hot gas. CGM baryon fractions as low as
30% of the cosmic value, as predicted by some strongly ejective feedback models
at this redshift, are not easy to reconcile with observations, under our
assumptions, unless both the QSO-hosting haloes at are more massive
than recent BOSS estimates based on clustering and the photo-ionized gas is
colder than expected in a standard QSO ionizing radiation field. We also
consider the option that the emission is dominated by photons scattered from
the QSO broad line region. In this scenario, a very stringent lower limit to
the baryon fraction can be obtained under the extreme assumption of optically
thin scattering. We infer in this case a baryon fraction of at least 70% of the
cosmic value, for fiducial parameters. Lower values require halo masses or gas
temperatures different than expected, or that some mechanism keeps the cold gas
systematically over-pressured with respect to the ambient medium.Comment: Accepted for publication in MNRAS. 22 pages, 10 Figure
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