arXiv subjects

Weishan Zhu

Publications and source records attributed to Weishan Zhu.

45 records · Page 3Linked to original sources

arXiv2017

Pre-heating of the intergalactic medium by gravitational collapse and ultraviolet background

The preheating of intergalactic medium(IGM) by structure collapsing and ultraviolet background(UVB) are investigated in cosmological hydrodynamical simulations. When gravitational collapsing is the sole heating mechanism, we find that (1) $60\%, 45\%$ of the IGM are heated up to $S>8, 17$ kev cm$^2$ respectively at $z=0$, but the fractions drop rapidly to a few percents at $z=2$, (2) the entropy of the circum-halo gas $S_{\rm{cir}}$ is higher than the virial entropy for more than $75 \%$ of the halos with masses $M<10^{11.5}$ $M_{\odot}$ since $z=2$, but the fraction higher than the entropy, $S_{\rm{pr}}$, required in preventive model of galaxies formation is only $15-20 \%$ for halos with $M<10^{10.5} M_{\odot}$ at $z=0$, and decreases as redshift increases, (3)assuming a metallicity of $Z \leq 0.03 Z_{\odot}$, the fraction of halos whose circum-halo gas having a cooling time longer than the Hubble time $t_{cool,cir}>t_{\rm{H}}$ is merely $5-10 \%$ at $z \lesssim 0.5$, and even less at $z \geq 1$ for halos with $M<10^{10.5} M_{\odot}$. (4) gas in the filaments undergoes the strongest preheating. Furthermore, we show that the UVB can not enhance the fraction of IGM with $S>17$ kev cm$^2$, but can increase the fraction of low mass halos($<10^{10.5} M_{\odot}$) that having $S_{\rm{cir}}>S_{\rm{pr}}$ to $\sim 70 \%$ at $z=0$, and that having $t_{\rm{cool, cir}}>t_{\rm{H}}$ to $15-30 \%$ at $z \lesssim 0.5$. Our results indicate that preheating due to gravitational collapsing and UVB are inadequate to fulfil the needs of preventative model, especially for halos with $10^{10.5}<M<10^{11.5} M_{\odot}$. Nevertheless, these two mechanisms might cause large scale galactic conformity.

astro-ph.GA↗

arXiv2017

Evolution of mass and velocity filed in comic web: comparison between baryonic and dark matter

We investigate the evolution of cosmic web since $z=5$ in grid based cosmological hydrodynamical simulations, focusing on the mass and velocity field of both baryonic and cold dark matter. The tidal tensor of density is used as the main method for web identification, with $λ_{th}=0.2-1.2$. The evolution trends in baryonic and dark matter are similar, while moderate differences are observed. Sheets appeared early and their large scale pattern may have been set up by $z=3$. In term of mass, filaments superseded sheets as the primary collapsing structures at $z\sim2-3$. Tenuous filaments assembled with each other to form prominent ones at $z<~2$. In accordance with the construction of the frame of sheets, the cosmic divergence velocity, $v_{div}$, had been well developed above 2-3 Mpc by z=3. Afterwards, curl velocity, $v_{curl}$, grown dramatically along with the rising of filaments, become comparable to $v_{div}$, for $<2-3 Mpc$ at $z=0$. The scaling of $v_{curl}$ can be described by the hierarchical turbulence model. The alignment between vorticity and eigenvectors of shear tensor in baryonic matter field resembles dark matter, and is even moderately stronger between $\vecω$ and $\vec{e}_1$, and $\vec{e}_3$. Compared with dark matter, mildly less baryonic matter is found residing in filaments and clusters, and its vorticity has been developed more significantly below $2-3 Mpc$. These differences may be underestimated due to the limited resolution and lack of star formation in our simulation. The impact of the change of dominant structures in over-dense regions at $z\sim2-3$ on galaxy formation and evolution is shortly discussed.

astro-ph.CO↗

arXiv2015

Vortical Motions of Baryonic Gas in the Cosmic Web: Growth History and Scaling Relation

The vortical motions of the baryonic gas residing in large scale structures are investigated by cosmological hydrodynamic simulations. Proceeding in the formation of the cosmic web, the vortical motions of baryonic matter are pumped up by baroclinity in two stages, i.e., the formation of sheets, and filaments. The mean curl velocity are about $< 1$, 1-10, 10-150, 5-50 km/s in voids, sheets, filaments and knots at $z=0$, respectively. The scaling of the vortical velocity of gas can be well described by the She-Leveque hierarchical turbulence model in the range of $l<0.65(1.50) h^{-1}$ Mpc in simulation of box size 25(100) $h^{-1}$ Mpc. The fractal Hausdorff dimension of vortical motions, $d$, revealed by velocity structure functions, is $\sim 2.1-2.3$($\sim 1.8-2.1$). It is slightly larger than the fractal dimension of mass distribution in filaments, $\textit{D}^f \sim 1.9-2.2$, and smaller than the fractal dimension of sheets, $\textit{D}^s \sim 2.4-2.7$. The vortical kinetic energy of baryonic gas is mainly transported by filaments. Both the scaling of mass distribution and vortical velocity increments show distinctive transition at the turning scale of $\sim 0.65(1.50) h^{-1}$ Mpc, which may be closely related to the characteristic radius of density filaments.

astro-ph.CO↗

arXiv2014

Cold stream stability during minor mergers

We use high-resolution Eulerian simulations to study the stability of cold gas flows in a galaxy size dark matter halo (10^12 Msun) at redshift z=2. Our simulations show that a cold stream penetrating a hot gaseous halo is stable against thermal convection and Kelvin-Helmholtz instability. We then investigate the effect of a satellite orbiting the main halo in the plane of the stream. The satellite is able to perturb the stream and to inhibit cold gas accretion towards the center of the halo for 0.5 Gyr. However, if the supply of cold gas at large distances is kept constant, the cold stream is able to re-establish itself after 0.3 Gyr. We conclude that cold streams are very stable against a large variety of internal and external perturbations.

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arXiv2013

Turbulence in the Inter-galactic Medium: Solenoidal and Dilatational Motions, and the Impact of Numerical Viscosity

We use a suite of cosmological hydrodynamic simulations, run by two fixed grid codes, to investigate the properties of solenoidal and dilatational motions of the intergalactic medium (IGM), and the impact of numerical viscosity on turbulence in a LCDM universe. The codes differ only in the spatial difference discretization. We find that (1) The vortical motion grows rapidly since $z=2$, and reaches $\sim 10 km/s -90 km/s$ at $z=0$. Meanwhile, the small-scale compressive ratio $r_{CS}$ drops from 0.84 to 0.47, indicating comparable vortical and compressive motions at present. (2) Power spectra of the solenoidal velocity possess two regimes, $\propto k^{-0.89}$ and $\propto k^{-2.02}$, while the total and dilatational velocity follow the scaling $k^{-1.88}$ and $k^{-2.20}$ respectively in the turbulent range. The IGM turbulence may contain two distinct phases, the supersonic and post-supersonic phases. (3) The non-thermal pressure support, measured by the vortical kinetic energy, is comparable with the thermal pressure for $ρ_b \simeq 10-100$, or $T <10^{5.5} K$ at $z=0.0$. The deviation of the baryon fraction from the cosmic mean shows a preliminary positive correlation with the turbulence pressure support. (4) A relatively higher numerical viscosity would dissipate both the compressive and vortical motions of the IGM into thermal energy more effectively, resulting in less developed vorticity, remarkably shortened inertial range, and leading to non-negligible uncertainty in the thermal history of gas accretion. Shocks in regions outside of clusters are significantly suppressed by numerical viscosity since $z=2$, which may directly cause the different levels of turbulence between two codes.

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arXiv2011

Intermittence of the Map of Kinetic Sunyaev-Zel'dovich Effect and Turbulence of IGM

We investigate the possibility of detecting the turbulent state of the IGM with the kinetic Sunyaev-Zel'dovich (kSZ) effect. Being sensitive to the divergence-free component of the momentum field of the IGM, the kSZ effect might be used to probe the vorticity of the turbulent IGM. With cosmological hydrodynamical simulation in the concordance $Λ$CDM universe, we find that the structure functions of 2D kSZ maps show strong intermittence, and the intermittent exponents follow a law similar to the She-Leveque scaling formula of fully developed turbulence. We also find that the intermittence is weak in the maps of thermal Sunyaev-Zel'dovich (tSZ) effect. Nevertheless, the superposition of the kSZ and tSZ effects still contain significant intermittence. We conclude that the turbulent behavior of the IGM may be revealed by the observation of SZ effect on angular scales equal to or less than 0.5 arcminute, corresponding to the multipole parameter $l\geq 2 \times10^4$.

astro-ph.CO↗

arXiv2010

Statistical and dynamical decoupling of the IGM from Dark Matter

The mean mass densities of cosmic dark matter is larger than that of baryonic matter by a factor of about 5 in the $Λ$CDM universe. Therefore, the gravity on large scales should be dominant by the distribution of dark matter in the universe. However, a series of observations incontrovertibly show that the velocity and density fields of baryonic matter are decoupling from underlying dark matter field. This paper shows our attemps to unveil the physics behind this puzzle. In linear approximation, the dynamics of the baryon fluid is completely governed by the gravity of the dark matter. Consequently, the mass density field of baryon matter $ρ_b({\bf r},t)$ will be proportional to that of dark matter $ρ_{\rm dm}({\bf r},t)$, even though they are different from each other initially. In weak and moderate nonlinear regime, the dynamics of the baryon fluid can be sketched by Burgers equation. A basic feature of the Burgers dynamics is to yield shocks. When the Reynolds number is large, the Burgers fluid will be in the state of Burgers turbulence, which consists of shocks and complex structures. On the other hand, the collisionless dark matter may not show such shock, but a multivalued velocity field. Therefore, the weak and moderate nonlinear evolution leads to the IGM-dark matter deviation. Yet, the velocity field of Burgers fluid is still irrotational, as gravity is curl-free. In fully nonlinear regime, the vorticity of velocity field developed, and the cosmic baryonic fluid will no longer be potential, as the dynamics of vorticity is independent of gravity and can be self maintained by the nonlinearity of hydrodynamics. In this case, the cosmic baryon fluid is in the state of fully developed turbulence, which is statistically and dynamically decoupling from dark matter. This scenario provides a mechanism of cohenent explanation of observations.

astro-ph.CO↗

arXiv2010

Log-Poisson Non-Gaussianity of Ly$α$ Transmitted Flux Fluctuations at High Redshift

We investigate the non-Gaussian features of the IGM at redshift $z\sim 5 - 6$ using Ly$α$ transmitted flux of quasar absorption spectra and cosmological hydrodynamic simulation of the concordance $Λ$CDM universe. We show that the neutral hydrogen mass density field and Ly$α$ transmitted flux fluctuations possess all the non-Gaussian features predicted by the log-Poisson hierarchy, which depends only on two dimensionless parameters $β$ and $γ$, describing, respectively, the intermittence and singularity of the random fields. We find that the non-Gaussianity of the Ly$α$ transmitted flux of quasars from $z=4.9$ to $z=6.3$ can be well reconstructed by the hydrodynamical simulation samples. Although the Gunn-Peterson optical depth and its variance underwent a significant evolution in the redshift range of $5 - 6$, the intermittency measured by $β$ is almost redshift-independent in this range. More interesting, the intermittency of quasar's absorption spectra on physical scales $0.1-1$ h$^{-1}$Mpc in redshift $5 - 6$ are found to be about the same as that on physical scales $1-10$ h$^{-1}$Mpc at redshifts $2 - 4$. Considering the Jeans length is less than 0.1 h$^{-1}$Mpc at $z\sim 5$, and $1$ h$^{-1}$Mpc at $z\sim 2$, these results imply that the nonlinear evolution in high and low redshifts will lead the cosmic baryon fluid to a state similar to fully developed turbulence. The log-Poisson high order behavior of current high redshift data of quasar's spectrum can be explained by uniform UV background in the redshift range considered. We also studied the log-Poisson non-Gaussianity by considering inhomogeneous background. With several simplified models of inhomogeneous background, we found the effect of the inhomogeneous background on the log-Poisson non-Gaussianity is not larger than 1-sigma.

astro-ph.CO↗

arXiv2010

Vorticity of IGM Velocity Field on Large Scales

We investigate the vorticity of the IGM velocity field on large scales with cosmological hydrodynamic simulation of the concordance model of LCDM. We show that the vorticity field is significantly increasing with time as it can effectively be generated by shocks and complex structures in the IGM. Therefore, the vorticity field is an effective tool to reveal the nonlinear behavior of the IGM, especially the formation and evolution of turbulence in the IGM. We find that the vorticity field does not follow the filaments and sheets structures of underlying dark matter density field and shows highly non- Gaussian and intermittent features. The power spectrum of the vorticity field is used to measure the development of turbulence in Fourier space. We show that the relation between the power spectra of vorticity and velocity fields is perfectly in agreement with the prediction of a fully developed homogeneous and isotropic turbulence from 0.2 to 3 h^{-1} Mpc at z~0. This indicates that cosmic baryonic field is in the state of fully developed turbulence on scales less than about 3 h^{-1} Mpc. The random field of the turbulent fluid yields turbulent pressure to prevent the gravitational collapsing of the IGM. The vorticity and turbulent pressure are strong inside and even outside of high density regions. In IGM regions with 10 times mean overdensity, the turbulent pressure can be on an average equivalent to the thermal pressure of the baryonic gas with a temperature of 10^5 K. The fully developed turbulence would prevent the baryons in the IGM from falling into the gravitational well of dark matter halos. Moreover, turbulent pressure is dynamical and non-thermal, which makes it different from pre-heating mechanism as it does not affect the thermal state and ionizing process of hydrogen in the IGM.

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