arXiv subjects

H. J. Mo

Publications and source records attributed to H. J. Mo.

169 records · Page 10Linked to original sources

arXiv1994

An analytic model for the gravitational clustering of dark matter haloes

We develop a simple analytic model for the gravitational clustering of dark haloes. The statistical properties of dark haloes are determined from the initial density field (assumed to be Gaussian) through an extension of the Press-Schechter formalism. Gravitational clustering is treated by a spherical model which describes the concentration of dark haloes in collapsing regions. We test this model against results from a variety of N-body simulations. The autocorrelation function of dark haloes in such simulations depends significantly on how haloes are identified. Our predictions agree well with results based on algorithms which break clusters into subgroups more efficiently than the standard friends-of-friends algorithm. The agreement is better than that found by assuming haloes to lie at the present positions of peaks of the linear density field. We use these techniques to study how the distribution of haloes is biased with respect to that of the mass. The initial (Lagrangian) positions of haloes identified at a given redshift and having circular velocities $v_c=v_c^*(z)$ (i.e. mass equal to the characteristic nonlinear mass $M^*$ at that redshift) are very weakly correlated with the linear density field or among themselves. As a result of dynamical evolution, however, the present-day correlations of these haloes are similar to those of the mass. Haloes with lower $v_c$ are biased toward regions with negative overdensity, while those with higher $v_c$ are biased toward regions with positive overdensity. Among the haloes identified at any given epoch, those with higher circular velocities are more strongly correlated today. Among the haloes of given circular velocity, those at higher redshifts are also more strongly clustered today. In the ``standard CDM'' model, haloes with

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arXiv1994

Substructures and Density Profiles of Clusters in Models of Galaxy Formation

In this paper we investigate, using high resolution N-body simulations, the density profiles and the morphologies of galaxy clusters in seven models of structure formation. We show that these properties of clusters are closely related to the occurrence of a significant merging event in the recent past. The seven models are: (1) the standard CDM model (SCDM) with $Ω_0 = 1$, $Λ_0=0$ and $h=0.5$; (2) a low-density flat model (FL03) with $Ω_0=0.3$, $Λ_0=0.7$ and $h=0.75$; (3) an open model (OP03) with $Ω_0=0.3$, $Λ_0=0$ and $h=0.75$; (4) a low-density flat model (FL02) with $Ω_0=0.2$, $Λ_0=0.8$ and $h=1$; (5) an open model (OP02) with $Ω_0=0.2$, $Λ_0=0$ and $h=1$; (6) a low-density flat model (FL01) with $Ω_0=0.1$ and $Λ_0=0.9$; (7) an open model (OP01) with $Ω_0=0.1$ and $Λ_0=0$. We find that the density profiles and morphologies of clusters depend both on $Ω_0$ and on $Λ_0$. For $Λ_0=0$, these properties are a monotonic function of $Ω_0$. Clusters in OP01 have the steepest density profiles, their density contours are the roundest and show the smallest center shifts. The other extreme case is SCDM, where clusters show the least steep density profiles and the most elongated contours. For a given $Ω_0$ ($<1$), clusters in the flat model (i.e. with $Λ_0=1-Ω_0$) have flatter density profiles and less substructures than in the corresponding open model. In particular, our results show that low-density flat models with $Ω_0\sim 0.3$, which are currently considered as a successful alternative to SCDM, can produce a substantial fraction of clusters with substructures. This is in contrast to the conception that this kind of models may have serious problem in this aspect.

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arXiv1994

Substructures of Clusters and Cosmological Models

We investigate, using high resolution N-body simulations, the density profiles and the morphologies of galaxy clusters in three models of structure formation. We show that these properties are closely related to the occurrence of a significant merging event to a cluster in the recent past. The three models are: (1) the standard CDM model (SCDM) with $Ω_0 = 1$, $Λ_0=0$ and $h=0.5$; (2) a low-density flat model (LCDM) with $Ω_0=0.3$, $Λ_0=0.7$ and $h=0.75$; (3) a CDM dominated open model (OCDM) with $Ω_0=0.1$, $Λ_0=0$. We find that the clusters in the OCDM model have the steepest density profiles, their density contours are the roundest and show the smallest center shifts, among the three models. The clusters in the SCDM model have contours that are most elongated and show the largest center shifts, indicating the existence of frequent mergers in the recent past. The clusters in the LCDM model have density profiles and center shifts (in their density contours) similar to those in the SCDM model, although the density contours tend to be rounder. Our results show that, although the density profiles and morphologies of clusters depend on models of structure formation, the LCDM model, which is currently considered as a successful alternative to the SCDM model, can do as well in producing a substantial fraction of clusters with substructures. This is in contrast to the conception that this model may have serious problem in this aspect.

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arXiv1994

Lyman alpha absorption at low redshifts and hot gas in galactic haloes

Motivated by recent observation of Lanzetta et al. that most luminous galaxies at low redshifts produce $\lya$ absorption at impact parameter $l\lsim 160 \kpch$, we propose that these absorbers are clouds confined by the pressure of ambient hot gas in galactic haloes. We determine the properties of this hot gas and of the absorption systems on the basis of observational and theoretical constraints. The absorbing clouds need to be replenished on about one orbital time ($\sim 10^9$ yrs) in the galactic halo. The pressure and temperature of the gas at radius $r\sim 100\kpc$ are $P=(10-100){\rm cm^{-3} K}$, $T=10^{(5.5-6.5)}{\rm K}$. The model requires that about 10 per cent of the gas in low-redshift galactic haloes is in the hot phase. Such gas in galactic haloes emits x-ray with bolometric luminosity of the order $10^{37-40}{\rm erg\,s^{-1}}$. The plausibility for such gas to exist in current models of galaxy formation is discussed.

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arXiv1994

Damped Lyman Alpha Systems and Galaxy Formation

We examine the constraints on theories of galaxy formation that are obtained from observations of damped $\lya$ (DL) systems, assuming they are gaseous protodisks in dark matter halos. Using the Press-Schechter formalism, we find that the mixed dark matter model, with $\ohdm = 0.3$, $\ocdm = 0.65$, $\obaryon = 0.05$, and $h=0.5$, is ruled out because the number of galactic halos at $z\simeq 3$ is too small to account for the total gaseous mass in DL systems, even under the assumption that all the gas in collapsed halos has settled into disks of neutral gas. The standard CDM model can account for the gas in DL systems if the bias is $b\lsim 2$; the same is true for the CDM model with a cosmological constant, if $b\lsim 1.5$ for $Λ= 0.8$. However, one still needs to assume that a fraction $\gsim 0.4$ of the baryons in collapsed halos at $z\simeq 3$ is in the form of neutral gas in disks. We also calculate the column density distribution $f(\nhi)$ of the DL systems, in terms of the surface density profiles of disks and the distribution of their central column densities. It is shown that the form of $f(\nhi)$ at the high end of column density is a diagnostic for the nature of DL systems.

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arXiv1993

Spatial Distribution of Low Surface Brightness Galaxies

Using redshift samples, we calculate the cross-correlation functions of LSB galaxies with normal galaxies in complete samples (i.e. CfA and IRAS), which enables us to compare directly the amplitudes and shapes of the correlation functions. For pair separations $r\gs 2\mpc$, we find that the shape is in agreement with that of the correlation functions for other galaxies. The amplitudes ($A$) of $\xiab (r)$ are lower than those of the autocorrelation functions for the CfA and IRAS samples, with $A_{\rm LSB-CfA}:A_{\rm CfA-CfA}\approx 0.4$ and $A_{\rm LSB-IRAS}:A_{\rm IRAS-IRAS}\approx 0.6$. These results suggest that LSB galaxies are imbedded in the same large scale structure as other galaxies, but are less strongly clustered. This offers the hope that LSB galaxies may be unbiased tracers of the mass density on large scales. For $r\ls 2\mpc$, the cross-correlation functions are significantly lower than that expected from the extrapolation of $\xiab$ on larger scales, showing that the formation and survival of LSB galaxies may be inhibited by interaction with neighboring galaxies. The model which matches these observations suggests that strong luminosity segregation in galaxy clustering is not a necessary consequence of biased galaxy formation, unless the effect of surface brightness is taken in to account. It is also implies a significant mass density in LSB galaxies.

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arXiv1993

The large-scale structure in a universe dominated by cold plus hot dark matter

Using numerical simulations, we investigate the large-scale gravitational clustering in a flat universe dominated by cold plus hot dark matter (i.e., $Ω_0=\ocdm+\ohdm+\obaryon=1$). Primordial density fluctuation spectrum is taken to have the Zel'dovich-Harrison form. Three models are studied, with Model I having $\ocdm=0.69$, $\obaryon=0.01$, and $\ohdm=0.30$ in one flavor of neutrinos; Model II having $\ocdm=0.60$, $\obaryon=0.10$, and $\ohdm=0.30$ in one flavor of neutrinos; Model III having $\ocdm=0.69$, $\obaryon=0.01$, and $\ohdm=0.30$ in three flavors of neutrinos. The initial density spectra are normalized by the COBE quadrupole measurement, and galaxies are identified from the peaks of initial density fields above a certain threshold chosen, to match the observed two-point correlation on scales $\ls 10\mpc$. Thus the clustering properties of both the mass and the galaxies are completely specified. The biasing parameter (for the `galaxies') determined in this way is $b_g\approx 1.2$ for Model I, 1.5 for Model II and 1.6 for Model III. The clustering and motions of the simulated `galaxies' are compared with recent observations. The spatial distributions of galaxies in the hybrid models are very frothy; filaments, sheets, voids etc. of sizes 10 -- 50$\mpc$ are frequently seen in the simulations.

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