arXiv subjects

H. J. Mo

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

At least 145 records · Page 8Linked to original sources

arXiv2002

Testing Theoretical Models for the Higher-Order Moments of Dark Halo Distribution

Using high--resolution N--body simulations, we test two theoretical models, based either on spherical or on ellipsoidal collapse model, for the higher--order moments of the dark matter halo distribution in CDM models. We find that a theoretical model based on spherical collapse describes accurately the simulated counts--in--cells moments for haloes of several mass ranges. It appears that the model using ellipsoidal collapse instead of spherical collapse in defining dark haloes is unable to improve the models for the higher--order moments of halo distribution, for haloes much smaller than $M^*$ (the mass scale on which the fluctuation of the density field has a rms about 1). Both models are particularly accurate for the descendants of haloes selected at high redshift, and so are quite useful in interpreting the high--order moments of galaxies. As an application we use the theoretical model to predict the higher--order moments of the Lyman break galaxies observed at $z\approx 3$ and their descendants at lower redshifts.

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arXiv2002

Galaxy Formation in Preheated Intergalactic Media

We outline a scenario of galaxy formation in which the gas in galaxy-forming regions was preheated to high entropy by vigorous energy feedback associated with the formation of stars in old ellipticals and bulges and with AGN activity. Such preheating likely occurred at redshifts z ~ 2-3, and can produce the entropy excess observed today in low-mass clusters of galaxies without destroying the bulk of the Lyman alpha forest. Subsequent galaxy formation is affected by the preheating, because the gas no longer follows the dark matter on galaxy scales. The hot gas around galaxy haloes has very shallow profiles and emits only weakly in the X-ray. Cooling in a preheated halo is not inside-out, because the cooling efficiency does not change significantly with radius. Only part of the gas in a protogalaxy region can cool and be accreted into the final galaxy halo. The accreted gas is likely in diffuse clouds and so does not lose angular momentum to the dark matter. Cluster ellipticals are produced by mergers of stellar systems formed prior to the preheating, while large galaxy disks form in low-density environments where gas accretion can continue to the present time.

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arXiv2002

An analytical model for the non-linear redshift-space power spectrum

We use N-body simulations to test the predictions of the redshift distortion in the power spectrum given by the halo model in which the clustering of dark matter particles is considered as a result both of the clustering of dark halos in space and of the distribution of dark matter particles in individual dark halo. The predicted redshift distortion depends sensitively on several model parameters in a way different from the real-space power spectrum. An accurate model of the redshift distortion can be constructed if the following properties of the halo population are modelled accurately: the mass function of dark halos, the velocity dispersion among dark halos, and the non-linear nature of halo bias on small scales. The model can be readily applied to interpreting the clustering properties and velocity dispersion of different populations of galaxies once a cluster-weighted bias (or equivalently an halo occupation number model) is specified for the galaxies. Some non-trivial bias features observed from redshift surveys of optical galaxies and of IRAS galaxies relative to the standard low-density cold dark matter model can be easily explained in the cluster weighted bias model. The halo model further indicates that a linear bias can be a good approximation only on for k <= 0.1 hMpc^{-1}.

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arXiv2002

Constraining Galaxy Formation and Cosmology with the Conditional Luminosity Function of Galaxies

We use the conditional luminosity function (CLF), which gives the number of galaxies with luminosities in the range [L, L+dL] that reside in a halo of mass M, to link the distribution of galaxies to that of dark matter haloes. We seek the CLF that reproduces the galaxy luminosity function and the luminosity dependence of the galaxy clustering strength and test the models by comparing the resulting mass-to-light ratios (M/L) with constraints from the Tully-Fisher (TF) relation. We obtain a number of stringent constraints on both galaxy formation and cosmology. In particular, this method can break the degeneracy between Omega_0 and the power-spectrum normalization sigma_8, inherent in current weak-lensing and cluster-abundance studies. For flat LCDM cosmogonies with sigma_8 normalized by recent weak lensing observations, the best results are obtained for Omega_0~0.3; Omega_0 < 0.2 leads to too large galaxy correlation lengths, while Omega_0 > 0.4 gives too high M/L. The best-fit model for the LCDM concordance cosmology (Omega_0=0.3) predicts M/L that are slightly too high. We discuss a number of possible effects that might remedy this problem, including small modification of cosmological parameters, warm in stead of cold dark matter, systematic errors in current observational data, and the existence of dark galaxies. Finally we use the CLF to predict several statistics about the distribution of galaxy light in the local Universe.

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arXiv2002

Modelling the IGM and the Lyalpha forest at high redshift from the dark matter distribution

A variety of approximate schemes for modelling the low-density Intergalactic Medium (IGM) in the high-redshift Universe is compared to the results of a large high-resolution hydro-dynamical simulation. These schemes use either an analytical description of the dark matter distribution and the IGM or numerical simulations of the DM distributions combined with different approximate relations between dark matter field and the gas distribution. Schemes based on a filtering of the dark matter distribution with a global Jeans scale result in a rather poor description of the gas distribution. An adaptive filtering which takes into account the density/temperature dependence of the Jeans scale is required. A reasonable description of the gas distribution can be achieved using a fit of the mean relation between the dark matter and gas densities in the hydro-dynamical simulation to relate dark matter and gas distribution. In the hydro-dynamical simulations deviations from this mean relation are correlated with gradients in the dark matter peculiar velocity field indicative of shocks in the gas component. A scheme which takes into account this correlation results in a further improved gas distribution. Such adaptive filtering schemes applied to dark matter simulations will be very well suited for studies of statistical properties of the Lyalpha forest which investigate the IGM and the underlying dark matter distribution and require a large dynamic range and/or an extensive parameter study.

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arXiv2002

Understanding the Results of Galaxy-Galaxy Lensing using Galaxy-Mass Correlation in Numerical Simulations

McKay et al. (2002) have recently used measurements of weak galaxy-galaxy lensing in the Sloan Digital Sky Survey to estimate the cross correlation between galaxies and the projected dark matter density field. They derive a relation between aperture mass within a radius of $260\kpch$, $M_{260}$, and lens galaxy luminosity, that does not depend on galaxy luminosity, type or environment. In this paper, we study the cross-correlation between galaxies and dark matter using galaxy catalogs constructed from a high-resolution N-body simulation of a $Λ$CDM Universe. We show that our simulations reproduce the McKay et al. results reasonably well. In the simulation, $M_{260}$ is approximately equal to the halo virial mass for $L_*$ galaxies. $M_{260}$ overestimates the virial mass for fainter galaxies and underestimates it for brighter galaxies. We use the simulations to show that under certain circumstances the halo virial mass may be recovered by fitting an NFW profile to the projected galaxy-mass correlation function. If we apply our method to the observations, we find that $L_*$ galaxies typically reside in halos of $\sim 2\times 10^{12} h^{-1} \msun$, consistent with halo masses estimated from the observed Tully-Fisher relation. In the simulations, the halo virial mass scales with galaxy luminosity as $L^{1.5}$ for central galaxies in halos and for galaxies in low-density regions. For satellite galaxies, and for galaxies in high-density regions, there is no simple relation between galaxy luminosity and halo mass and care must be exercised when interpreting the lensing results.

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arXiv2002

The abundance and clustering of dark haloes in the standard Lambda CDM cosmogony

Much evidence suggests that we live in a flat Cold Dark Matter universe with a cosmological constant. Accurate analytic formulae are now available for many properties of the dark halo population in such a Universe. Assuming current ``concordance'' values for the cosmological parameters, we plot halo abundance against redshift as a function of halo mass, of halo temperature, of the fraction of cosmic matter in haloes, of halo clustering strength, and of the clustering strength of the z=0 descendants of high redshift haloes. These plots are useful for understanding how nonlinear structure grows in the model. They demonstrate a number of properties which may seem surprising, for example: 10^9 solar mass haloes are as abundant at z=20 as L_* galaxies are today; 10^6K haloes are equally abundant at z=8 and at z=0; 10% of all matter is currently in haloes hotter than 1 keV, while more than half is in haloes too cool to trap photo-ionized gas; 1% of all matter at z=15 is in haloes hot enough to ionise hydrogen; haloes of given mass or temperature are more clustered at higher redshift; haloes with the abundance of present-day L_* galaxies are equally clustered at all z<20; the metals produced by star-formation at z>10 are more clustered at z=0 than are L_* galaxies.

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arXiv2002

On the Distribution of Haloes, Galaxies and Mass

The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function $P_V(N_h|δ_m)$ which gives the probability of finding $N_h$ haloes in a volume $V$ with mass density contrast $δ_m$. We study the properties of this function using high-resolution $N$-body simulations, and find that $P_V(N_n|δ_m)$ is significantly non-Poisson. The ratio between the variance and the mean goes from $\sim 1$ (Poisson) at $1+δ_m\ll 1$ to $<1$ (sub-Poisson) at $1+δ_m\sim 1$ to $>1$ (super-Poisson) at $1+δ_m\gg 1$. The mean bias relation is found to be well described by halo bias models based on the Press-Schechter formalism. The sub-Poisson variance can be explained as a result of halo-exclusion while the super-Poisson variance at high $δ_m$ may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behavior of the variance as a function of $δ_m$. Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behavior. We discuss the implications of the stochasticity in halo bias to the modelling of higher-order moments of dark haloes and of galaxies.

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arXiv2001

Probing the Intergalactic Medium with the Lyman alpha forest along multiple lines of sight to distant QSOs

(abridged) We present an effective implementation of analytical calculations of the Lyalpha opacity distribution of the Intergalactic Medium (IGM) along multiple lines of sight (LOS) to distant quasars in a cosmological setting. This method fully accounts for the expected correlations between LOS and the cosmic variance in the large-scale modes of the dark matter distribution. Strong correlations extending up to ~300 kpc (proper) and more are found at redshift z~2->3, in agreement with observations. These correlations are investigated using the cross-correlation coefficient and the cross-power spectrum of the flux distribution along different LOS and by identifying coincident absorption features as fitted with a Voigt profile fitting routine. The cross-correlation coefficient between the LOS can be used to constrain the shape-parameter Gamma of the power spectrum if the temperature and the temperature density relation of the IGM can be determined indepedently. We also propose a new technique to recover the 3D linear dark matter power spectrum by integrating over 1D flux cross-spectra which is complementary to the usual `differentiation' of 1D auto-spectra. The cross-power spectrum suffers much less from errors uncorrelated in different LOS, like those introduced by continuum fitting. Investigations of the flux correlations in adjacent LOS should thus allow to extend studies of the dark matter power spectrum with the Lyalpha forest to significantly larger scales than is possible with flux auto-power spectra. 30 pairs with separation of 1-2 arcmin should be sufficient to determine the 1D cross-spectrum at scales of 60 Mpc/h with an accuracy of about 30% if the error is dominated by cosmic variance.

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arXiv2001

Constraining cosmological parameters with the clustering properties of galaxy clusters in optical and X-ray bands

(abridged) We use a theoretical model to predict the clustering properties of galaxy clusters. Our technique accounts for past light-cone effects on the observed clustering and follows the non-linear evolution of the dark matter correlation function and cluster bias factor. A linear treatment of redshift-space distortions is also included. We perform a maximum-likelihood analysis by comparing the theoretical predictions to a set of observational data, both in the optical (APM and EDCC catalogues) and X-ray band (RASS1 Bright Sample, BCS, XBACs, REFLEX). In the framework of CDM models, we compute the constraints on cosmological parameters. Our results show that X-ray data are more powerful than optical ones, allowing smaller regions in the parameter space. We find that the clustering properties of clusters are almost independent of the matter density parameter and of the presence of a cosmological constant, while they appear to be strongly dependent on the shape parameter. Using the X-ray data only, we obtain Gamma~0.1 and 0.4<sigma_8<1.1 for the Einstein-de Sitter model, while 0.14<Gamma<0.22 and 0.6<sigma_8<1.3 for open and flat models with Omega_0m=0.3. Finally, we use our model to make predictions on the correlation length of galaxy clusters expected in future surveys. In particular, we show the results for an optical catalogue with characteristics similar to the EIS project and for a very deep X-ray catalogue with the characteristics of the XMM/LSS survey. We find that clusters at high redshifts are expected to have larger a correlation length than local ones.

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arXiv2001

The Fundamental Plane of Spiral Galaxies: Theoretical Expectations

Current theory of disk galaxy formation is used to study fundamental-plane (FP) type of relations for disk galaxies. We examine how the changes in model parameters affect these relations and explore the possibility of using such relations to constrain theoretical models. The distribution of galaxy disks in the space of their fundamental properties are predicted to be concentrated in a plane, with the Tully-Fisher (TF) relation (a relation between luminosity $L$ and maximum rotation velocity $V_m$) being an almost edge-on view. Using rotation velocities at larger radii generally leads to larger TF scatter. In searching for a third parameter, we find that both the disk scale-length $R_d$ (or surface brightness) and the rotation-curve shape are correlated with the TF scatter. The FP relation in the $(\Log L, \Log V_m, \Log R_d)$-space obtained from the theory is $L\propto R_d^{α'} V_m^{β'}$, with ${α'}\sim 0.50$ and ${β'}\sim 2.60$, consistent with the preliminary result we obtain from observational data. Among the model parameters we probe, variation in any of them can generate significant scatter in the TF relation, but the effects of the spin parameter and halo concentration can be reduced significantly by introducing $R_d$ while the scatter caused by varying $m_d$ (the ratio between disk mass and halo mass) is most effectively reduced by introducing the parameters which describes the rotation-curve shape. The TF and FP relations combined should therefore provide useful constraints on models of galaxy formation.

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arXiv2001

The Host Haloes of Lyman Break Galaxies and Sub-millimeter Sources

We use the observed sizes and star formation rates of Lyman-break galaxies (LBGs) to understand their properties expected in the hierarchical scenario of galaxy formation. The observed size distribution constrains the masses of the host haloes of LBGs from below, while the observed star-formation distribution constrains the masses from above. Assuming a flat CDM model with a cosmological constant ($Ω_Λ=0.7$), we find that consistency with observations requires the circular velocities of LBG host haloes to be in the range 100--$300 {\rm km s^{-1}}$. The predicted comoving correlation length of these objects is $\sim 3 h^{-1}{\rm Mpc}$, and the predicted velocity dispersion of their stellar contents is typically $70 {\rm km s^{-1}}$. The same prescription applied to larger haloes in the CDM cosmogony predicts the existence of galaxies with star formation rates $\sim 1000 {\rm M_\odot yr^{-1}}$ at redshift $z\sim 3$. We explore the possibility of identifying these galaxies to be the bright sub-millimeter (sub-mm) sources detected by SCUBA. The model predicts that the host haloes of these sub-mm sources are massive. These galaxies are predicted to be strongly correlated, with a comoving correlation length of $\sim 7 h^{-1}{\rm Mpc}$. The descendants of the bright sub-mm sources should reside in clusters of galaxies at the present time, and it is likely that these objects are the progenitors of giant ellipticals. The model predicts the existence of a relatively bright and red population of galaxies at $z\sim 3$, which may be observed in the K-band.

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arXiv2001

Observational signatures of feedback in QSO absorption spectra

Models for the formation of galaxies and clusters of galaxies require strong feedback in order to explain the observed properties of these systems. We investigate whether such feedback has observational consequences for the intergalactic medium, as probed in absorption towards background quasars. A typical quasar sight-line intersects one proto-cluster per unit redshift, and significant feedback from forming galaxies or AGN, heating the proto-cluster gas, will result in a large clearing of reduced absorption in the Lyman alpha-forest. Such a gap could be detected at redshift > 3 when the mean opacity is high. Feedback from Lyman-break galaxies in proto-clusters can be probed by the absorption lines produced in their winds. Strong feedback from galaxies has a major impact on the number and properties of absorption lines with neutral hydrogen column densities of 10^16/ cm^2. This feedback can be probed with multiple sight-lines and by studying the unsaturated higher-order lines of the Lyman series. Galactic winds from dwarf galaxies should break-up into clouds, in order not to over produce the number of absorption lines. These clouds can then coast to large distances.

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arXiv2000

Low Redshift QSO Lyman alpha Absorption Line Systems Associated with Galaxies

In this paper we present Monte-Carlo simulations of Lyman alpha absorption systems which originate in galactic haloes, galaxy discs and dark matter (DM) satellites around big central haloes. It is found that for strong Lyman alpha absorption lines galactic haloes and satellites can explain ~20% and 40% of the line number density of QSO absorption line key project respectively. If big galaxies indeed possess such large numbers of DM satellites and they possess gas, these satellites may play an important role for strong Lyman alpha lines. However the predicted number density of Lyman-limit systems by satellites is \~0.1 (per unit redshift), which is four times smaller than that by halo clouds. Including galactic haloes, satellites and HI discs of spirals, the predicted number density of strong lines can be as much as 60% of the HST result. The models can also predict all of the observed Lyman-limit systems. The average covering factor within 250 kpc/h is estimated to be ~0.36. And the effective absorption radius of a galaxy is estimated to be ~150 kpc/h. The models predict W_r propto rho^{-0.5} L_B^{0.15} (1+z)^{-0.5}. We study the selection effects of selection criteria similar to the imaging and spectroscopic surveys. We simulate mock observations through known QSO lines-of-sight and find that selection effects can statistically tighten the dependence of line width on projected distance. (abridged)

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arXiv2000

Tully-Fisher Relation and its Implications for Halo Density Profile and Self-interacting Dark Matter

We show that the Tully-Fisher relation observed for spiral galaxies can be explained in the current scenario of galaxy formation without invoking subtle assumptions, provided that galactic-sized dark haloes have shallow, core-like central profiles, with a core radius proportional to halo circular velocity. In such a system, both disk and halo contribute significantly to the maximum rotation of the disk, and the interaction between the disk and halo components acts to reduce the scatter in the Tully-Fisher relation. With model parameters chosen in plausible ranges, the model can well accommodate the zero-point, slope, scatter of the observed Tully-Fisher relation, as well as the large range of disk sizes. The model predicts that LSB disks obey a Tully-Fisher relation similar to that of normal disks, if disk mass-to-light ratio is properly taken into account. The halo profile required by the Tully-Fisher relation is as shallow as that required by the rotation curves of faint disks, but much shallower than that predicted by conventional CDM models. Our results cannot be explained by some of the recent proposals for resolving the conflict between conventional CDM models and the rotation-curve shapes of faint galaxies. If dark matter self-interaction (either scattering or annihilation) is responsible for the shallow profile, the observed Tully-Fisher relation requires the interaction cross section σ_X to satisfy <σ_{X}|v|>/m_{X}~10^{-16} cm^3/s/GeV.

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arXiv1999

Ellipsoidal collapse and an improved model for the number and spatial distribution of dark matter haloes

The Press--Schechter, excursion set approach allows one to make predictions about the shape and evolution of the mass function of bound objects. It combines the assumption that objects collapse spherically with the assumption that the initial density fluctuations were Gaussian and small. While the predicted mass function is reasonably accurate at the high mass end, it has more low mass objects than are seen in simulations of hierarchical clustering. This discrepancy can be reduced substantially if bound structures are assumed to form from an ellipsoidal, rather than a spherical collapse. In the spherical model, a region collapses if the initial density within it exceeds a threshold value, delta_sc. This value is independent of the initial size of the region, and since the mass of the collapsed object is related to its initial size, delta_sc is independent of final mass. In the ellipsoidal model, the collapse of a region depends on the surrounding shear field, as well as on its initial overdensity. Therefore, there is a relation between the density threshold value required for collapse, and the mass of the final object. We provide a fitting function to this delta_ec(m) relation for initially Gaussian fields which simplifies the inclusion of ellipsoidal dynamics in the excursion set approach. We discuss the relation between the excursion set predictions and the halo distribution in high resolution N-body simulations, and show that our simple parametrization of the ellipsoidal collapse model represents a significant improvement on the spherical model on an object-by-object basis. Finally, we show that the associated statistical predictions, the mass function and the large scale halo-to-mass bias relation, are also more accurate than the standard predictions.

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arXiv1998

The Formation and Evolution of Disk Galaxies

We review some of our recent progress in modelling the formation of disk galaxies in the framework of hierarchical structure formation. Our model is not only consistent with the local observations, but also provides a good description of disk evolution out to redshift z~1. We use this model to interpret recent observational results on Lyman-break galaxies (LBGs) at z~3. Assuming that these galaxies are associated with the most massive halos and adopting an empirical law for their star formation rates, we find that many properties of the LBG population, including their correlations, sizes and kinematics can be accommodated in the model.

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arXiv1998

The Nature of the Host Galaxies for Gamma-Ray Bursts

It has been suggested recently that the rate of gamma-ray bursts (GRBs) is proportional to the star formation rate in the universe. In this paper, we study the nature of GRB hosts expected in this scenario. We improve upon previous studies by incorporating a luminosity function for the GRBs, as required by observations. This model provides a good match to the observed number counts of GRBs as a function of peak-count rate. The model predicts that the host galaxies have their redshift distribution peaked around z~1, and about 15 percent have z>2.5. This high-redshift fraction have the same properties as the star-forming galaxies recently discovered by the Lyman-break technique. At z<~1, many of the GRBs may be hosted by faint blue galaxies. Using a photometric redshift sample of galaxies from the Hubble Deep Field, we find that the host galaxies have magnitudes in the range from 21.5 to 28 in the I-band, and about 90 percent of them have semi-major axis smaller than 1.3 arcsec. Assuming isotropic emission, the typical peak-luminosity and total energy of GRBs are in an Einstein-de Sitter universe with H_0=100km/s/Mpc. We also discuss further observational tests of this scenario.

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