arXiv subjects

H. J. Mo

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

At least 163 records · Page 9Linked to original sources

arXiv1998

The Structure and Clustering of Lyman Break Galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs) are consistent with them being the central galaxies of the most massive dark halos present at z~3. This conclusion holds in all currently popular hierarchical models for structure formation, and is almost independent of the global cosmological parameters. We examine whether the sizes, luminosities, kinematics and star-formation rates of LBGs are also consistent with this identification. Simple formation models tuned to give good fits to low redshift galaxies can predict the distribution of these quantities in the LBG population. The LBGs should be small (with typical half-light radii of 0.6-2 kpc/h), should inhabit haloes of moderately high circular velocity (180-290 km/s) but have low stellar velocity dispersions (70-120 km/s) and should have substantial star formation rates (15-100 Msun/yr). The numbers here refer to the predicted median values in the LBG sample of Adelberger et al. (1998); the first assumes an Omega=1 universe and the second a flat universe with Omega=0.3. For either cosmology these predictions are consistent with the current (rather limited) observational data. Following the work of Kennicutt (1998) we assume stars to form more rapidly in gas of higher surface density. This predicts that LBG samples should preferentially contain objects with low angular momentum, and so small size, for their mass. In contrast, samples of damped Lyman alpha systems (DLSs), should be biased towards objects with large angular momentum. Bright LBGs and DLSs may therefore form distinct populations, with very different sizes and star formation rates, LBGs being smaller and more metal-rich than DLSs of similar mass and redshift.

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arXiv1998

Observational constraints on disk galaxy formation

We use data from the literature to constrain theoretical models of galaxy formation. We show how to calculate the dimensionless spin parameter $λ$ of the halos of disk galaxies and we compare the distribution of $λ$ with that observed in cosmological $N$-body simulations. The agreement is excellent, which provides strong support for the hierarchical picture of galaxy formation. Assuming only that the radial surface density distribution of disks is exponential, we estimate crudely the maximum-disk mass-to-light ratio in the $I$-band and obtain $ \la 3.56h$, for a Hubble constant of $100h~\kms \mpc^{-1}$. We discuss this result and its limitations in relation to other independent determinations of $M/L$. We also define a dimensionless form of the Tully-Fisher relation, and use it to derive a value of the baryon fraction in disk galaxies. For galaxies with circular velocity $\vmax>100\kms$, the median value is $md = 0.086 (M/L/3.56h)$. Assuming that the gas fraction in galactic halos is at most as large as that in clusters, we also conclude that $ \la 2.48h^{-1/2}$.

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arXiv1998

The Origin of the Hubble Sequence for Spiral Galaxies

We suggest that the bulge-to-disc ratios of spiral galaxies are primarily determined by the angular momenta of their host haloes predicted in current hierarchical clustering models for structure formation. Gas with low specific angular momentum becomes self-gravitating and presumably forms stars before it can settle into a rotationally supported disc. We assume this part of the gas in a dark halo to form a bulge, while the rest is assumed to settle into a rotationally supported disc. With these assumptions the predicted bulge-to-disc ratios in mass and in size, and other correlations between the bulge and disc components can match current observational results. This model predicts the existence of a population of low-surface-brightness galaxies which are bulgeless. The model also predicts that the bulge component has many properties in common with the disc component, because both form through similar processes. In particular, many bulges should be supported (at least partially) by rotation.

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arXiv1997

Spatial correlation function and pairwise velocity dispersion of galaxies: CDM models versus the Las Campanas Survey

We show, with the help of large N-body simulations, that the real-space two-point correlation function and pairwise velocity dispersion of galaxies can both be measured reliably from the Las Campanas Redshift Survey. The real-space correlation function is well fitted by the power law $ξ(r)=(r_0/r)^γ$ with $r_0=(5.06\pm0.12)\mpc$ and $γ=1.862\pm 0.034$, and the pairwise velocity dispersion at $1\mpc$ is $(570\pm 80)\kms$. A detailed comparison between these observational results and the predictions of current CDM cosmogonies is carried out. We construct 60 mock samples for each theoretical model from a large set of high resolution N-body simulations, which allows us to include various observational selection effects in the analyses and to use exactly the same methods for both real and theoretical samples. We demonstrate that such a procedure is essential in the comparison between models and observations. The observed two-point correlation function is significantly flatter than the mass correlation function in current CDM models on scales $\la 1\mpc$. The observed pairwise velocity dispersion is also lower than that of dark matter particles in these models. We propose a simple antibias model to explain these discrepancies. This model assumes that the number of galaxies per unit dark matter mass, $N/M$, decreases with the mass of dark haloes. The predictions of CDM models with $σ_8Ω_0^{0.6}\sim 0.4$-0.5 and $Ω_0 h\sim 0.2$ are in agreement with the observational results, if the trend of $N/M$ with $M$ is at the level already observed for rich clusters of galaxies. Thus CDM models with cluster-abundance normalization are consistent with the observed correlation function and pairwise velocity dispersion of galaxies. A high level of velocity bias is not required in these models.

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arXiv1997

The Evolution of Galactic Disks

We use recent observations of high-redshift galaxies to study the evolution of galactic disks over the redshift range 0<z<1. The data are inconsistent with models in which disks were already assembled at z=1 and have evolved only in luminosity since that time. Assuming that disk properties change with redshift as powers of 1+z and analysing the observations assuming an Einstein-de Sitter universe, we find that for given rotation speed, disk scalelength decreases with z as ~(1+z)^{-1}, total B-band mass-to-light ratio decreases with z as ~(1+z)^{-1}, and disk luminosity (again in B) depends only weakly on z. These scalings are consistent with current data on the evolution of disk galaxy abundance as a function of size and luminosity. Both the scalings and the abundance evolution are close to the predictions of hierarchical models for galaxy formation. If different cosmogonies are compared, the observed evolution in disk-size and disk abundance favours a flat low-density universe over an Einstein-de Sitter universe.

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arXiv1997

A Minihalo Model for the Lyman Limit Absorption Systems at High Redshift

We propose that a large fraction of the QSO Lyman limit absorption systems (LLS) observed at high redshift (z > 3) originate from gas trapped in small objects, such as minihalos, that form prior to reionization. In the absence of a strong UV flux, the gas is predominantly neutral and may form clouds with HI column density NHI > 10^18 cm^-2. Due to their high densities and high HI column densities, these clouds are not destroyed by the onset of the UV background at a later time. Thus, if not disrupted by other processes, such as mergers into larger systems or `blow away' by supernovae, they will produce LLS. We show that the observed number density of LLS at high redshifts can well be reproduced by the survived `minihalos' in hierarchical clustering models such as the standard cold dark matter model. The number density of LLS in such a population increases with z even beyond the redshifts accessible to current observations and dies off quickly at z < 2. This population is distinct from other populations because the absorbing systems have small velocity widths and a close to primordial chemical composition. The existence of such a population requires that the reionization of the universe occurs late, at z < 20.

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arXiv1997

Global stability and the mass-to-light ratio of galactic disks

We examine the global stability of an exponential stellar disk embedded in a dark matter halo and constrain the mass-to-light ratio of observed disks in the I-band, Y_I. Assuming only that the radial surface density distribution of disks is exponential, we derive an analytic upper limit: Y_I <~ 2.4h, for a Hubble constant of 100h~\kms\mpc^{-1}. Using N-body simulations we derive a stability criterion significantly different from that of previous authors. Using this criterion we argue that, almost independent of the concentration of the halo, Y_I <~ 1.9h. We discuss this result in relation to other independent determinations of Y_I, its limitations and its implications for theories of disk formation and barred galaxies.

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arXiv1997

Gaseous Galactic Halos and QSO Absorption Line Systems

Recent observations have shown that (some) metal line absorption systems in QSO spectra arise from gas in galactic halos with radii much larger than the optical radii of galaxies. We show that the observed galaxy-absorber connections are natural results of galaxy formation in hierarchical cosmogonies, where galaxies form from gas cooling and condensation in dark matter halos.

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arXiv1997

On the Physical Connections between Galaxies of Different Types

Galaxies can be classified in two broad sequences which are likely to reflect their formation mechanism. The `main sequence', consisting of spirals, irregulars and all dwarf galaxies, is probably produced by gas settling within dark matter haloes. We show that the sizes and surface densities along this sequence are primarily determined by the distributions of the angular momentum and formation time of dark haloes. They are well reproduced by current cosmogonies provided that galaxies form late, at $z \la 2$. In this scenario, dwarf ellipticals were small `disks' at $z\sim 1$ and become `ellipticals' after they fall into cluster environments. The strong clustering of dwarf ellipticals is then a natural by-product of the merging and transformation process. The number of dwarf galaxies predicted in a cluster such as Virgo is in good agreement with the observed number. On the other hand, the `giant branch', consisting of giant ellipticals and bulges, is probably produced by the merging of disk galaxies. Based on the observed phase-space densities of galaxies, we show that the main bodies of all giant ellipticals can be produced by dissipationless mergers of high-redshift disks. However, high-redshift disks, although denser than present-day ones, are still not compact enough to produce the high central phase space density of some low-luminosity ellipticals. Dissipation must have occurred in the central parts of these galaxies during the merger which formed them.

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arXiv1997

The Formation of Galactic Disks

We study the population of galactic disks expected in current hierarchical clustering models for structure formation. A rotationally supported disk with exponential surface density profile is assumed to form with a mass and angular momentum which are fixed fractions of those of its surrounding dark halo. We assume that haloes respond adiabatically to disk formation, and that only stable disks can correspond to real systems. With these assumptions the predicted population can match both present-day disks and the damped Lyman alpha absorbers in QSO spectra. Good agreement is found provided: (i) the masses of disks are a few percent of those of their haloes; (ii) the specific angular momenta of disks are similar to those of their haloes; (iii) present-day disks were assembled recently (at z<1). In particular, the observed scatter in the size-rotation velocity plane is reproduced, as is the slope and scatter of the Tully-Fisher relation. The zero-point of the TF relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values derived from disk dynamics. High redshift disks are predicted to be small and dense, and could plausibly merge together to form the observed population of elliptical galaxies. In many (but not all) currently popular cosmogonies, disks with rotation velocities exceeding 200 km/s can account for a third or more of the observed damped Lyman alpha systems at z=2.5. Half of the lines-of-sight to such systems are predicted to intersect the absorber at r>3kpc/h and about 10% at r>10kpc/h. The cross-section for absorption is strongly weighted towards disks with large angular momentum and so large size for their mass. The galaxy population associated with damped absorbers should thus be biased towards low surface brightness systems.

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arXiv1997

Analytical Approximations to Galaxy Clustering

We discuss some recent progress in constructing analytic approximations to the galaxy clustering. We show that successful models can be constructed for the clustering of both dark matter and dark matter haloes. Our understanding of galaxy clustering and galaxy biasing can be greatly enhanced by these models.

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arXiv1996

Analytical Approximations to the Low-Order Statistics of Dark Matter Distribution

We show that in a hierarchical clustering model the low-order statistics of the density and the peculiar velocity fields can all be modelled semianalytically for a given cosmology and an initial density perturbation power spectrum $P(k)$. We present such models for the two-point correlation function $ξ(r)$, the amplitude $Q$ of the three-point correlation function, the mean pairwise peculiar velocity $< v_{12}(r)> $, the pairwise peculiar velocity dispersion $< v_{12}^2(r)>$, and the one-point peculiar velocity dispersion $< v_1^2 >$. We test our models against results derived from N-body simulations. These models allow us to understand in detail how these statistics depend on $P(k)$ and cosmological parameters. They can also help to interpret, and maybe correct for, sampling effects when these statistics are estimated from observations. The dependence of the small-scale pairwise peculiar velocity dispersion on rich clusters in the sample, for instance, can be studied quantitatively. There are also significant implications for the reconstruction of the cosmic density field from measurements in redshift space.

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arXiv1996

The Evolution of Correlation Functions and Power Spectra in Gravitational Clustering

Hamilton et al. (1991) proposed a simple formula relating the nonlinear autocorrelation function of the mass distribution to the primordial spectrum of density fluctuations for gravitational clustering in an $Ω=1$ universe. High resolution N-body simulations show this formula to work well for scale-free spectra $P(k) \propto k^n$ when the spectral index $n\sim 0$, but not when $n \lsim -1$. We show that a modified version of the formula can work well provided its form depends on $n$. This dependence can be derived from a simple physical model for collapse from Gaussian initial conditions. Our modified formula is easy to apply and is an excellent fit to N-body simulations with $0 \leq n \leq -2$. It can also be applied to non-power law initial spectra such as that of the standard Cold Dark Matter model by using the local spectral index at the current nonlinear scale as the effective value of $n$ at any given redshift. We give analytic expressions both for the nonlinear correlation function and for the nonlinear power spectrum.

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arXiv1996

Constraints on the Cosmic Structure Formation Models from Early Formation of Giant Galaxies

A recent observation of Steidel et al. indicates that a substantial fraction of giant galaxies were formed at an epoch as early as redshift $z>3-3.5$. We show that this early formation of giant galaxies gives strong constraints on models of cosmic structure formation. Adopting the COBE normalization for the density perturbation spectrum, we argue that the following models do not have large enough power on galactic scales to yield the observed abundance: (i) standard cold dark matter (CDM) models (where mass density $Ω_0=1$ and power index $n=1$) with the Hubble constant $h\lsim 0.35$; (ii) tilted CDM models with $h=0.5$ and $n\lsim 0.75$; (iii) open CDM models with $h\lsim 0.8$ and $Ω_0 \lsim 0.3$, and (iv) mixed dark matter models with $h=0.5$ and $Ω_ν\gsim 0.2$. Flat CDM models with a cosmological constant $λ_0 \sim 0.7$ are consistent with the observation, provided that $h\gsim 0.6$. Combined with constraints from large-scale structure formation, these results imply that the flat CDM model with a low $Ω_0$ is the only one that is fully consistent with observations. We predict that these high-redshift galaxies are more strongly clustered than normal galaxies observed today.

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arXiv1996

High-Order Correlations of Peaks and Halos: a Step toward Understanding Galaxy Biasing

We develop an analytic model for the hierarchical correlation amplitudes S_j(R)= \bxi_j(R)/\bxi_2^{j-1}(R) of density peaks and dark matter halos in the quasi-linear regime. The statistical distribution of density peaks and dark matter halos within the initial density field are determined by the peak formalism and by an extension of the Press-Schechter formalism, respectively. Modifications of these distributions caused by gravitationally induced motions are treated using a spherical collapse model. We test our model against results from a variety of N-body simulations. The model works well for peaks and for halos that are identified earlier than the time when the moments are calculated. Because halos are spatially exclusive at the time of their identification, our model is only qualitatively correct for halos identified at the same time as the moments are calculated. The S_j depend only weakly on the bias parameter b for large b but increase rapidly with decreasing b at b\sim 1. Thus if galaxies are associated with peaks in the initial density field, or with dark halos formed at high redshifts, a measurement of S_j in the quasilinear regime should determine whether galaxies are significantly biased relative to the mass. We use our model to interpret the observed high order correlation functions of galaxies and clusters. We find that if the values of S_j for galaxies are as high as those given by the APM survey, then APM galaxies should not be significantly biased.

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arXiv1996

Gaseous Galactic Halos and QSO Absorption Line Systems

A model of Lyman limit QSO absorption systems is investigated where they are produced in gaseous galactic halos with a two-phase structure: a hot phase at the halo virial temperature in approximate pressure equilibrium, and a cold, photoionized phase in the form of clouds confined by the pressure of the hot medium, and falling through it to the halo center. We show that the masses of these clouds must be over a relatively narrow range so that they are stable against gravitational collapse and evaporation. These masses, near $10^6 \msun$, also lead to a covering factor near unity. The hot phase is required to have a core radius such that its cooling time in the core is equal to the age of the halo, and the mass in the cold phase is determined by the rate at which the hot gas cools. We calculate the number of Lyman limit systems arising in halos of different masses in a CDM model and their impact parameters implied by these assumptions, and find them to be in reasonable agreement with observations. The observed low ionization systems such as MgII are well reproduced as arising from the photoionized phase, and are mostly in halos around massive galaxies, while CIV selected systems are predicted to be more commonly associated with lower mass galaxies and larger impact parameters. The hot phase may give rise to detectable absorption lines in OVI, while the column densities predicted for other highly ionized species are low and difficult to observe.

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arXiv1996

The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe

We show that if a sample of galaxy clusters is complete above some mass threshold, then hierarchical clustering theories for structure formation predict its autocorrelation function to be determined purely by the cluster abundance and by the spectrum of linear density fluctuations. Thus if the shape of the initial fluctuation spectrum is known, its amplitude $σ_8$ can be estimated directly from the correlation length of a cluster sample in a way which is independent of the value of $Ω_0$. If the cluster mass corresponding to the sample threshold is also known, it provides an independent estimate of the quantity $σ_8Ω_0^{0.6}$. Thus cluster data should allow both $σ_8$ and $Ω_0$ to be determined observationally. We explore these questions using N-body simulations together with a simple but accurate analytical model based on extensions of Press-Schechter theory. Applying our results to currently available data we find that if the linear fluctuation spectrum has a shape similar to that suggested by the APM galaxy survey, then a correlation length $r_0$ in excess of $20\mpch$ for Abell clusters would require $σ_8>1$, while $r_0<15\mpch$ would require $σ_8<0.5$. With conventional estimates of the relevant mass threshold these imply $Ω_0\la 0.3$ and $Ω_0\ga 1$ respectively.

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arXiv1995

An analytic model for the spatial clustering of dark matter haloes

We develop a simple analytic model for the gravitational clustering of dark matter haloes to understand how their spatial distribution is biased relative to that of the mass. The statistical distribution of dark haloes within the initial density field (assumed Gaussian) is determined by an extension of the Press-Schechter formalism. Modifications of this distribution caused by gravitationally induced motions are treated using a spherical collapse approximation. We test this model against results from a variety of N-body simulations, and find that it gives an accurate description of a bias function. This bias function is sufficient to calculate the cross-correlation between dark haloes and mass, and again we find excellent agreement between simulation results and analytic predictions. Because haloes are spatially exclusive, the variance in the count of objects within spheres of fixed radius and overdensity is significantly smaller than the Poisson value. This seriously complicates any analytic calculation of the autocorrelation function of dark halos. Our simulation results show that this autocorrelation function is proportional to that of the mass over a wide range in $R$, even including scales where both functions are significantly greater than unity. The constant of proportionality is very close to that predicted on large scales by the analytic model. This result permits an entirely analytic estimate of the autocorrelation function of dark haloes. We use our model to study how the distribution of galaxies may be biased with respect to that of the mass. In conjunction with other data these techniques should make it possible to measure the amplitude of cosmic mass fluctuations and the density of the Universe.

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