arXiv subjects

Matthias Steinmetz

Publications and source records attributed to Matthias Steinmetz.

At least 163 records · Page 9Linked to original sources

arXiv1999

The Cosmological Origin of Disk Galaxy Scaling Laws

We discuss possible origins of scaling laws relating structural properties of disk galaxies within the context of hierarchically clustering theories of galaxy formation. Using gasdynamical simulations that incorporate the effects of star formation we illustrate these global trends and highlight the difficulties faced by models that envision disk galaxies as the final outcome of a hierarchical sequence of merger events. In particular, we focus on the cosmological origin of the Tully-Fisher relation, and argue that this correlation between the total luminosity and rotation speed of disk galaxies is a natural result of the approximately scale free formation process of the massive halos they inhabit. Although the slope and scatter of the Tully-Fisher relation can be readily reproduced in hierarchical formation scenarios, the observed zero-point of the relation is inconsistent with simulations of galaxy formation in Cold Dark Matter universes, a difficulty that can be traced to the high central mass concentration of dark halos formed in this cosmogony. This result indicates that substantial revision to the new ``standard'' model of structure formation ($Λ$CDM) may be needed in order to accommodate observations on the scale of individual disk galaxies.

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arXiv1999

Numerical Simulations of Galaxy Formation

The current status of numerical simulations of galaxy formation is reviewed. After a short description of the main numerical simulation techniques, three sample applications illustrate how numerical simulations have provided deeper insight in the galaxy formation process and how they have illuminated success and failure of the hierarchical galaxy formation paradigm: N-body simulations demonstrate that the density profiles of dark matter halos that form in hierarchical clustering scenarios follow a characteristic law. A comparison with the kinematics of disk galaxies however unravels that these density profiles are too concentrated. Hydrodynamical simulation show that the highly irregular velocity field of merging subclumps at redshift $z\approx 3$ can easily account for the observed asymmetry in the absorption profiles of low ionization species in damped \Lya absorption systems. The built-up of galaxies due to mergers is however also cause for one of the major inconsistencies of hierarchical structure formation models, the failure to reproduce the sizes of the present day disk galaxies due to excessive transport of angular momentum from the baryonic to the dark matter component. Hydrodynamical simulations that include star formation show that scaling laws like the Tully-Fisher relation can readily be reproduced in hierarchical scenarios, however the high concentration of dark matter halos results in a zero-point of the simulated Tully-Fisher relation that is incompatible wi th observations.

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arXiv1999

The core density of dark matter halos: a critical challenge to the Lambda-CDM paradigm?

We compare the central mass concentration of Cold Dark Matter halos found in cosmological N-body simulations with constraints derived from the Milky Way disk dynamics and from the Tully-Fisher relation. For currently favored values of the cosmological parameters ($Ω_0 \sim 0.3$; $Λ_0=1-Ω_0 \sim 0.7$; $h \sim 0.7$; COBE- and cluster abundance-normalized $σ_8$; Big-Bang nucleosynthesis $Ω_b$), we find that halos with circular velocities comparable to the rotation speed of the Galaxy have typically {\it three times} more dark matter inside the solar circle than inferred from observations of Galactic dynamics. Such high central concentrations of dark matter on the scale of galaxy disks also imply that stellar mass-to-light ratios much lower than expected from population synthesis models must be assumed in order to reproduce the zero-point of the Tully-Fisher relation. Indeed, even under the extreme assumption that {\it all} baryons in a dark halo are turned into stars, disks with conventional $I$-band stellar mass-to-light ratios ($M/L_I \sim 2 \pm 1 (M/L_I)_{\odot}$) are about two magnitudes fainter than observed at a given rotation speed. We examine several modifications to the $Λ$CDM model that may account for these discrepancies and conclude that agreement can only be accomplished at the expense of renouncing other major successes of the model. Reproducing the observed properties of disk galaxies thus appears to demand substantial revision to the currently most successful model of structure formation.

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arXiv1998

The cosmological origin of the Tully-Fisher relation

We use high-resolution cosmological simulations that include the effects of gasdynamics and star formation to investigate the origin of the Tully-Fisher relation in the standard Cold Dark Matter cosmogony. Luminosities are computed for each model galaxy using their full star formation histories and the latest spectrophotometric models. We find that at z=0 the stellar mass of model galaxies is proportional to the total baryonic mass within the virial radius of their surrounding halos. Circular velocity then correlates tightly with the total luminosity of the galaxy, reflecting the equivalence between mass and circular velocity of systems identified in a cosmological context. The slope of the relation steepens slightly from the red to the blue bandpasses, and is in fairly good agreement with observations. Its scatter is small, decreasing from \~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular cosmological model we explore here seems unable to account for the zero-point of the correlation. Model galaxies are too faint at z=0 (by about two magnitudes) if the circular velocity at the edge of the luminous galaxy is used as an estimator of the rotation speed. The Tully-Fisher relation is brighter in the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent observations of z~1 galaxies. We conclude that the slope and tightness of the Tully-Fisher relation can be naturally explained in hierarchical models but that its normalization and evolution depend strongly on the star formation algorithm chosen and on the cosmological parameters that determine the universal baryon fraction and the time of assembly of galaxies of different mass.

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arXiv1998

How universal are the density profiles of dark halos?

We investigate the formation of virialized halos due to the gravitational collapse of collisionless matter using high-resolution N-body simulations. A variety of formation scenarios are studied, ranging from hierarchical clustering to monolithic radial collapse. The goal of these experiments was to study departures from the universal density profiles recently found to arise in cosmological settings. However, we found that even for models which exhibit quite a different formation history, the density and velocity dispersion profiles of the virialized halos are strikingly similar. Power law density profiles do not result even in models with initial power law profiles and no initial substructure or non-radial motions. Such initial conditions give rise to a radial orbit instability which leads to curved velocity dispersion and density profiles. The shapes of the density profiles in all our models are well parameterized by the profiles of halos formed in a generic cosmological setting. Our results show that the universality of dark halo density profiles does not depend crucially on hierarchical merging as has been suggested recently in the literature. Rather it arises because apparently different collapse histories produces a near universal angular momentum distribution among the halo particles. We conclude that the density and velocity dispersion profiles of virialized halos in an expanding universe are robust outcomes of gravitational collapse, nearly independent of the initial conditions and the formation history.

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arXiv1998

Photometric Evolution of Galaxies in Cosmological Scenarios

The photometric evolution of galaxies in a hierarchically clustering universe is investigated. The study is based on high resolution numerical simulations which include the effects of gas dynamics, shock heating, radiative cooling and a heuristic star formation scheme. The outcome of the simulations is convolved with photometric models which enables us to predict the appearance of galaxies in the broad band colors U, B, V, R, I and K. We demonstrate the effect of the mutual interplay of the hierarchical build-up of galaxies, photometric evolution, k-correction, and intervening absorption on the appearance of forming disk galaxies at redshift one to three. We also discuss to what extend the numerical resolution of current computer simulations is sufficient to make quantitative predictions on surface density profiles and color gradients.

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arXiv1998

Model Predictions for Clustering and Morphologies at HDF depths

The current status of numerical simulations of the formation of galaxies is reviewed. Success and failure of modeling galaxies at low and high redshift is demonstrated using a variety of examples, such as the Tully-Fisher relation, the appearance of high-redshift galaxies and the kinematics of damped Lya systems. The relationship between the clustering properties of high-z galaxies and the present generation of galaxies is emphasized.

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arXiv1997

The relation between QSO absorption systems and high redshift galaxies

The relation between high redshift galaxies and QSO absorption systems is discussed in the context of hierarchical galaxy formation. It is demonstrated that imprints of the reionization history of the universe are detectable in the $b$-parameter distribution of the \Lya forest at redshifts two to four, favoring models in which hydrogen and helium are reionized simultaneously at or before redshift five by a quasar-like spectrum of UV photons. Hydrodynamical simulations including star formation, feedback due to supernovae and chemical enrichment are also presented. Energy feedback in form of kinetic energy can give rise to an efficient transport of metals out to distances of a few hundred kpc. The observational signature of this metal transport mechanism compared to simple homogeneous enrichment models is discussed. Finally, it is shown that present day $L^*$ galaxies typically have several progenitors at $z\approx3$ spread over a few hundred kpc. These progenitors are closely associated with damped \Lya and Ly-limit systems, and have velocity dispersions, luminosities and colors comparable to U-dropout galaxies at $z\approx 3$.

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arXiv1997

Probing the thermal history of the Intergalactic Medium with Lyman-alpha absorption lines

The Doppler parameter distribution of Lyman-alpha absorption is calculated for a set of different reionization histories. The differences in temperature between different reionization histories are as large as a factor three to four depending on the spectrum of the ionizing sources and the redshift of helium reionization. These temperature differences result in observable differences in the Doppler parameter distribution. Best agreement with the observed Doppler parameter distribution between redshift two and four is found if hydrogen and helium are reionized simultaneously at or before redshift five with a quasar-like spectrum.

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arXiv1997

Damped Lyman alpha absorbers at high redshift -- large disks or galactic building blocks?

We investigate the nature of the physical structures giving rise to damped Lyman alpha absorption systems (DLAS) at high redshift. In particular, we examine the suggestion that rapidly rotating large disks are the only viable explanation for the characteristic observed asymmetric profiles of low ionization absorption lines. We demonstrate using hydrodynamic simulations of galaxy formation in a cosmological context that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity of the dark matter halo of the forming protogalactic clump (v_{wid} ~ 0.6 times v_{vir} for the median values with a large scatter of order a factor two between different lines-of-sight). The typical virial velocity of the halos required to give rise to the DLAS population is about 100 km/s and most standard hierarchical structure formation scenarios can easily account even for the largest observed velocity widths. We conclude that the evidence that DLAS at high redshift are related to large rapidly rotating disks with v_circ >= 200 km/s is not compelling.

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arXiv1997

Energy Input and Mass Redistribution by Supernovae in the Interstellar Medium

We present the results of numerical studies of supernova remnant evolution and their effects on galactic and globular cluster evolution. We show that parameters such as the density and the metallicity of the environment significantly influence the evolution of the remnant, and thus change its effects on the global environment (e.g., globular clusters, galaxies) as a source of thermal and kinetic energy. We conducted our studies using a one-dimensional hydrodynamics code, in which we implemented a metallicity dependent cooling function. Global time-dependent quantities such as the total kinetic and thermal energies and the radial extent are calculated for a grid of parameter sets. The quantities calculated are the total energy, the kinetic energy, the thermal energy, the radial extent, and the mass. We distinguished between the hot, rarefied bubble and the cold, dense shell, as those two phases are distinct in their roles in a gas-stellar system. We also present power-law fits to those quantities as a function of environmental parameters after the extensive cooling has ceased. The power-law fits enable simple incorporation of improved supernova energy input and matter redistribution (including the effect of the local conditions) in galactic/globular cluster models. Our results for the energetics of supernova remnants in the late stages of their expansion give total energies ranging from 9e49 to 3e50 ergs, with a typical case being 1e50 erg, depending on the surrounding environment. About 8.5e49 erg of this energy can be found in the form of kinetic energy. Supernovae play an important role in the evolution of the interstellar medium

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arXiv1996

QSO Metal Absorption Systems at High Redshift and the Signature of Hierarchical Galaxy Formation

In hierarchical cosmogonies at redshift three the matter content of a typical present-day galaxy is dispersed over several individual clumps embedded in sheet-like structures, often aligned along filaments. We have used hydrodynamical simulations to investigate the spatial distribution and absorption properties of metal enriched gas in such regions of ongoing galaxy formation. Strong CII and SiIV absorption occurs for lines-of-sight passing the densest regions close to the center of the protogalactic clumps, while CIV is a good tracer of the prominent filamentary structures and OVI becomes the strongest absorption feature for lines-of-sight through low-density regions far away from fully-collapsed objects. Observed column density ratios can be well reproduced if a mean metallicity [Z/H] = -2.5, relative abundances as found in metal-poor stars, a UV background with intensity J_{-22} = 3 at the Lyman limit, and either a power law spectrum (alpha = -1.5) or the spectral shape proposed by Haardt & Madau (1996) are assumed. The observed scatter in [C/H] is about a magnitude larger than that in the simulations suggesting inhomogeneous metal distribution. The detailed small and large scale kinematics observed in CIV and HI are also well reproduced.

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arXiv1996

Two--body heating in numerical galaxy formation experiments

We show that discreteness effects related to classical two-body relaxation produce spurious heating of the gaseous component in numerical simulations of galaxy formation. A simple analytic model demonstrates that this artificial heating will dominate radiative cooling in any simulation where the mass of an individual dark matter particle exceeds a certain critical value. This maximum mass depends only on the cooling function of the gas, on the fraction of the material in gaseous form, and (weakly) on typical temperatures in the gas. It is comparable to, or smaller than, the dark matter particle masses employed in most published simulations of cosmological hydrodynamics and galaxy formation. Any simulation which violates this constraint will be unable to follow cooling flows, although catastrophic cooling of gas may still occur in regions with very short cooling times. We use a series of N--body/smoothed particle hydrodynamics simulations to explore this effect. In simulations which neglect radiative cooling, two--body heating causes a gradual expansion of the gas component. When radiative effects are included, we find that gas cooling is almost completely suppressed for dark matter particle masses above our limit. Although our test simulations use smoothed particle hydrodynamics, similar effects, and a similar critical mass, are expected in any simulation where the dark matter is represented by discrete particles.

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arXiv1996

Non-equilibrium effects on line-of-sight size estimates of QSO absorption systems

Estimates of the linear extent of heavy-element absorption systems along the line-of-sight to a QSO often assume that the cloud is photoionized and that the temperature takes the equilibrium value where photo-heating balances line cooling. We show that rather small deviations from this photoionization equilibrium temperature caused by additional heating processes will lead to an overestimate of the neutral hydrogen fraction and thus to an underestimate of the thickness of the absorber by about two orders of magnitude. Such temperature deviations are indicated both by observations and numerical simulations. This interpretation reconciles the discrepancy between the rather small extent of heavy-element-absorption systems parallel to the line-of-sight obtained from a standard photoionization analysis and the much larger transverse sizes estimates inferred from the observation of common absorption in the spectra of close quasar pairs.

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arXiv1996

Numerical Simulations of Galaxy Formation

An overview over the current status of modeling galaxies by means of numerical simulations is given. After a short description of how galaxies form in hierarchically clustering scenarios, success and failures of current simulations are demonstrated using three different applications: the morphology of present day galaxies; the appearance of high redshift galaxies; and the nature of the Ly-alpha forest and metal absorption lines. It is shown that current simulations can qualitatively account for many observed features of galaxies. However, the objects which form in these simulations suffer from a strong overcooling problem. Star formation and feedback processes are likely to be indispensable ingredients for a realistic description even of the most basic parameters of a galaxy. The progenitors of todays galaxies are expected to be highly irregular and concentrated, as supported by recent observations. Though they exhibit a velocity dispersion similar to present day L > L^* galaxies, they may be much less massive. The filamentary distribution of the gas provides a natural explanation for Ly-alpha and metal absorption systems. Furthermore, numerical simulations can be used to avoid misinterpretations of observed data and are able to alleviate some apparent contradictions in the size estimates of Ly-alpha absorption systems.

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arXiv1996

The Effects of a Photoionizing UV Background on the Formation of Disk Galaxies

We use high resolution N-body/gasdynamical simulations to investigate the effects of a photoionizing UV background on the assembly of disk galaxies in hierarchically clustering universes. We focus on the mass and rotational properties of gas that can cool to form centrifugally supported disks in dark matter halos of different mass. Photoheating can significantly reduce the amount of gas that can cool in galactic halos. Depending on the strength of the UV background field, the amount of cooled gas can be reduced by up to $50\%$ in systems with circular speeds in the range $80$-$200$ \kms. The magnitude of the effect, however, is not enough to solve the ``overcooling'' problem that plagues hierarchical models of galaxy formation if the UV background is chosen to be consistent with estimates based on recent observations of QSO absorption systems. Photoionization has little effect on the collapse of gas at high redshift and affects preferentially gas that is accreted at late times. Since disks form inside-out, accreting higher angular momentum gas at later times, disks formed in the presence of a UV background have spins that are even smaller than those formed in simulations that do not include the effects of photoionization. This exacerbates the angular momentum problem that afflicts hierarchical models of disk formation. We conclude that photoionization cannot provide the heating mechanism required to reconcile hierarchically clustering models with observations. Energy feedback and enrichment processes from the formation and evolution of stars must therefore be indispensable ingredients for any successful model of the formation of disk galaxies.

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arXiv1996

A Comparison of X-ray and Strong Lensing Properties of Simulated X-ray Clusters

We use gas-dynamical simulations of galaxy clusters to compare their X-ray and strong lensing properties. Special emphasis is laid on mass estimates. The cluster masses range between 6 x 10^14 solar masses and 4 x 10^15 solar masses, and they are examined at redshifts between 1 and 0. We compute the X-ray emission of the intracluster gas by thermal bremsstrahlung, add background contamination, and mimic imaging and spectral observations with current X-ray telescopes. Although the beta model routinely provides excellent fits to the X-ray emission profiles, the derived masses are typically biased low because of the restricted range of radii within which the fit can be done. For beta values of ~ 2/3, which is the average in our numerically simulated sample, the mass is typically underestimated by ~ 40 per cent. The masses of clusters which exhibit pronounced substructure are often substantially underestimated. We suggest that the ratio between peak temperature and emission-weighted average cluster temperature may provide a good indicator for ongoing merging and, therefore, for unreliable mass estimates. X-ray mass estimates are substantially improved if we fit a King density profile rather than the beta model to the X-ray emission, thereby dropping the degree of freedom associated with beta. Clusters selected for their strong lensing properties are typically dynamically more active than typical clusters. Bulk flows in the intracluster gas contain a larger than average fraction of the internal energy of the gas in such objects, hence the measured gas temperatures are biased low. The bulk of the optical depth for arc formation is contributed by clusters with intermediate rather than high X-ray luminosity. Arcs occur predominantly in clusters which exhibit substructure and are not in an equilibrium state. Finally we explain why the

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arXiv1995

CIV Absorption From Galaxies in the Process of Formation

We investigate the heavy element QSO absorption systems caused by gas condensations at high redshift which evolve into galaxies with circular velocity of 100 to 200 km/s at the present epoch. Artificial QSO spectra were generated for a variety of lines-of-sight through regions of the universe simulated with a hydrodynamics code. The CIV and HI absorption features in these spectra closely resemble observed CIV and HI absorption systems over a wide range in column density. CIV absorption complexes with multiple-component structure and velocity spreads up to about 600 km/s are found. The broadest systems are caused by lines-of-sight passing through groups of protogalactic clumps with individual velocity dispersions of less than 150 km/s aligned along filamentary structures. The temperature of most of the gas does not take the photoionization equilibrium value. This invalidates density and size estimates derived from thermal equilibrium models. Consequences for metal abundance determinations are briefly discussed. We predict occasional exceptionally large ratios of CIV to HI column density (up to a third) for lines-of-sight passing through compact halos of hot gas with temperature close to 3 10^5 K. Our model may be able to explain both high-ionization multi-component heavy-element absorbers and damped Lyman alpha systems as groups of small protogalactic clumps.

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