arXiv subjects

Matthias Steinmetz

Publications and source records attributed to Matthias Steinmetz.

At least 145 records · Page 8Linked to original sources

arXiv2005

Internal Alignment of the Halos of Disk Galaxies in Cosmological Hydrodynamic Simulations

Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1 r_vir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientation of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The disconnect between these two regions of the halo should be taken into account when modelling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.

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arXiv2005

Internal and External Alignment of the Shapes and Angular Momenta of LCDM Halos

(Abridged) We investigate how the shapes and angular momenta of galaxy and group mass dark matter halos in a LCDM N-body simulation are correlated internally, and how they are aligned with respect to the location and properties of surrounding halos. We explore these relationships down to halos of much lower mass (10^11/h Msun) than previous studies. The halos are triaxial, with c/a ratios of 0.6+-0.1. More massive halos are more flattened. The principal axes are very well aligned within 0.6 r_vir. The angular momentum vectors are also reasonably well aligned except between the very outermost and very innermost regions of the halo. The angular momentum vectors tend to align with the minor axes, with a mean misalignment of ~25 degrees, and lie perpendicular to the major and intermediate axes. The properties of a halo at 0.4 r_vir are quite characteristic of the properties at most other radii within the halo. There is a very strong tendency for the minor axes of halos to lie perpendicular to large scale filaments. This alignment extends to much larger separations for group and cluster mass halos than for galaxy mass halos. As a consequence, the intrinsic alignments of galaxies are likely weaker than previous predictions, which were based on the shapes of cluster mass halos. The angular momenta of the highest concentration halos tend to point toward other halos. The angular momenta of galaxy mass halos point parallel to filaments, while those of group and cluster mass halos show a very strong tendency to point perpendicular to the filaments. This suggests that group and cluster mass halos acquire most of their angular momentum from major mergers along filaments, while the accretion history of mass and angular momentum onto galaxy mass halos has been smoother.

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arXiv2005

How Concentrated Are The Haloes Of Low Surface Brightness Galaxies In The Cold Dark Matter Model?

We investigate whether a correlation exists between the concentration and spin parameters of dark matter haloes forming in LCDM N-body simulations. In particular, we focus on haloes with virial masses characteristic of the hosts of Low Surface Brightness (LSB) and High Surface Brightness (HSB) galaxies. Our analysis demonstrates that concentration and spin are anti-correlated. If we assume that a galaxy disk forms in one of these haloes from baryons that approximately conserved angular momentum during their dissipation into the rotationally supported disk, then it is possible to estimate the disk's central surface density. For an appropriate choice of mass-to-light ratio, we can place constraints on the central surface brightness of the disk and hence identify the analogues of the host haloes of LSB and HSB galaxies. We find that our LSB galaxy analogues occupy haloes that have lower concentrations than might be expected based on consideration of the mass-concentration relation for the LCDM cosmology. The distribution of concentrations peaks at c~6, in good agreement with observational data, although there are important differences between the shapes of the simulated and observationally inferred distributions. This suggests that LSB galaxies inhabit a biased subsample of the halo population, and that this bias may be an important ingredient for resolving the current debate regarding the structure of LSB galaxy dark matter haloes.

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arXiv2005

Accretion relicts in the solar neighbourhood: debris from omegaCen's parent galaxy

We use numerical simulations to investigate the orbital characteristics of tidal debris from satellites whose orbits are dragged into the plane of galactic disks by dynamical friction before disruption. We find that these satellites may deposit a significant fraction of their stars into the disk components of a galaxy, and use our results to motivate the search for accretion relicts in samples of metal-poor disk stars in the vicinity of the Sun. Satellites disrupted on very eccentric orbits coplanar with the disk are expected to shed stars in "trails" of distinct orbital energy and angular momentum during each pericentric passage. To an observer located between the pericenter and apocenter of such orbits, these trails would show as distinct groupings of stars with low vertical velocity and a broad, symmetric, often double-peaked distribution of Galactocentric radial velocities. One group of stars with these characteristics stands out in available compilations of nearby metal-poor stars. These stars have specific angular momenta similar to that of the globular cluster omegaCen, long hypothesized to be the nucleus of a dwarf galaxy disrupted by the Milky Way tidal field. In addition to their kindred kinematics, stars in the omegaCen group share distinct chemical abundance characteristics, and trace a well-defined track in the [alpha/Fe] versus [Fe/H] plane, consistent with simple closed-box enrichment models and a protracted star formation history. The dynamical and chemical coherence of this group suggests that it consists of stars that once belonged to the dwarf that brought omegaCen into the Galaxy. The presence of this and other "tidal relicts" in the solar neighbourhood suggest an extra-Galactic origin for the presence of nearby stars with odd kinematics and chemistry.

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arXiv2004

Tidal Torques and the Orientation of Nearby Disk Galaxies

We use numerical simulations to investigate the orientation of the angular momentum axis of disk galaxies relative to their surrounding large scale structure. We find that this is closely related to the spatial configuration at turnaround of the material destined to form the galaxy, which is often part of a coherent two-dimensional slab criss-crossed by filaments. The rotation axis is found to align very well with the intermediate principal axis of the inertia momentum tensor at this time. This orientation is approximately preserved during the ensuing collapse, so that the rotation axis of the resulting disk ends up lying on the plane traced by the protogalactic material at turnaround. This suggests a tendency for disks to align themselves so that their rotation axis is perpendicular to the minor axis of the structure defined by surrounding matter. One example of this trend is provided by our own Galaxy, where the Galactic plane is almost at right angles with the supergalactic plane (SGP) drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole is just 6 degrees. We have searched for a similar signature in catalogs of nearby disk galaxies, and find a significant excess of edge-on spirals (for which the orientation of the disk rotation axis may be determined unambiguously) highly inclined relative to the SGP. This result supports the view that disk galaxies acquire their angular momentum as a consequence of early tidal torques acting during the expansion phase of the protogalactic material.

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arXiv2004

The Angular Momentum Distribution of Gas and Dark Matter in Galactic Halos

(Abridged) We report results of a series of non radiative N-body/SPH simulations in a LCDM cosmology. We find that the spin of the baryonic component is on average larger than that of the dark matter (DM) component and we find this effect to be more pronounced at lower redshifts. A significant fraction f of gas has negative angular momentum and this fraction is found to increase with redshift. We describe a toy model in which the tangential velocities of particles are smeared by Gaussian random motions. This model is successful in explaining some of the angular momentum properties. We compare and contrast various techniques to determine the angular momentum distributions (AMDs). We show that broadening of velocity dispersions is unsuitable for making comparisons between gas and DM. We smooth the angular momentum of the particles over a fixed number of neighbors. We find that an analytical function based on gamma distribution can be used to describe a wide variety of profiles, with just one parameter α. The distribution of the shape parameter $α$ for both gas and DM follows roughly a log-normal distribution. The mean and standard deviation of log(α) for gas is -0.04 and 0.11 respectively. About 90-95% of halos have α<1.3, while exponential disks in NFW halos would require 1.3<α<1.6. This implies that a typical halo in simulations has an excess of low angular momentum material as compared to that of observed exponential disks, a result which is consistent with the findings of earlier works. αfor gas is correlated with that of DM but they have a significant scatter <α_Gas/α_DM>=1.09 \pm 0.2. α_Gas is also biased towards slightly higher values compared to α_DM.

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arXiv2004

Figure Rotation of Cosmological Dark Matter Halos

We have analyzed galaxy and group-sized dark matter halos formed in a high resolution LCDM numerical N-body simulation in order to study the rotation of the triaxial figure, a property in principle independent of the angular momentum of the particles themselves. Such figure rotation may have observational consequences, such as triggering spiral structure in extended gas disks. The orientation of the major axis is compared at 5 late snapshots of the simulation. Halos with significant substructure or that appear otherwise disturbed are excluded from the sample. We detect smooth figure rotation in 278 of the 317 halos in the sample. The pattern speeds follow a log normal distribution centred at 0.148 h km/s/kpc with a width of 0.83. These speeds are an order of magnitude smaller than required to explain the spiral structure of galaxies such as NGC 2915. The axis about which the figure rotates aligns very well with the halo minor axis, and also reasonably well with its angular momentum vector. The pattern speed is correlated with the halo spin parameter, but shows no correlation with the halo mass. The halos with the highest pattern speeds show particularly strong alignment between their angular momentum vectors and their figure rotation axes. The figure rotation is coherent outside 0.12 r_vir. The measured pattern speed and degree of internal alignment of the figure rotation axis drops in the innermost region of the halo, which may be an artifact of the numerical force softening. The axis ratios show a weak tendency to become more spherical with time.

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arXiv2003

Coupling between satellite dwarfs and the Milky Way warp

The perturbations of satellite galaxies, in particular the Large Magellanic Cloud (LMC), have been repeatedly proposed and discounted as the cause of the Milky Way warp. While the LMC may excite a wake in the Galactic dark matter halo that could provide sufficient torque to excite a warp of the observed magnitude, its orbit may be incompatible with the orientation of the warp's line of nodes. The Sgr dSph galaxy has an appropriately-oriented orbit, and due to its closer orbit may produce a stronger tidal effect than the LMC. Evidence that Sgr may be responsible for the warp comes from its orbital angular momentum, which has the same magnitude as the angular momentum of the warped component of the Galactic disk and is anti-aligned with it. We have run high resolution numerical N-body simulations of Sgr-sized satellites around a Milky Way-sized disk to test this idea. Preliminary results suggest that Sgr-sized satellites can indeed excite warps with properties similar to that observed for the Milky Way.

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arXiv2003

Simulations of Galaxy Formation in a Lambda CDM Universe III: The Dissipative Formation of an Elliptical Galaxy

We examine in detail the dynamical structure of an elliptical galaxy simulated in the Lambda CDM scenario. The morphology of the galaxy evolves dramatically over time in response to the mode and timing of mass accretion; smooth deposition of cooled gas leads to the formation of centrifugally supported disks, whilst major mergers disperse stellar disks into spheroids. These two modes of accretion alternate successively until z~0.6, when the galaxy undergoes one last major (1:2) merger that consumes much of the remaining gas into stars. Little gas cools and accretes subsequently and, as a result, most stars at z=0 are in a spheroidal component that resembles present-day elliptical galaxies. Dynamically, the galaxy is well approximated by an E4 oblate rotator. Boxy isophotes are obtained when the galaxy is seen face-on and Vrot/sigma<<1. On the other hand, disky isophotes are found for inclinations which maximize Vrot/sigma. The line-of-sight velocity distribution is nearly Gaussian at all radii. The sign of the Gauss-Hermite skewness parameter h3 is anti-correlated with the apparent sense of rotation, in agreement with observed trends. The simulated galaxy has much higher effective surface brightness than normal ellipticals of similar luminosity, in a way reminiscent of the less common type of compact ellipticals. Our simulation shows that repeated episodes of dissipational collapse, followed by merger events, lead to stellar spheroids that are only mildly triaxial and of relatively simple kinematic structure. This is in better agreement with observation than earlier models based on dissipationless mergers of stellar disks, and a positive step towards reconciling the observed structure of ellipticals with the hierarchical merging scenario.

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arXiv2003

On the nature of the ring-like structure in the outer Galactic disk

We examine the tidal disruption of satellite galaxies in a cosmological simulation of the formation of a disk galaxy in the $Λ$CDM scenario. We find that the disruption of satellite galaxies in orbits roughly coplanar with the disk leads naturally to the formation of ring-like stellar structures similar to that recently discovered in the outer disk of the Milky Way. Two interpretations appear plausible in this context. One is that the ring is a transitory, localized radial density enhancement reflecting the apocenter of particles stripped from a satellite during a recent pericentric passage (a ``tidal arc'' reminiscent of the tidal arms seen in disk galaxy mergers). In the second scenario, the ring is analogous to the ``shells'' found around some elliptical galaxies, and would result from a minor merger that took place several Gyr ago. The two interpretations differ in several ways. Tidal arcs are expected to span a limited longitude range; may carry a substantial fraction of the original mass of the satellite; should exhibit a significant velocity gradient with Galactic longitude; and are in all likelihood asymmetric above and below the plane of the disk. Shells, on the other hand, may comprise at most a small fraction of the original mass of the satellite and, due to their more relaxed state, ought to be symmetric above and below the plane, with no discernible velocity gradients across the structure. If confirmed as a tidal feature, the ring discovered by SDSS in the outer Galactic disk would strengthen the view--supported by numerical simulations--that minor mergers have played a significant role building up not only the stellar halo, but also the disk components of the Galaxy.

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arXiv2002

Simulations of Galaxy Formation in a Lambda CDM Universe II: The Fine Structure of Simulated Galactic Disks

We present a detailed analysis of the dynamical properties of a simulated disk galaxy assembled hierarchically in the Lambda CDM cosmogony. At z=0, two distinct dynamical components are identified on the basis of the orbital parameters of stars in the galaxy: a slowly rotating, centrally concentrated spheroid and a disk-like component largely supported by rotation. These components are also recognized in the surface brightness profile of the galaxy, which can be very well approximated by the superposition of an R^{1/4} spheroid and an exponential disk. However, neither does the dynamically-identified spheroid follow de Vaucouleurs' law nor is the disk purely exponential, a resultwhich calls for caution when estimating the importance of the disk from traditional photometric decomposition techniques. The disk may be further decomposed into a thin, dynamically cold component with stars on nearly circularorbits and a hotter, thicker component with orbital parameters transitional between the thin disk and the spheroid. The spheroid is old, and has essentiallyno stars younger than the time elapsed since the last major accretion event ~8 Gyr ago. The majority of thin disk stars, form after the merging activity is over, although a significant fraction ~15% of thin-disk stars are old enough to predate the last major merger event. This unexpected population of old disk stars consists mainly of the tidal debris of satellites whose orbital plane was coincident with the disk and whose orbits were circularized by dynamical friction prior to full disruption. Our results highlight the role of satellite accretion events in shaping the disk and the spheroidal component and reveal some of the clues to the assembly process of a galaxy preserved in the detailed dynamics of old stellar populations.

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arXiv2002

RAVE: The RAdial Velocity Experiment

RAVE (RAdial Velocity Experiment) is an ambitious program to conduct an all-sky survey (complete to V = 16) to measure the radial velocities, metallicities and abundance ratios of 50 million stars using the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO), together with a northern counterpart, over the period 2006 - 2010. The survey will represent a giant leap forward in our understanding of our own Milky Way galaxy, providing a vast stellar kinematic database three orders of magnitude larger than any other survey proposed for this coming decade. RAVE will offer the first truly representative inventory of stellar radial velocities for all major components of the Galaxy. The survey is made possible by recent technical innovations in multi-fiber spectroscopy; specifically the development of the 'Echidna' concept at the AAO for positioning fibers using piezo-electric ball/spines. A 1m-class Schmidt telescope equipped with an Echidna fiber-optic positioner and suitable spectrograph would be able to obtain spectra for over 20 000 stars per clear night. Although the main survey cannot begin until 2006, a key component of the RAVE survey is a pilot program of 100 000 stars which may be carried out using the existing 6dF facility in unscheduled bright time over the period 2003--2005.

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arXiv2002

Simulations of Galaxy Formation in a Lambda CDM Universe I: Dynamical and Photometric Properties of a Simulated Disk Galaxy

We analyze the properties of a disk galaxy simulated with unprecedented numerical resolution in the Lambda CDM cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z~1 which lead to the formation of two distinct dynamical components: a spheroid and a disk. The surface brightness profile is very well approximated by the superposition of an R^{1/4} spheroid and an exponential disk. The surface brightness profile is remarkably similar to that of Sab galaxy UGC615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies ~ 1 mag off the I-band Tully-Fisher relation of late-type spirals, but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. The angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This reflects the dominance of the slowly-rotating, dense spheroidal component. The disk component, on the other hand, has properties rather similar to those of late-type spirals. This suggests that a different form of feedback than adopted in this simulation is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as Lambda CDM remains a challenging, yet so far elusive, proposition.

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arXiv2002

Tidal Torques and Galactic Warps

(Abridged) We investigate how galactic disks react to external tidal torques. We calculate the strength and radial dependence of torques on disks that arise from a misalignment between the disk and the main axis system of a flattened dark matter halo. We find that except for in the very inner regions, the torques are well-described by a power law of the form $τ\propto r^{-2.5}$. The magnitude of the torque is large enough for the entire disk to react to the torque in less than the Hubble time. We demonstrate analytically that disks which are originally located in the $xy$-plane and which are subjected to a torque around the x-axis tilt around the y-axis, as also found in fully non-linear N-body simulations. We further demonstrate that that the torque causes the radius of a chosen particle to increase with time. Investigations of tilting disks which treat the disk as a set of solid rings thus may systematically overestimate the effects of the torque by a factor of two. For torques of the form we investigate, the inner regions of the disk react to the torque faster than the outer regions, resulting in a trailing warp. We then study the effect of the self-gravity of the disk in such a scenario using numerical N-body models. Self-gravity flattens out the inner regions of the disk, but these regions are tilted with respect to their initial plane followed by a non-flat outer region whose tilt decreases with radius. The ``warp radius,'' which marks the end of the inner flat disk, grows throughout the disk at a rate that depends only on the strength of the torque and the local surface density of the disk.

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arXiv2002

Galaxy Formation Now and Then

I review the current state of our understanding of the galaxy formation and evolution process from the modeler's perspective. With the advent of the cold dark matter model and the support of fast computers and advanced simulation techniques, there has been considerable progress in explaining the growth of structure on the largest scales and in reproducing some of the basic properties of galaxies and their evolution with redshift. However, many properties of galaxies are still only poorly understood or appear to be in conflict with the prediction of the cold dark matter model. I discuss in what direction the next generation of galaxy formation models may go and why a large space-based optical-UV telescope could be critical for the calibration and testing of these advanced models.

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arXiv2002

Numerical studies of galaxy formation using special purpose hardware

I review recent progress in numerically simulating the formation and evolution of galaxies in hierarchically clustering universes. Special emphasis is given to results based on high-resolution gas dynamical simulations using the N-body hardware integrator GRAPE. Applications address the origin of the spin of disk galaxies, the structure and kinematics of damped Lyman-alpha systems, and the origin of galaxy morphology and of galaxy scaling laws.

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arXiv2000

Dark Halo and Disk Galaxy Scaling Laws in Hierarchical Universes

We use cosmological N-body/gasdynamical simulations that include star formation and feedback to examine the proposal that scaling laws between the total luminosity, rotation speed, and angular momentum of disk galaxies reflect analogous correlations between the structural parameters of their surrounding dark matter halos. The numerical experiments follow the formation of galaxy-sized halos in two Cold Dark Matter dominated universes: the standard Omega=1 CDM scenario and the currently popular LCDM model. We find that the slope and scatter of the I-band Tully-Fisher relation are well reproduced in the simulations, although not, as proposed in recent work, as a result of the cosmological equivalence between halo mass and circular velocity: large systematic variations in the fraction of baryons that collapse to form galaxies and in the ratio between halo and disk circular velocities are observed in our numerical experiments. The Tully-Fisher slope and scatter are recovered in this model as a direct result of the dynamical response of the halo to the assembly of the luminous component of the galaxy. We conclude that models that neglect the self-gravity of the disk and its influence on the detailed structure of the halo cannot be used to derive meaningful estimates of the scatter or slope of the Tully-Fisher relation. Our models fail, however, to match the zero-point of the Tully-Fisher relation, as well as that of the relation linking disk rotation speed and angular momentum. These failures can be traced, respectively, to the excessive central concentration of dark halos formed in the Cold Dark Matter cosmogonies we explore and to the formation of galaxy disks as the final outcome of a sequence of merger events. (abridged)

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arXiv1999

Damped Lyman alpha absorber and the faint end of the galaxy luminosity function at high redshift

We combine predictions for several hierarchical cosmogonies with observational evidence on damped Lyman alpha systems to establish a correspondence between the high redshift galaxy population and the properties of damped Lyman alpha systems. We assume that high redshift galaxies and damped Lyman alpha systems are hosted by the same dark matter halos and require consistency between the predicted halo space density, the rate of incidence and the velocity width distribution of damped Lyman alpha systems, and the observed galaxy luminosity function at the bright end. We arrive at the following results: (1) predicted impact parameters between the damped absorption system and the luminous part of the absorbing galaxy are expected to be very small (0.3 - 1arcsec) for most galaxies; (2) luminosities of galaxies causing damped absorption are generally fainter than m_R = 25 and damped Lyman alpha systems are predicted to sample preferentially the outer regions of galaxies at the faint end of the galaxy luminosity function at high redshift. Therefore, DLAS should currently provide the best probe of the progenitors of normal present-day galaxies.

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