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We use the Millennium Simulation (MS) to study the statistics of LCDM halo concentrations at z = 0. Our results confirm that the average halo concentration declines monotonically with mass; a power-law fits well the concentration-mass relation for over 3 decades in mass, up to the most massive objects to form in a LCDM universe (~ 10^15 h^-1 Msol). This is in clear disagreement with the predictions of the model proposed by Bullock et al. for these rare objects, and agrees better with the original predictions of Navarro, Frenk, & White. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allow us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. About one in a hundred cluster haloes (M200 >~ 3x10^14 h^-1 Msol) have concentrations exceeding c200 = 7.5, a result that may be used to interpret the likelihood of unusually strong massive gravitational lenses, such as Abell 1689, in the LCDM cosmogony. A similar fraction (1 in 100) of galaxy-sized haloes (M200 ~ 10^12 h^-1 Msol) have c200 < 4.5, an important constraint on models that attempt to reconcile the rotation curves of low surface-brightness galaxies by appealing to haloes of unexpectedly low concentration. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium are removed from the sample. Compared to their relaxed brethren, the concentrations of out-of-equilibrium haloes tend to be lower and to have more scatter, while their spins tend to be higher... (continue), 14 pages, 13 eps figures; Submitted to MNRAS

We use a semianalytic galaxy catalogue constructed from the Millennium Simulation to study the satellites of isolated galaxies in the LCDM cosmogony. This sample (~80,000$ bright primaries, surrounded by ~178,000 satellites) allows the characterization, with minimal statistical uncertainty, of the dynamical properties of satellite/primary galaxy systems in a LCDM universe. We find that, overall, the satellite population traces the dark matter rather well: its spatial distribution and kinematics may be approximated by an NFW profile with a mildly anisotropic velocity distribution. Their spatial distribution is also mildly anisotropic, with a well-defined ``anti-Holmberg'' effect that reflects the misalignment between the major axis and angular momentum of the host halo. The isolation criteria for our primaries picks not only galaxies in sparse environments, but also a number of primaries at the centre of ''fossil'' groups. We find that the abundance and luminosity function of these unusual systems are in reasonable agreement with the few available observational constraints. We recover the expected L_{host} \sigma_{sat}^3 relation for LCDM models for truly-isolated primaries. Less strict primary selection, however, leads to substantial modification of the scaling relation. Our analysis also highlights a number of difficulties afflicting studies that rely on blind stacking of satellite systems to constrain the mean halo mass of the primary galaxies., Comment: 18 pages, 14 figures, MNRAS in press. Accepted version with minor changes. Version with high resolution figures available at: http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.html

We examine the dynamics of stellar systems embedded within cold dark matter (CDM) halos in order to assess observational constraints on the dark matter content of Local Group dwarf spheroidals (dSphs). Our analysis shows that the total mass within the luminous radius is reasonably well constrained and approximately independent of the luminosity of the dwarf, highlighting the poor correspondence between luminosity and halo mass. This result implies that the average density of dark matter is substantially higher in physically small systems such as Draco and Sculptor than in larger systems such as Fornax. For example, our results imply that Draco formed in a halo 5 times more massive than Fornax's despite being roughly 70 times fainter. Stellar velocity dispersion profiles, sigma_p(R), provide further constraints; flat sigma_p(R) profiles imply that stars are deeply embedded within their cold dark matter halos and so quite resilient to tidal disruption. We estimate that halos would need to lose more than 90% of their original mass before tides begin affecting the kinematics of stars. We estimate that V_max is about 3 times higher than the central velocity dispersion of the stars, which alleviates significantly the CDM ``substructure crisis''. We use these results to interpret the size differences between the M31 and Milky Way (MW) dSph population. Our modeling indicates that this difference should be reflected in their kinematics, and predicts that M31 dwarfs should have velocity dispersions up to a factor of ~ 2 higher than their MW counterparts. This CDM-motivated prediction may be verified with present observational capabilities., Comment: ApJ accepted. Additional figure and extended discussion added

(Abridged) We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically-motivated model where dSphs are approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, R_c decreases by just a factor of ~2. Interestingly, tides tend to make dSphs more dark-matter dominated because the tightly bound central dark matter ``cusp'' is more resilient to disruption than the ``cored'' King profile. We examine whether the extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have been caused by tidal stripping of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold., Comment: 17 pages, 12 figs., accepted by ApJ

We study the population of satellite galaxies formed in a suite of N-body/gasdynamical simulations of galaxy formation in a LCDM universe. We find little spatial or kinematic bias between the dark matter and the satellite population. The velocity dispersion of the satellites is a good indicator of the virial velocity of the halo: \sigma_{sat}/V_{vir}=0.9 +/- 0.2. Applied to the Milky Way and M31 this gives V_{vir}^{MW}=109 +/- 22$ km/s and V_{vir}^{M31} = 138 +/- 35 km/s, respectively, substantially lower than the rotation speed of their disk components. The detailed kinematics of simulated satellites and dark matter are also in good agreement. By contrast, the stellar halo of the simulated galaxies is kinematically and spatially distinct from the population of surviving satellites. This is because the survival of a satellite depends on mass and on time of accretion; surviving satellites are biased toward low-mass systems that have been recently accreted by the galaxy. Our results support recent proposals for the origin of the systematic differences between stars in the Galactic halo and in Galactic satellites: the elusive ``building blocks'' of the Milky Way stellar halo were on average more massive, and were accreted (and disrupted) earlier than the population of dwarfs that has survived self-bound until the present., Comment: 13 pages, 11 figures, MNRAS in press. Accepted version with minor changes. Version with high resolution figures available at: http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.html

We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than being only the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event, and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy reflects primarily the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation., 7 pages, 3 figures, ApJ Letters, submitted. Comments welcome

The dwarf spheroidal (dSph) satellites of the Local Group have long been thought to be simple spheroids of stars embedded within extended dark matter halos. Recently, however, evidence for the presence of spatially and kinematically distinct stellar populations has been accumulating. Here, we examine the influence of such components on dynamical models of dwarf galaxies embedded in cold dark matter halos. We begin by constructing a model of Andromeda II, a dSph satellite of M31 which shows evidence for spatially distinct stellar components. We find that the two-component model predicts an overall velocity dispersion profile that remains approximately constant at $\sim 10 - 11$ km s$^{-1}$ out to $\sim 1$ kpc from the center; this is despite wide kinematic and spatial differences between the two individual components. The presence of two components may also help to explain oddities in the velocity dispersion profiles of other dSphs; we show that velocity dispersion profiles which appear to rise from the center outwards before leveling off--such as those of Leo I, Draco, and Fornax--can result from the gradual transition from a dynamically cold, concentrated component to a second, hotter, and more spatially extended one, both in equilibrium within the same dark halo. Dwarf galaxies with two stellar components generally have a leptokurtic line-of-sight velocity distribution which is well described by a double Maxwellian. Interestingly, we find that multiple equilibrium components could also provide a potential alternative origin for ``extra-tidal'' stars (normally ascribed to tidal effects) in situations where corroborating evidence for tides may be lacking., Accepted by MNRAS Letters. Revised version, with addition of new section and expanded discussion

We study the kinematics of gaseous disks in triaxial dark matter halos using the closed-loop orbit solutions in non-axisymmetric potentials. The orbits are in general non-circular and, for given triaxiality, their ellipticity depends on the ratio of escape to circular velocities, V_esc^2/V_c^2. This ratio increases steeply towards the center for cold dark matter (CDM) halo density profiles, implying that even minor deviations from spherical symmetry may induce large deviations from circular orbits in the velocity field of a gaseous disk, especially near the center. This result suggests that caution should be exercised when interpreting constraints on the presence of density cusps in the dark halo derived from the innermost velocity profile. Simulated long-slit rotation curves vary greatly in shape, depending primarily on the viewing angle of the disk and on its orientation relative to the principal axes of the potential. "Solid-body" rotation curves - typically interpreted as a signature of a constant density core in the dark matter distribution - are often obtained when the slit samples velocities near the major axis of the closed loop orbits. Triaxial potentials imprint specific symmetries in 2D velocity fields, generally inducing "twists" in the isovelocity contours and anti-symmetric patterns in opposite quadrants. We suggest that triaxial halos may be responsible for the variety of shapes of long-slit rotation curves of low surface brightness (LSB) galaxies, as well as for the complex central kinematics of LSBs, which are sometimes ascribed to the presence of "radial motions" in the gas. We argue that LSB rotation curves might be reconciled with the structure of CDM halos once the effects of halo triaxiality on the dynamics of gaseous disks are properly taken into account., fixed color bar in Fig. 4. 9 pages, 4 figures, accepted for publication in MNRAS, high resolution version avaliable at http://www.mpa-garching.mpg.de/~ehayashi/hiding/

We use a set of cosmological N-body simulations to investigate the structural shape of galaxy-sized cold dark matter (CDM) halos. Unlike most previous work on the subject - which dealt with shapes as measured by the inertia tensor - we focus here on the shape of the gravitational potential, a quantity more directly relevant to comparison with observational probes. A further advantage is that the potential is less sensitive to the effects of substructure and, as a consequence, the isopotential surfaces are typically smooth and well approximated by concentric ellipsoids. Our main result is that the asphericity of the potential increases rapidly towards the center of the halo. The radial trend is more pronounced than expected from constant flattening in the mass distribution, and reflects a strong tendency for dark matter halos to become increasingly aspherical inwards. Near the center the halo potential is approximately prolate ((c/a)_0=0.72 +/- 0.04, (b/a)_0=0.78 +/- 0.08), but it becomes increasingly spherical in the outer regions. The principal axes of the isopotential surfaces remain well aligned, and in most halos the angular momentum tends to be parallel to the minor axis and perpendicular to the major axis. This suggests that galactic disks may form in a plane where the potential is elliptical and where its ellipticity varies rapidly with radius. This can result in significant deviations from circular motion in systems such as low surface brightness galaxies (LSBs), even for relatively minor deviations from circular symmetry. Simulated long-slit rotation curves can appear similar to those of LSBs often cited as evidence for constant density "cores". This suggests that taking into account the 3D shape of the dark mass distribution might help to reconcile such evidence with the cuspy mass profile of CDM halos., Comment: 18 pages, 10 figures, accepted by MNRAS after minor revisions, high resolution version avaliable at http://www.mpa-garching.mpg.de/~ehayashi/potshape

(Abridged) We use numerical simulations to investigate the origin and structure of the luminous halos that surround isolated galaxies. These stellar structures extend out to several hundred kpc away from a galaxy, and consist of stars shed by merging subunits during the many accretion events that characterize the hierarchical assembly of galaxies. Such origin suggests that outer luminous halos are ubiquitous and that they should appear as an excess of light over extrapolations of the galaxy's inner profile beyond its traditional luminous radius. The mass profile of the accreted stellar component is well approximated by a model where the logarithmic slope steepens monotonically with radius; from -3 at the luminous edge of the galaxy to -4 or steeper near the virial radius of the system. Such spatial distribution is consistent with that of Galactic and M31 globular clusters, suggesting that many of the globulars were brought in by accretion events, in a manner akin to the classic Searle-Zinn scenario. The outer stellar spheroid is supported by a velocity dispersion tensor with a substantial and radially increasing radial anisotropy. These properties distinguish the stellar halo from the dark matter component, which is more isotropic in velocity space, as well as from some tracers of the outer spheroid such as satellite galaxies. Most stars in the outer halo formed in progenitors that have since merged with the central galaxy; very few stars in the halo are contributed by satellites that survive as self-bound entities at the present. These features are in reasonable agreement with recent observations of the outer halo of the MW, of M31, and of other isolated spirals, and suggest that all of these systems underwent an early period of active merging, as envisioned in hierarchical models of galaxy formation., Submitted to MNRAS, 13 pages, 12 figures

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., To be published in ApJL, 5 pages, 3 figures

We explore similarities in the luminosity distribution of early type galaxies and the mass profiles of LCDM halos. The spatial structure of these systems may be accurately described by a simple law where the logarithmic slope of the projected density is a power law of radius; the Sersic law. We show that this law provides a significantly better fit than a three-parameter generalization of the NFW profile and derive the best-fitting Sersic parameters for a set of high-resolution LCDM halos spanning a wide range in mass. The mean Sersic n values are 3.0 for dwarf- and galaxy-sized halos and 2.4 for cluster-sized halos, similar to the values that characterize luminous elliptical galaxies. We discuss possible reasons why the same law should describe dark and luminous systems that span a range of over seven decades in mass., Comment: 4 pages

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., 5 pages, 2 figures, accepted for publication in ApJL

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., Comment: 11 pages, 11 color figures, new title and minor changes to match MNRAS accepted version

I review the results of recent cosmological simulations of galaxy formation that highlight the importance of satellite accretion in the formation of galactic disks. Tidal debris of disrupted satellites may contribute to the disk component if they are compact enough to survive the decay and circularization of the orbit as dynamical friction brings the satellite into the disk plane. This process may add a small but non-negligible fraction of stars to the thin and thick disks, and reconcile the presence of very old stars with the protracted merging history expected in a hierarchically clustering universe. I discuss various lines of evidence which suggest that this process may have been important during the formation of the Galactic disk.

I report on recent numerical simulations designed to study the inner structure of cold-dark-matter (CDM) haloes. This work confirms and extends earlier indications that the mass profiles of CDM haloes are roughly independent of mass and that the dark-matter density appears to diverge in a central "cusp". Our simulations also show that CDM halo-density profiles get increasingly shallow inwards down to the innermost radius (r(min)) resolved by the simulations, with no evidence for convergence to a well-defined asymptotic value (beta(0)) of the logarithmic slope. The dark mass contained within r(min) places strong constraints on beta(0); cusps as steep as beta(0) approximately -1.5 are clearly ruled out in our highest resolution simulations, although beta(0) less, similar -1 is consistent with the numerical data. The circular-velocity curves of the simulated CDM haloes are generally consistent with the rotation curves of low-surface-brightness (LSB) galaxies in the literature, but there are also some clearly discrepant cases. This disagreement has been interpreted as excluding the presence of cusps, but I argue that it may just reflect the difference between circular velocity and rotation speed likely to arise in gaseous discs embedded within realistic triaxial haloes. The perceived challenge to the CDM paradigm stemming from LSB rotation curves may thus plausibly result from the complex three-dimensional structure of CDM haloes, and might be resolved without appeal to a drastic revision of the paradigm.

We investigate the mass profile of LambdaCDM halos using a suite of numerical simulations spanning five decades in halo mass, from dwarf galaxies to rich galaxy clusters. Our analysis confirms the proposal of Navarro, Frenk & White (NFW) that the shape of LambdaCDM halo mass profiles differs strongly from a power law and depends little on mass. The logarithmic slope of the spherically-averaged density profile, as measured by beta=-dln(rho)/dln(r), decreases monotonically towards the center and becomes shallower than isothermal (beta, Minor editing changes to match version accepted for publication in MNRAS

We examine the tidal disruption of satellite galaxies in a cosmological simulation of the formation of a disk galaxy in the $\Lambda$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., Comment: 4 pages, 2 figures, ApJ Letters in press. High resolution Fig. 2 available from http://www.astro.uu.nl/~helmi/ring. More thorough discussion of each scenario, and of observability of shells. Conclusions unchanged

Numerical simulations have revealed the presence of long-lived substructure in Cold Dark Matter (CDM) halos. These surviving cores of past merger and accretion events vastly outnumber the known satellites of the Milky Way. This finding has prompted suggestions that substructure in cold dark matter (CDM) halos may be incompatible with observation and in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of a disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolution cosmological CDM simulations indicate that substructure plays only a minor dynamical role in the heating of the disk. This is because the orbits of satellites seldom take them near the disk, where their tidal effects are greatest. We conclude that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way., Presented at the Yale Symposium "The Shape of Galaxies and their Halos", ed. P.Natarajan

We determine the Galactic potential in the solar neigbourhood from RAVE observations. We select red clump stars for which accurate distances, radial velocities, and metallicities have been measured. Combined with data from the 2MASS and UCAC catalogues, we build a sample of 4600 red clump stars within a cylinder of 500 pc radius oriented in the direction of the South Galactic Pole, in the range of 200 pc to 2000 pc distances. We deduce the vertical force and the total mass density distribution up to 2 kpc away from the Galactic plane by fitting a distribution function depending explicitly on three isolating integrals of the motion in a separable potential locally representing the Galactic one with four free parameters. Because of the deep extension of our sample, we can determine nearly independently the dark matter mass density and the baryonic disc surface mass density. We find (i) at 1kpc Kz/(2piG) = 68.5 pm 1.0 Msun/pc2, and (ii) at 2 kpc Kz/(2piG) = 96.9 pm 2.2 Msun/pc2. Assuming the solar Galactic radius at R0 = 8.5 kpc, we deduce the local dark matter density rhoDM (z=0) = 0.0143 pm 0.0011Msun pc3 = 0.542 pm 0.042 Gev/cm3 and the baryonic surface mass density Sigma = 44.4 pm 4.1 Msun/pc2 . Our results are in agreement with previously published Kz determinations up to 1 kpc, while the extension to 2 kpc shows some evidence for an unexpectedly large amount of dark matter. A flattening of the dark halo of order 0.8 can produce such a high local density in combination with a circular velocity of 240 km/s . Another explanation, allowing for a lower circular velocity, could be the presence of a secondary dark component, a very thick disc resulting either from the deposit of dark matter from the accretion of multiple small dwarf galaxies, or from the presence of an effective phantom thick disc in the context of effective galactic-scale modifications of gravity., Comment: 14 pages, 13 figures, accepted to Astronomy and Astrophysics

I report on recent progress in our understanding of the structure of CDM halos, and in particular of the inner mass profile of galaxy-sized systems. Numerical simulations have consistently shown that the density profiles of CDM halos steepen monotonically from the center outwards, with slopes shallower than isothermal near the center and steeper than isothermal near the virial radius. Ongoing debate centers on the precise radial dependence of the logarithmic slope, as well as on whether it approaches a well defined asymptotic central value. The latest high-resolution simulations suggest that the circular velocity profile is well approximated by the model proposed by Navarro, Frenk & White (NFW). On the other hand, the radial dependence of the slope of the density profile differs modestly, but significantly, from the model proposed by NFW. As a result, NFW fits tend to underestimate the density at radii just inside the scale radius. Rather than implying a very steep (-1.5) inner divergent slope, I argue that the data is actually best represented by a model where the density profile becomes increasingly shallow with radius, with little sign of approach to a well-defined asymptotic value. A model where the {\it phase-space density profile} is a power law accounts well for these results and suggests that the innermost slope may be as shallow as -0.75. These conclusions are supported by a thorough numerical convergence study that elucidates the effect of numerical parameters such as the timestep, gravitational softening, and particle number, on the mass profile of simulated dark matter halos., Invited review presented at IAU Symposium 208, Astrophysical SuperComputing using Particles, J. Makino & P.Hut, eds

High-resolution N-body simulations are used to examine the power spectrum dependence of the concentration of galaxy-sized dark matter halos. It is found that dark halo concentrations depend on the amplitude of mass fluctuations as well as on the ratio of power between small and virial mass scales. This finding is consistent with the original results of Navarro, Frenk & White (NFW), and allows their model to be extended to include power spectra substantially different from Cold Dark Matter (CDM). In particular, the single-parameter model presented here fits the concentration dependence on halo mass for truncated power spectra, such as those expected in the warm dark matter scenario, and predicts a stronger redshift dependence for the concentration of CDM halos than proposed by NFW. The latter conclusion confirms recent suggestions by Bullock et al., although this new modeling differs from theirs in detail. These findings imply that observational limits on the concentration, such as those provided by estimates of the dark matter content within individual galaxies, may be used to constrain the amplitude of mass fluctuations on galactic and subgalactic scales. The constraints on $\Lambda$CDM models posed by the dark mass within the solar circle in the Milky Way and by the zero-point of the Tully-Fisher relation are revisited, with the result that neither dataset is clearly incompatible with the `concordance' ($\Omega_0=0.3$, $\Lambda_0=0.7$, $\sigma_8=0.9$) $\Lambda$CDM cosmogony. This conclusion differs from that reached recently by Navarro & Steinmetz, a disagreement that can be traced to inconsistencies in the normalization of the $\Lambda$CDM power spectrum used in that work., Comment: 12 pages including 7 figures using emulateapj, submitted to ApJ

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), Comment: submitted to The Astrophysical Journal

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few Gyrs, the typical gas consumption timescale of disk galaxies in the field. Such behaviour might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a Lambda-CDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as two virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems, and that the cessation of star formation may have taken place gradually over a few Gyrs., Comment: 11 pages emulateapj style, 4 embedded postscript figures Accepted for publication in ApJ

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 ($\Omega_0 \sim 0.3$; $\Lambda_0=1-\Omega_0 \sim 0.7$; $h \sim 0.7$; COBE- and cluster abundance-normalized $\sigma_8$; Big-Bang nucleosynthesis $\Omega_b$), 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 $\Lambda$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., Comment: accepted for publication in The Astrophysical Journal

We examine gravitational lensing constraints on the structure of galaxy clusters and compare them with the results of cosmological N-body simulations of cluster formation in cold dark matter (CDM) dominated universes. We find that cluster core masses, as measured by the observed location of giant tangential arcs, generally exceed those of dark matter halos of similar velocity dispersion. The magnitude of the discrepancy is a strong function of cluster mass. Arc properties in the most massive clusters in the sample (i.e. those with velocity dispersion, \sigma \sim 1500-2000 km/s) are essentially consistent with the N-body predictions. On the other hand, giant arcs in \sigma \sim 1000 km/s clusters can only be reconciled with CDM cluster halos if their lensing power has been increased substantially by the presence of a massive (\sim 3 \times 10^{12} h^{-1} M_{\odot}) central galaxy and of significant substructure. Best agreement is found if the mass of the central galaxy and the effects of substructure are approximately independent of cluster mass. Massive central galaxies with steep inner density profiles are also needed to explain a clear trend observed in our dataset between the radial thickness of giant tangential arcs and the velocity dispersion of the cluster lens. The position and redshift of radial arcs may be used as independent tests of these results, but at present the dataset available is too limited to have a significant impact on these conclusions. Our results depend only weakly on the cosmological model adopted, and suggest that structural parameters of clusters derived from strong lensing studies cannot usefully constrain the values of cosmological parameters., Comment: 26 pages, including 7 figs, LaTeX

We present results of N-body/gasdynamical simulations designed to investigate the evolution of X-ray clusters in a flat, low-density, cold dark matter (CDM) cosmogony. The density profile of the dark matter component can be fitted rather accurately by the simple formula originally proposed by Navarro, Frenk & White to describe the structure of clusters in a CDM universe with $\Omega=1$. In projection, the shape of the dark matter radial density profile and the corresponding line-of-sight velocity dispersion profile are in very good agreement with the observed profiles for galaxies in the CNOC sample of clusters. The gas in our simulated clusters is less centrally concentrated than the dark matter, and its radial density profile is well described by the familiar $\beta$-model. The total mass and velocity dispersion of our clusters can be accurately inferred (with $\sim 15%$ uncertainty) from their X-ray emission-weighted temperature. We generalize Kaiser's scaling relations for scale-free universes and show that the clusters in our simulations generally follow these relations. The agreement between the simulations and the analytical results provides a convincing demonstration of the soundness of our gasdynamical numerical techniques. The slope of the luminosity-temperature relation implied by the scaling relations, and obeyed by the simulations, is in disagreement with observations. This suggests that non-gravitational effects such as preheating or cooling must have played an important role in determining the properties of the observed X-ray emission from galaxy clusters., Comment: Minor changes, final version in press in the ApJ. 43 pages with 17 figures. Full PS file can be obtained from http://penedes.as.arizona.edu/~jfn/preprints/lamclus.ps.gz

RAVE, the unbiased magnitude limited survey of the southern sky stars, contained 456,676 medium-resolution spectra at the time of our analysis. Spectra cover the CaII IRT range which is a known indicator of chromospheric activity. Our previous work (Matijevi\v{c} et al. 2012) classified all spectra using locally linear embedding. It identified 53,347 cases with a suggested emission component in calcium lines. Here we use a spectral subtraction technique to measure the properties of this emission. Synthetic templates are replaced by the observed spectra of non-active stars to bypass the difficult computations of non-LTE profiles of the line cores and stellar parameter dependence. We derive both the equivalent width of the excess emission for each calcium line on a 5\AA\ wide interval and their sum EW_IRT for ~44,000 candidate active dwarf stars with S/N>20 and with no respect to the source of their emission flux. From these ~14,000 show a detectable chromospheric flux with at least 2\sigma\ confidence level. Our set of active stars vastly enlarges previously known samples. Atmospheric parameters and in some cases radial velocities of active stars derived from automatic pipeline suffer from systematic shifts due to their shallower calcium lines. We re-estimate the effective temperature, metallicity and radial velocities for candidate active stars. The overall distribution of activity levels shows a bimodal shape, with the first peak coinciding with non-active stars and the second with the pre main-sequence cases. The catalogue will be publicly available with the next RAVE public data releases.

The correlation function of galaxy clusters has often been used as a test of cosmological models. A number of assumptions are implicit in the comparison of theoretical expectations to data. Here we use an ensemble of ten large N-body simulations of the standard cold dark matter cosmology to investigate how cluster selection criteria and other uncertain factors influence the cluster correlation function. We consider the effects of varying the definition of a cluster, the mean number density (or equivalently the richness) in a catalogue, and the assumed normalisation of the model; we also examine the importance of redshift space distortions. We implement five different group-finding algorithms and construct cluster catalogues defined by mass, velocity dispersion or a measure of X-ray luminosity. We find that different cluster catalogues yield correlation functions which can differ by substantially more than the statistical errors in any one determination. For a given cluster selection criteria, the correlation length typically varies by $\sim 20\%$ in catalogues spanning the range of intercluster separations covered by the APM and Abell (richness class $\gsim 1$) catalogues. Distortions produced by peculiar velocities in redshift space enhance the correlation function at large separations and lead to a larger clustering length in redshift space than in real space. The sensitivity of the cluster correlation function to various uncertain model assumptions substantially weakens previous conclusions based on the comparison of model predictions with real data. Detailed modelling of cluster selection procedures including the effects of selecting from projected galaxy catalogues is required before the cluster correlation function can be regarded as a high precision constraint on cosmological models., Compressed postscript also available at ftp://star-ftp.dur.ac.uk/pub/preprints/ecfn.ps.gz

We use N-body simulations to investigate the structure of dark halos in the standard Cold Dark Matter cosmogony. Halos are excised from simulations of cosmologically representative regions and are resimulated individually at high resolution. We study objects with masses ranging from those of dwarf galaxy halos to those of rich galaxy clusters. The spherically averaged density profiles of all our halos can be fit over two decades in radius by scaling a simple ``universal'' profile. The characteristic overdensity of a halo, or equivalently its concentration, correlates strongly with halo mass in a way which reflects the mass dependence of the epoch of halo formation. Halo profiles are approximately isothermal over a large range in radii, but are significantly shallower than $r^{-2}$ near the center and steeper than $r^{-2}$ near the virial radius. Matching the observed rotation curves of disk galaxies requires disk mass-to-light ratios to increase systematically with luminosity. Further, it suggests that the halos of bright galaxies depend only weakly on galaxy luminosity and have circular velocities significantly lower than the disk rotation speed. This may explain why luminosity and dynamics are uncorrelated in observed samples of binary galaxies and of satellite/spiral systems. For galaxy clusters, our halo models are consistent both with the presence of giant arcs and with the observed structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM halos are too concentrated to be consistent with the halo parameters inferred for dwarf irregulars, and the predicted abundance of galaxy halos is larger than the observed abundance of galaxies., compressed and uuencoded PS file. 22 pages including 14 figures. 447875 bytes. Submitted to ApJ on August 1, 1995

The evolution of substructure in dark matter halos is investigated in a series of simulations of N = 105 satellite halos on elliptical orbits in the gravitational potential of a much larger host system. The bound mass of the satellite decreases with each pericentric passage and most of the mass is lost from the outer region of the satellite halo. We parameterize the change in its density profile by modifying the initial profile by a factor proportional to (1 + r-3), which results in reasonable fits to the mass profiles of tidally stripped subhalos.

We investigate recent suggestions that substructure in cold dark matter (CDM) halos has potentially destructive effects on galactic disks. N-body simulations of disk/bulge models of the Milky Way, embedded in a dark matter halo with substructure similar to that found in cosmological simulations, show that tides from substructure halos play only a minor role in the dynamical heating of the stellar disk. This suggests that substructure might not preclude CDM halos from being acceptable hosts of thin stellar disks.

Radial velocity surveys such as the Radial Velocity Experiment (RAVE) provide us with measurements of hundreds of thousands of nearby stars most of which belong to the Galactic thin, thick disk or halo. Ideally, to study the Galactic disks (both thin and thick) one should make use of the multi-dimensional phase-space and the whole pattern of chemical abundances of their stellar populations. In this paper, with the aid of the RAVE Survey, we study the thin and thick disks of the Milky Way, focusing on the latter. We present a technique to disentangle the stellar content of the two disks based on the kinematics and other stellar parameters such as the surface gravity of the stars. Using the Padova Galaxy Model, we checked the ability of our method to correctly isolate the thick disk component from the Galaxy mixture of stellar populations. We introduce selection criteria in order to clean the observed radial velocities from the Galactic differential rotation and to take into account the partial sky coverage of RAVE. We developed a numerical technique to statistically disentangle thin and thick disks from their mixture. We deduce the components of the solar motion relative to the Local Standard of Rest (LSR) in the radial and vertical direction, the rotational lag of the thick disk component relative to the LSR, and the square root of the absolute value of the velocity dispersion tensor for the thick disk alone. The analysis of the thin disk is presented in another paper. We find good agreement with previous independent parameter determinations. In our analysis we used photometrically determined distances. In the Appendix we show that similar values can be found for the thick disk alone as derived in the main sections of our paper even without the knowledge of photometric distances., accepted on A&A, please see companion paper "THIN disk kinem..."

Aims. We study the Milky Way thin disk with the Radial Velocity Experiment (RAVE) survey. We consider the thin and thick disks as different Galactic components and present a technique to statistically disentangle the two populations. Then we focus our attention on the thin disk component. Methods. We disentangle the thin disk component from amixture of the thin and thick disks using a data set providing radial velocities, proper motions, and photometrically determined distances. Results. We present the trend of the velocity dispersions in the thin disk component of the Milky Way (MW) in the radial direction above and below the Galactic plane using data from the RAdial Velocity Experiment (RAVE). The selected sample is a limited subsample from the entire RAVE catalogue, roughly mapping up to 500 pc above and below the Galactic plane, a few degrees in azimuthal direction and covering a radial extension of 2.0 kpc around the solar position. The solar motion relative to the local standard of rest is also re-determined with the isolated thin disk component. Major results are the trend of the velocity mean and dispersion in the radial and vertical direction. In addition the azimuthal components of the solar motion relative to the local standard of rest and the velocity dispersion are discussed., Comment: accepted on A&A, please see companion paper "THICK disk kinem..."

[Abbreviated] We have investigated the color-magnitude diagram of Omega Centauri and find that the blue main sequence (bMS) can be reproduced only by models that have a of helium abundance in the range Y=0.35-$0.40. To explain the faint subgiant branch of the reddest stars ("MS-a/RG-a" sequence), isochrones for the observed metallicity ([Fe/H]\approx0.7) appear to require both a high age (~13Gyr) and enhanced CNO abundances ([CNO/Fe]\approx0.9$). Y~0.35 must also be assumed in order to counteract the effects of high CNO on turnoff colors, and thereby to obtain a good fit to the relatively blue turnoff of this stellar population. This suggest a short chemical evolution period of time ( =6.8M_sun, M_He,core>=1.245M_sun) predict too large N-enhancements, which limits their role in contributing to the extreme populations. We show quantitatively that highly He- and N-enriched AGB ejecta have particularly efficient cooling properties. Based on these results and on the reconstruction of the orbit of Omega Cen with respect to the Milky Way we propose the galactic plane passage gas purging scenario for the chemical evolution of this cluster. Our model addresses the formation and properties of the bMS population (including their central location in the cluster). We follow our model descriptively through four passage events, which could explain not only some key properties of the bMS, but also of the MS-a/RGB-a and the s-enriched stars.

Abundance data on solar neighbourhood stars suggest the presence of chemically-distinct stellar components in the solar neighbourhood. When the abundances of Fe, α elements, and the r -process element Eu are considered together, stars separate neatly into two groups that delineate the thin and thick disk components of the Milky Way. The group akin to the thin disk is traced by stars of relatively high Fe content and low [α/Fe] ratios. The thick disk-like group overlaps the thin disk in [Fe/H] but has higher abundances of α elements and Eu. Fe-poor stars with low [α/Fe] ratios, however, seem to belong to a separate, dynamically-cold, non-rotating component likely associated with debris from past accretion events. The kinematically-hot stellar halo dominates the sample at the metal-poor end. These results suggest that it may be possible to define the main dynamical components of the solar neighbourhood using only their chemistry, an approach with a number of interesting consequences. For example, the average rotation speed and velocity dispersion of thin disk stars is roughly independent of metallicity, a result unexpected in most current theories of thin-disk formation. In this scenario, the familiar increase in the velocity dispersion of disk stars with decreasing metallicity is the result of the increasing prevalence of the thick disk at lower metallicities, rather than of the sustained operation of a dynamical heating mechanism. The substantial overlap in [Fe/H] and, likely, stellar age, of the various components might affect other reported trends in the properties of stars in the solar neighbourhood. A purely chemical characterization of these components allows the use of their kinematics to assess their origin, an powerful approach denied to traditional ways of apportioning stars to the various Galactic components.

We study the formation of galaxies by using $N$-body/hydrodynamics simulations to investigate how baryons collect at the centre of dark matter halos. We treat the dark matter as a collisionless fluid and the baryons as an ideal gas. We include the effects of gravity, pressure gradients, hydrodynamical shocks, and radiative energy losses, but we neglect star formation. Our initial conditions assume a flat universe dominated by cold dark matter with a mean baryon abundance of 10\% by mass. Typical halos form through the merging of a few smaller systems which had themselves formed in a similar manner at higher redshift. The gas collects at the bottom of dark matter potential wells as soon as these are properly resolved by our simulations. There it settles into cold, tightly bound disks, and it remains cold during subsequent evolution. As their halos coalesce, these disks merge on a timescale that is consistent with dynamical friction estimates based on their {\it total} (gas + surrounding dark matter) mass. Both the merger rates of the disks and their mass spectrum are in remarkably good agreement with recent analytic models that describe the evolution of {\it dark halos} in a hierarchical universe. This very simple model of galaxy formation suffers from serious shortcomings. It predicts that most baryons should be locked up in galaxies, whereas in the real universe most baryons are thought to lie outside visible galaxies. In addition, it predicts the specific angular momentum of a disk to be only about 20\% that of its surrounding halo, corresponding to a radius smaller than that of observed spiral galaxy disks., 11 pages, uuencoded file, fig. from dust0.dur.ac.uk (129.234.8.70), /pub/jfn/gxass_fig_ps.tar.Z

We predict the observable properties of the galaxy population in popular hierarchical models of galaxy formation. We employ a detailed semianalytic procedure which incorporates the formation and merging of dark matter halos, the shock heating and radiative cooling of gas, self-regulated star formation, the merging of galaxies within dark matter halos, and the spectral evolution of the stellar populations. We contrast the standard CDM cosmogony with variants of the CDM model having either a low value of H_0, or a low value of Omega with or without a cosmological constant. In addition, we compare galaxy formation in these CDM universes with a CHDM model. We find that although the models have some success in remedying the shortcomings of the standard CDM cosmogony, none of these new models produce broad agreement with the whole range of observations. Although the low-Omega and Omega+Lambda=1 CDM models reduce the discrepancy between the predicted and observed Tully-Fisher relations (the main weakness of galaxy formation in standard CDM), these models predict an inverted colour-magnitude relation and do not produce an exponential cut-off at the bright end of the galaxy luminosity function. All of our models predict recent star formation and exhibit galaxy colours bluer than observed, but this problem is far more severe in the CHDM model which produces colours about two magnitudes too blue in B-K. Unlike in the variants of the CDM model in the CHDM case this result is not dependent on our model of stellar feedback, but is instead directly caused by the late epoch of structure formation in this model., (postscript by anonymous ftp from dust0.dur.ac.uk as /pub/preprint/gf_heyl.ps)

We simulate the formation and evolution of X-ray clusters in a Cold Dark Matter dominated universe using a 3D hydro dynamical code which includes the effects of gravity, pressure gradients, and hydro dynamical shocks. Radiative cooling is neglected in these calculations. The equilibrium density profiles of gas and dark matter appear to be self-similar; a unique density profile fits clusters of different mass when properly scaled. The same profile also fits remarkably well clusters identified at different redshifts, at least in the regions not affected by numerical limitations. The slope of the profile steepens gradually from the center outwards and becomes steeper than an isothermal profile near the virial radius. A single temperature profile also fits all clusters. The gas is essentially isothermal near the center but its temperature drops substantially at large radii. The temperature at the virial radius is found to be typically one-half of that at the center. This temperature dependence on radius should be detectable with current X-ray missions.

We report the discovery of eight lithium-rich field giants found in a high resolution spectroscopic sample of over 700 metal-poor stars ([Fe/H], Comment: 17 pages, 8 figures, accepted for publication in ApJ

We characterize substructure in the simulated stellar halos of Cooper et al. (2010) which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius Project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared to the value estimated for the Milky Way from main sequence turn-off stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in-situ., Comment: 5 pages, 5 color figures, accepted on April 8, 2011 with a change in title. A high-resolution version of the paper may be found at: http://www.astro.rug.nl/~ahelmi/aquarius.html

The Pan Andromeda Archeological Survey (PAndAS) CFHT Megaprime survey of the M31-M33 system has found a star stream which extends about 120 kpc NW from the center of M31. The great length of the stream, and the likelihood that it does not significantly intersect the disk of M31, means that it is unusually well suited for a measurement of stream gaps and clumps along its length as a test for the predicted thousands of dark matter sub-halos. The main result of this paper is that the density of the stream varies between zero and about three times the mean along its length on scales of 2 to 20 kpc. The probability that the variations are random fluctuations in the star density is less than 10^-5. As a control sample we search for density variations at precisely the same location in stars with metallicity higher than the stream, [Fe/H]=[0, -0.5] and find no variations above the expected shot noise. The lumpiness of the stream is not compatible with a low mass star stream in a smooth galactic potential, nor is it readily compatible with the disturbance caused by the visible M31 satellite galaxies. The stream's density variations appear to be consistent with the effects of a large population of steep mass function dark matter sub-halos, such as found in LCDM simulations, acting on an approximately 10Gyr old star stream. The effects of a single set of halo substructure realizations are shown for illustration, reserving a statistical comparison for another study., ApJ revised version submitted

In Figure 6 we inadvertently labeled the proxy circular velocity as the virial velocity of the dark matter halo instead of what is actually plotted, the maximum circular velocity of the dark matter halo. The maximum halo circular velocity is a much better estimate of the disk V_c than is V_vir. This confusion influenced the discussion of the Tully-Fisher relation in our paper. In fact, Figure 6 demonstrates that it is possible to simultaneously reproduce both the local Tully-Fisher relation and luminosity function using semi-analytic techniques applied to the standard LCDM cosmology, thus contradicting previous studies of this issue and our own discussion in Section 3.6.

We use numerical simulations to study the dynamical effect of substructure in Cold Dark Matter (CDM) halos on stellar galactic disks. Substructure halos with masses, densities and orbits derived from high-resolution cosmological CDM simulations play only a minor dynamical role in the heating of the disk over several Gyrs. This is because the orbits of dark satellites in CDM halos seldom take them near the disk, where their tidal effects are greatest. The most noticeable effects of substructure in our simulation may be traced to the most massive sub-halo and include: (i) the forcing of short-lived stellar warps in the disk as a result of tidal shocks that arise during each pericentric passage, and (ii) the tilting of the disk caused by the orbital decay of the satellite. These results imply that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way, and that the ubiquity of minor stellar warps may be associated with the recurrent tidal influence on the disk of the most massive substructure halos.

We use cosmological gas dynamic simulations to investigate the accuracy of galaxy cluster mass estimates based on X-ray observations. The experiments follow the formation of clusters in different cosmological models and include the effects of gravity, pressure gradients, and hydrodynamical shocks. A subset of our ensemble also allows for feedback of mass and energy from galactic winds into the intracluster medium. We find that mass estimates based on the hydrostatic, isothermal beta-model are remarkably accurate when evaluated at radii where the cluster mean density is between 500-2500 times the critical density. Applied to 174 artificial ROSAT images constructed from the simulations, the distribution of the estimated-to-true mass ratio is nearly unbiased and has a standard deviation of 14-29%. The scatter can be considerably reduced (to 8-15%) by using an alternative mass estimator that exploits the tightness of the mass-temperature relation found in the simulations. The improvement over beta-model estimates is due to the elimination of the variance contributed by the gas outer slope parameter. We discuss these findings and their implications for recent measurements of cluster baryon fractions., Comment: TeX, 24p; 11 Postscript figs. Submitted to the Astrophysical Journal

The dynamical evolution of both collisionless dark particles and a dissipative gaseous (baryonic) component in a flat universe are simulated in order to investigate the formation process of the luminous components of galaxies at the center of galactic dark halos. The initial density fluctuations are assumed to be Gaussian with a power spectrum of the form P(k) ∞ k −1 . The models assume that the gas amounts to 10% of the mass of the universe, and that both the gas and the dark matter are identifically distributed in phase space at high redshifts.

SeveralN-body experiments were performed in order to simulate the dynamical behaviour of systems of galaxies gravitationally dominated by a massive dark background. We discuss mass estimates from the dynamics of the luminous component (M VT) under the influence of such a background, assuming a constant dark/luminous mass ratio (M D/M L) and plausible physical conditions. We extend in this way previous studies (Smith, 1980, 1984) about the dependence ofM VT on the relative distributions of dark and luminous matter (Limber, 1959). We found that the observed ratio of the virial theorem mass to luminosity (M VT/L) in systems of galaxies of different sizes could be the result of different stages of their post-virialisation evolution as was previously suggested by White and Rees (1978) and Barnes (1983). This evolution is mainly the result of the dynamical friction that dark matter exerts on the luminous component. Thus our results give support to the idea that compact groups of galaxies are dynamically more evolved than large clusters, which is expected from the ‘hierarchical clustering’ picture for the formation of such structures.

We have performed a series ofN-body experiments including the effects of a massive dominant background which follows Schuster's density law in order to simulate clusters of galaxies in which a smoothly distributed dark component is present. The existence of this background is inferred from the weak luminosity segregation observed in clusters which, however, show several characteristics of well-relaxed systems. The comparison of the velocity dispersion profiles of three clusters of galaxies (Coma, Perseus, and Virgo) with those obtained in the numerical experiments allows us to place some constraints on both the distribution and amount of distributed dark material in these clusters. The profiles are rather insensitive to variations in the ratio of the background mass to the mass attached to galaxies (Mb/Mg), but exhibit a strong dependence on their relative concentration. We conclude that highly concentrated background models are not consistent with observations. We find a maximum value for the ratio of the gravitational radius of the galaxies and the background (Rg/Rb) (approximately 0.6) and using previous results (Navarroet al., 1986) we conclude that virial theorem masses underestimate the total mass (Mb+Mg) of the clusters. As a final result, we derive a minimum value for theMb/Mg ratio. All these conclusions could apply in general if Coma, Perseus, and Virgo constitute a fair sample of the rich clusters of galaxies in the Universe.

Several N-body experiments were performed in order to analyze the importance of smoothly distributed dark matter on the merging processes in groups of galaxies. We simulated groups of galaxies as self-gravitating systems of softened point masses with average intergalactic distance (Dₘ) ranging between 10 and 30 in units of the mean galactic half mass radius, and a cluster velocity dispersion to mean galactic internal velocity dispersion ratio (V_cl/σ_g) of about 2. A giant central galaxy ('cannibal') develops in less than a free fall time τ_ff when no background of dark matter is present. Only the galaxies which could acquire escape velocity, and thus leave the system, can avoid merging with the cannibal. This situation arises regardless of the initial dynamical conditions were imposed, which ranged from 'cold' models (all galaxies at rest) to equilibrium models (started with galaxies in virial equilibrium). We also tried initial conditions in which galaxies were moving in random directions with random velocity (rnd models) and found no difference in the time-scale of the process mentioned above. Thus groups of galaxies with no background cannot avoid merging into a single unit in a time-scale (∼ 1 τ_ff) much shorter than the expected life of such systems. The efficiency of the merging process is remarkably diminished when the effects of a massive background are included in the simulations. The more important the background in the dynamics of the system, the longer the system can stand without being cannibalized by a continuously growing galaxy, although keeping similar initial values for Dₘ and V_cl/σ_g. However when such a galaxy has 'eaten' about half of the mass in form of a galaxy, the process accelerates itself and the cannibal is rapidly formed. So, it must be concluded that such a dominant background is actually present if groups of galaxies are, as commonly assumed, older than five crossing times, because systems in which most of the mass is concentrated in galaxies appear to be largely unstable to merging., Asociación Argentina de Astronomía