Showing papers by "Julio F. Navarro published in 2008"

The Aquarius Project: the subhaloes of galactic haloes

Abstract: We have performed the largest ever particle simulation of a Milky Way sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far We have also simulated a sample of six ultrahighly resolved Milky Way sized haloes, allowing us to estimate the halo-to-halo scatter in substructure statistics In our largest simulation, we resolve nearly 300 000 gravitationally bound subhaloes within the virialized region of the halo Simulations of the same object differing in mass resolution by factors of up to 1800 accurately reproduce the largest subhaloes with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures We detect up to four generations of subhaloes within subhaloes, but contrary to recent claims, we find less substructure in subhaloes than in the main halo when regions of equal mean overdensity are compared The overall substructure mass fraction is much lower in subhaloes than in the main halo Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3 per cent within 100 kpc, and to be below 01 per cent within the solar circle The inner density profiles of subhaloes show no sign of converging to a fixed asymptotic slope and are well fitted by gently curving profiles of Einasto form The mean concentrations of isolated haloes are accurately described by the fitting formula of Neto et al down to maximum circular velocities of 15 km s(-1), an extrapolation over some five orders of magnitude in mass However, at equal maximum circular velocity, subhaloes are more concentrated than field haloes, with a characteristic density that is typically similar to 26 times larger and increases with decreasing distance from halo centre

The Aquarius Project: the subhalos of galactic halos

Abstract: We have performed the largest ever particle simulation of a Milky Way-sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far. We have also simulated a sample of 6 ultra-highly resolved Milky-way sized halos, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300,000 gravitationally bound subhalos within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors up to 1800 accurately reproduce the largest subhalos with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subhalos within subhalos, but contrary to recent claims, we find less substructure in subhalos than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3% within 100 kpc, and to be below 0.1% within the Solar Circle. The inner density profiles of subhalos show no sign of converging to a fixed asymptotic slope and are well fit by gently curving profiles of Einasto form. The mean concentrations of isolated halos are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km/s, an extrapolation over some 5 orders of magnitude in mass. However, at equal maximum circular velocity, subhalos are more concentrated than field halos, with a characteristic density that is typically ~2.6 times larger and increases towards the halo centre.

The Diversity and Similarity of Simulated Cold Dark Matter Halos

Abstract: We study the mass, velocity dispersion, and anisotropy profiles of $\Lambda$CDM halos using a suite of N-body simulations of unprecedented numerical resolution (the {\it Aquarius Project}). Our analysis confirms a number of results claimed by earlier work, and clarifies a few issues where conflicting claims may be found in the recent literature. The spherically-averaged density profile becomes progressively shallower inwards and, at the innermost resolved radius, the logarithmic slope is $\gamma \equiv -$d$\ln\rho/$d$\ln r \simlt 1$. Asymptotic inner slopes as steep as the recently claimed $\rho \propto r^{-1.2}$ are clearly ruled out. The radial dependence of $\gamma$ is well approximated by a power-law, $\gamma \propto r^{\alpha}$ (the Einasto profile). The shape parameter, $\alpha$, varies slightly but significantly from halo to halo, implying that the mass profiles of $\Lambda$CDM halos are not strictly universal: different halos cannot, in general, be rescaled to look identical. Departures from similarity are also seen in velocity dispersion profiles and correlate with those in density profiles so as to preserve a power-law form for the spherically averaged pseudo-phase-space density, $\rho/\sigma^3\propto r^{-1.875}$. Our conclusions are reliable down to radii below 0.4% of the virial radius, providing well-defined predictions for halo structure when baryonic effects are neglected, and thus an instructive theoretical template against which the modifications induced by the baryonic components of real galaxies can be judged.

The redshift dependence of the structure of massive Λ cold dark matter haloes

Abstract: We use two very large cosmological simulations to study how the density profiles of relaxedCDM dark halos depend on redshift and on halo mass. We confirm that these profiles deviate slightly but systematically from the NFW form and are better approximated by the empirical formula, dlog ρ/dlog r ∝ r � , first used by Einasto to fit star counts in the Milky Way. The best-fit value of the additional shape parameter, α, increases gradually with mass, from α ∼ 0.16 for present-day galaxy halos to α ∼ 0.3 for the rarest and most massive clusters. Halo concentrations depend only weakly on mass at z = 0, and this dependence weakens further at earlier times. At z ∼ 3 the average concentration of relaxed halos does not vary appreciably over the mass range accessible to our simulations (M ∼3×10 11 h −1 M⊙). Furthermore, in our biggest simulation, the average concentration of the most massive, relaxed halos is constant at h c200i ∼ 3.5 to 4 for 0 ≤ z ≤ 3. These results agree well with those of Zhao et al (2003b) and support the idea that halo densities reflect the density of the universe at the time they formed, as proposed by Navarro, Frenk & White (1997). With their original parameters, the NFW prescription overpredicts halo concentrations at high redshift. This shortcoming can be reduced by modifying the definition of halo formation time, although the evolution of the concentrations of Milky Way mass halos is still not reproduced well. In contrast, the much-used revisions of the NFW prescription by Bullock et al. (2001) and Eke, Navarro & Steinmetz (2001) predict a steeper drop in concentration at the highest masses and stronger evolution with redshift than are compatible with our numerical data. Modifying the parameters of these models can reduce the discrepancy at high masses, but the overly rapid redshift evolution remains. These results have important implications for currently planned surveys of distant clusters.

Prospects for detecting supersymmetric dark matter in the Galactic halo

Abstract: Dark matter is the dominant form of matter in the Universe, but its nature is unknown. It is plausibly an elementary particle, perhaps the lightest supersymmetric partner of known particle species. In this case, annihilation of dark matter in the halo of the Milky Way should produce gamma-rays at a level that may soon be observable. Previous work has argued that the annihilation signal will be dominated by emission from very small clumps (perhaps smaller even than the Earth), which would be most easily detected where they cluster together in the dark matter haloes of dwarf satellite galaxies. Here we report that such small-scale structure will, in fact, have a negligible impact on dark matter detectability. Rather, the dominant and probably most easily detectable signal will be produced by diffuse dark matter in the main halo of the Milky Way. If the main halo is strongly detected, then small dark matter clumps should also be visible, but may well contain no stars, thereby confirming a key prediction of the cold dark matter model.

The Cold Dark Matter Halos of Local Group Dwarf Spheroidals

Abstract: We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically motivated model in which dSphs are approximated by a King model embedded in 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, -->Rc 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 mass-to-light ratios of the newly discovered ultrafaint 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 ultrafaint 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.

The Diversity and Similarity of Cold Dark Matter Halos

Abstract: We study the mass, velocity dispersion, and anisotropy profiles of $\Lambda$CDM halos using a suite of N-body simulations of unprecedented numerical resolution (the {\it Aquarius Project}). Our analysis confirms a number of results claimed by earlier work, and clarifies a few issues where conflicting claims may be found in the recent literature. The spherically-averaged density profile becomes progressively shallower inwards and, at the innermost resolved radius, the logarithmic slope is $\gamma \equiv -$d$\ln\rho/$d$\ln r \simlt 1$. Asymptotic inner slopes as steep as the recently claimed $\rho \propto r^{-1.2}$ are clearly ruled out. The radial dependence of $\gamma$ is well approximated by a power-law, $\gamma \propto r^{\alpha}$ (the Einasto profile). The shape parameter, $\alpha$, varies slightly but significantly from halo to halo, implying that the mass profiles of $\Lambda$CDM halos are not strictly universal: different halos cannot, in general, be rescaled to look identical. Departures from similarity are also seen in velocity dispersion profiles and correlate with those in density profiles so as to preserve a power-law form for the spherically averaged pseudo-phase-space density, $\rho/\sigma^3\propto r^{-1.875}$. Our conclusions are reliable down to radii below 0.4% of the virial radius, providing well-defined predictions for halo structure when baryonic effects are neglected, and thus an instructive theoretical template against which the modifications induced by the baryonic components of real galaxies can be judged.

Diffuse interstellar bands in RAVE survey spectra

Abstract: We have used spectra of hot stars from the RAVE Survey in order to investigate the visibility and properties of five diffuse interstellar bands previously reported in the literature. The RAVE spectroscopic survey for Galactic structure and kinematics records CCD spectra covering the 8400–8800 A wavelength region at 7500 resolving power. The spectra are obtained with the UK Schmidt at the AAO, equipped with the 6dF multi-fiber positioner. The DIB at 8620.4 A is by far the strongest and cleanest of all DIBs occurring within the RAVE wavelength range, with no interference by underlying absorption stellar lines in hot stars. It correlates so tightly with reddening that it turns out to be a reliable tool to measure it, following the relation EB−V = 2.72(±0.03) × EW (A), valid throughout the general interstellar medium of our Galaxy. The presence of a DIB at 8648 A is confirmed. Its intensity appears unrelated to reddening, in agreement with scanty and preliminary reports available in the literature, and its measurability is strongly compromised by severe blending with underlying stellar He I doublet at 8649 A. The two weak DIBs at 8531 and 8572 A do not appear real and should actually be blends of underlying stellar lines. The very weak DIB at 8439 A cannot be resolved within the profile of the much stronger underlying hydrogen Paschen 18 stellar line.

Diffuse interstellar bands in RAVE Survey spectra

Abstract: We have used spectra of hot stars from the RAVE Survey in order to investigate the visibility and properties of five diffuse interstellar bands previously reported in the literature. The RAVE spectroscopic survey for Galactic structure and kinematics records CCD spectra covering the 8400-8800 Ang wavelength region at 7500 resolving power. The spectra are obtained with the UK Schmidt at the AAO, equipped with the 6dF multi-fiber positioner. The DIB at 8620.4 Ang is by far the strongest and cleanest of all DIBs occurring within the RAVE wavelength range, with no interference by underlying absorption stellar lines in hot stars. It correlates so tightly with reddening that it turns out to be a reliable tool to measure it, following the relation E(B-V) = 2.72 (+/- 0.03) x E.W.(Ang), valid throughout the general interstellar medium of our Galaxy. The presence of a DIB at 8648 Ang is confirmed. Its intensity appears unrelated to reddening, in agreement with scanty and preliminary reports available in the literature, and its measurability is strongly compromised by severe blending with underlying stellar HeI doublet at 8649 Ang. The two weak DIBS at 8531 and 8572 Ang do not appear real and should actually be blends of underlying stellar lines. The very weak DIB at 8439 Ang cannot be resolved within the profile of the much stronger underlying hydrogen Paschen 18 stellar line.

Local group dwarf galaxies in the ΔCDM paradigm

Abstract: We report the results of two theoretical studies that 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). (i) Firstly, approximating the stellar and dark components by King and NFW models, respectively, we calculate the parameters of dark halos consistent with the kinematics and spatial distribution of stars in dSphs as well as with cosmological N-body simulations. (ii) Subsequently, N-body realization of these models are constructed to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. The analytical estimates highlight the poor correspondence between luminosity and halo mass. In systems where data exist, the stellar velocity dispersion profiles remains flat almost to the nominal “tidal” radius, implying that stars are deeply embedded within the dwarf halos and are therefore quite resilient to tidal disruption. This is confirmed by our N-body experiments: halos need to lose more than 90% of their original mass before stars can be stripped. As tidal mass loss proceeds, the stellar luminosity, L, velocity dispersion, σ0, central surface brightness, Σ0, and core radius, Rc, decrease monotonically. Remarkably, the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius, which permit us to derive a tidal evolutionary track for each of them. This information is used to examine whether the newly-discovered ultra-faintMilkyWay dwarfs are tidally-stripped versions of the “classical”, bright dwarfs. 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. 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. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Unorthodox Orbits of Substructure Halos

Abstract: (Abridged) We use cosmological N-body simulations to study the properties of substructure halos in galaxy-sized dark matter halos. We extend prior work on the subject by considering the whole population of subhalos physically associated with the main system. These are defined as subhalos that have at some time in the past been within the virial radius of the halo's main progenitor and that have survived as self-bound entities to $z=0$. We find that this population extends beyond {\it three times} the virial radius, and contains objects on extreme orbits. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups of subhalos, which results in the ejection of some subhalos along tidal streams. Ejected subhalos are primarily low-mass systems, leading to mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time, the less centrally concentrated and kinematically hotter their descendant population. The bias is strongest amongst the most massive subhalos, but disappears at the low-mass end. Our findings imply that subhalos identified within the virial radius represent an incomplete census of the substructure physically related to a halo: only about {\it one half} of all associated subhalos are found today within the virial radius of a halo. These results may explain the age dependence of the clustering of low-mass halos, and has implications for (i) the interpretation of the structural parameters and assembly histories of halos neighboring massive systems; (ii) the existence of low-mass dynamical outliers in the Local Group; and (iii) the presence of evidence for evolutionary effects well outside the traditional virial boundary of a galaxy system.

Erratum: "The Cold Dark Matter Halos of Local Group Dwarf Spheroidals" (ApJ, 672, 904 [2008])

Abstract: 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). Approximating the stellar and dark components byKing andNFWmodels, respectively, we identify the parameters of dark halos consistent with the kinematics and spatial distribution of stars in dSphs, as well as with cosmological N-body simulations. Our analysis shows that the total mass within the luminous radius is reasonably well constrained and approximately independent of the luminosity of the dwarf. Becausemassive CDMhalos are denser than low-mass ones at all radii, 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. We find that the flat stellar velocity dispersion profiles, p(R), observed in dSphs 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 Vmax is about 3 times higher than the central velocity dispersion of the stars, which significantly alleviates the CDM ‘‘substructure crisis.’’ We use these results to interpret the size differences between the M31 andMilkyWay (MW) dSph populations. 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. 1460 The Astrophysical Journal, 687:1460, 2008 November 10 # 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.

Galactic kinematics from RAVE to Gaia-RVS Data

Abstract: RAVE data has provided new results on Galactic kinematics like the kinematical decomposition of the Galactic disk. This decomposition permits to identify the different components of the disk and to characterize them in terms of scale height and scale length. With the data provided by Gaia and in particular the RVS, we will have a completly renewed view of the Galaxy. The precision of the RVS will permit to undertake a precise analysis of the kinematics of the Galactic disks. This knowledge will provide significant clues to constrain the scenarios of the Galactic disk formation.