arXiv subjects

Julio F. Navarro

Publications and source records attributed to Julio F. Navarro.

At least 181 records · Page 10Linked to original sources

arXiv2011

The Density and Pseudo-Phase-Space Density Profiles of CDM halos

Cosmological N-body simulations indicate that the spherically-averaged density profiles of cold dark matter halos are accurately described by Einasto profiles, where the logarithmic slope is a power-law of adjustable exponent, γ=dlnρ/dlnr ~ r^α$. The pseudo-phase-space density (PPSD) profiles of CDM halos also show remarkable regularity, and are well approximated by simple power laws, Q(r)=ρ/σ^3 ~ r^-χ. We show that this is expected from dynamical equilibrium considerations, since Jeans' equations predict that the pseudo-phase-space density profiles of Einasto halos should resemble power laws over a wide range of radii. For the values of αtypical of CDM halos, the inner Q profiles of equilibrium halos deviate significantly from a power law only very close to the center, and simulations of extremely high-resolution would be needed to detect such deviations unambiguously. We use an ensemble of halos drawn from the Millennium-II simulation to study which of these two alternatives describe best the mass profile of CDM halos. Our analysis indicates that at the resolution of the best available simulations, both Einasto and power-law PPSD profiles (with adjustable exponents αand χ, respectively) provide equally acceptable fits to the simulations. A full account of the structure of CDM halos requires understanding how the shape parameters that characterize departures from self-similarity, like αor χ, are determined by evolutionary history, environment or initial conditions.

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arXiv2010

The Dawning of the Stream of Aquarius in RAVE

We identify a new, nearby (0.5 < d < 10 kpc) stream in data from the RAdial Velocity Experiment (RAVE). As the majority of stars in the stream lie in the constellation of Aquarius we name it the Aquarius Stream. We identify 15 members of the stream lying between 30 < l < 75 and -70< b <-50, with heliocentric line-of-sight velocities V_los~-200 km/s. The members are outliers in the radial velocity distribution, and the overdensity is statistically significant when compared to mock samples created with both the Besançon Galaxy model and newly-developed code Galaxia. The metallicity distribution function and isochrone fit in the log g - T_eff plane suggest the stream consists of a 10 Gyr old population with [m/H]~-1.0. We explore relations to other streams and substructures, finding the stream cannot be identified with known structures: it is a new, nearby substructure in the Galaxy's halo. Using a simple dynamical model of a dissolving satellite galaxy we account for the localization of the stream. We find that the stream is dynamically young and therefore likely the debris of a recently disrupted dwarf galaxy or globular cluster. The Aquarius stream is thus a specimen of ongoing hierarchical Galaxy formation, rare for being right in the solar suburb.

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arXiv2010

Through Thick and Thin: Kinematic and Chemical Components in the Solar Neighbourhood

We search for the existence of chemically-distinct stellar components in the solar neighbourhood using published data. Extending earlier work, we show that when the abundances of Fe, alpha elements, and the r-process element Eu are considered, stars separate neatly into two groups that delineate the traditional thin and thick disk of the Milky Way. The group akin to the thin disk is traced by stars with [Fe/H]>-0.7 and alpha/Fe<0.2. The thick disk-like group overlaps the thin disk in [Fe/H] but has higher abundances of alpha elements and Eu. Stars in the range -1.5<[Fe/H]<-0.7 with low [alpha/Fe] ratios, however, seem to belong to a separate, dynamically-cold, non-rotating component that we associate with tidal debris. The kinematically-hot stellar halo dominates the sample for [Fe/H]<-1.5. 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. The kinematics of thin disk stars is then independent of metallicity: their average rotation speed remains roughly constant in the range -0.7<[Fe/H]<+0.4, a result that argues against radial migration having played a substantial role in the evolution of the thin disk. The velocity dispersion of stars assigned to the thin disk is also independent of [Fe/H], implying that the familiar increase in velocity dispersion with decreasing metallicity is the result of the increased 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, probably, stellar age, of the various components might affect other reported trends in the properties of stars in the solar neighbourhood.

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arXiv2010

Feedback and the Structure of Simulated Galaxies at redshift z=2

We study the properties of simulated high-redshift galaxies using cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations (OWLS) project. The runs contrast several feedback implementations of varying effectiveness: from no-feedback, to supernova-driven winds to powerful AGN-driven outflows. These different feedback models result in large variations in the abundance and structural properties of bright galaxies at z=2. We find that feedback affects the baryonic mass of a galaxy much more severely than its spin, which is on average roughly half that of its surrounding dark matter halo in our runs. Feedback induces strong correlations between angular momentum content and galaxy mass that leave their imprint on galaxy scaling relations and morphologies. Encouragingly, we find that galaxy disks are common in moderate-feedback runs, making up typically ~50% of all galaxies at the centers of haloes with virial mass exceeding 1e11 M_sun. The size, stellar masses, and circular speeds of simulated galaxies formed in such runs have properties that straddle those of large star-forming disks and of compact early-type galaxies at z=2. Once the detailed abundance and structural properties of these rare objects are well established it may be possible to use them to gauge the overall efficacy of feedback in the formation of high redshift galaxies.

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arXiv2010

The Sphericalization of Dark Matter Halos by Galaxy Disks

Cosmological simulations indicate that cold dark matter (CDM) halos should be triaxial. Verifying observationally this theoretical prediction is, however, less than straightforward because the assembly of galaxies is expected to modify the halo shapes and to render them more axisymmetric. We use a suite of N-body simulations to investigate quantitatively the effect of the growth of a central disk galaxy on the shape of triaxial dark matter halos. As expected, the halo responds to the presence of the disk by becoming more spherical. The net effect depends only weakly on the orientation of the disk relative to the halo principal axes or the timescale of disk assembly, but strongly on the overall gravitational importance of the disk. Our results show that exponential disks whose contribution peaks at less than ~50% of their circular velocity are unable to modify noticeably the shape of the gravitational potential of their surrounding halos. Many dwarf and low surface brightness galaxies are expected to be in this regime, and therefore their detailed kinematics could be used to probe halo triaxiality, one of the basic predictions of the CDM paradigm. We argue that the complex disk kinematics of the dwarf galaxy NGC 2976 might be the reflection of a triaxial halo. Such signatures of halo triaxiality should be common in galaxies where the luminous component is subdominant.

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arXiv2010

Secondary Infall and the Pseudo-Phase-Space Density Profiles of Cold Dark Matter Halos

We use N-body simulations to investigate the radial dependence of the density and velocity dispersion in cold dark matter (CDM) halos. In particular, we explore how closely Q rho/sigma^3, a surrogate measure of the phase-space density, follows a power-law in radius. Our study extends earlier work by considering, in addition to spherically-averaged profiles, local Q-estimates for individual particles, Q_i; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, Q_i(r'); and by carefully removing substructures in order to focus on the profile of the smooth halo, Q^s. The resulting Q_i^s(r') profiles follow closely a power law near the center, but show a clear upturn from this trend near the virial radius, r_{200}. The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, Q \propto r^{-1.875} in the inner, virialized regions, but departures from a power-law occur near r_{200} because of the proximity of this radius to the location of the first shell crossing - the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of $Q$ profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM halos beyond their characteristic scale radius.

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arXiv2009

The remnants of galaxy formation from a panoramic survey of the region around M31

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach $10 - 100$ times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than $M_V < -6$ await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

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arXiv2009

The Origin of Extended Disk Galaxies at z=2

Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological N-body/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m_d=M_gal/M_vir, depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M_vir>1e11 h^{-1}M_sun). The angular momentum of a galaxy, j_d=J_gal/J_vir, correlates with m_d in a manner that is insensitive to feedback and that deviates strongly from the simple j_d = m_d assumption often adopted in semi-analytic models of galaxy formation. The m_d-j_d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (~20%-30%) of all baryons within the virial radius. At z=2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z=2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter.

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arXiv2009

The Diversity and Similarity of Simulated Cold Dark Matter Halos

We study the mass, velocity dispersion, and anisotropy profiles of $Λ$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 $γ\equiv -$d$\lnρ/$d$\ln r \simlt 1$. Asymptotic inner slopes as steep as the recently claimed $ρ\propto r^{-1.2}$ are clearly ruled out. The radial dependence of $γ$ is well approximated by a power-law, $γ\propto r^α$ (the Einasto profile). The shape parameter, $α$, varies slightly but significantly from halo to halo, implying that the mass profiles of $Λ$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, $ρ/σ^3\propto r^{-1.875}$. 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.

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arXiv2009

Disk galaxies at z=2 in OWLS

We use the OWLS (OverWhelmingly Large Simulations) set of cosmological Nbody/gasdynamical simulations to study the properties of simulated galaxies at z=2. We focus on the effect of supernova feedback from evolving stars on the baryonic mass and angular momentum content of galaxies that assemble at the center of 1e11-3e12 h^{-1}M_\odot halos. Our main finding is that the mass and angular momentum of such galaxies are strongly coupled, in a way that is approximately independent of feedback: varying the feedback implementation leads, in a given halo, to large variations in galaxy mass but leaves the galaxy mass-angular momentum correlation largely unaltered. In particular, the ratio between the angular momentum of a galaxy and that of its surrounding halo (j_d=J_gal/J_vir) correlates closely with the galaxy mass (expressed in units of the virial mass of the halo; m_d=M_gal/M_vir). This correlation differs substantially from the m_d=j_d assumption commonly adopted in semianalytic models of galaxy formation. We use these results to infer the sizes of disk galaxies at z=2 expected in the LCDM scenario and to interpret recent observations of extended disks at z~2 by the SINS collaboration

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arXiv2009

The Signature of Galactic Tides in Local Group Dwarf Spheroidals

(Abridged) We use N-body simulations to explore the effects of tidal stripping on the structure of dwarf spheroidal galaxies (dSphs). We model dSphs as King spheres embedded in NFW dark halos that orbit the Galactic potential on eccentric orbits. We find that episodes of stellar mass loss remove the initial cutoff of the bound stellar core. Once equilibrium has been re-established the outer mass profile approaches a power-law in projection that is well described by a simple Plummer model. Tides also result in transient features in the outer density profile. As the system re-equilibrates, an outward-moving "excess" of stars is found at radii where the local crossing time exceeds the time elapsed since pericenter. If the orbit is known, these results provide a simple way to assess whether "breaks" and "bumps" in the profile of dSphs are actually tidal in origin. We apply this to the Sagittarius dwarf and, encouragingly, identify two features in the profile that may be traced to its two last pericentric passages. Applied to Leo I, we find that any tidal break would locate beyond the area surveyed by current data, casting doubt on recent claims of the detection of tidal debris around this galaxy. For Carina, the tidal break should occur at a radius twice farther than observed, suggesting that the outer excess of stars is not tidal in origin unless its orbit is in error. A similar comment applies to Sculptor, whose pericenter appears too large for Galactic tides to be important but whose outer profile, like that of Draco, nonetheless follows closely a Plummer-law. Fornax and Leo II show no sign of a power-law outer profile, suggesting that they have not suffered stellar tidal stripping. Published profiles for other, fainter Milky Way dSph companions do not extend sufficiently far to allow for conclusive assessment.

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arXiv2009

Galaxy-Induced Transformation of Dark Matter Halos

We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its dark halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component and a second one where a baryonic component is added. These simulations include radiative cooling but neglect star formation and feedback, leading most baryons to collect at the halo center in a disk which is too small and too massive when compared with typical spiral. This unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their dark halos. We find that the shape of the halo becomes more axisymmetric: halos are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the "adiabatic contraction" hypothesis. The reduced contraction helps to reconcile LambdaCDM halos with constraints on the dark matter content inside the solar circle and should alleviate the long-standing difficulty of matching simultaneously the scaling properties of galaxy disks and the luminosity function. The halo contraction is also less pronounced than found in earlier simulations, a disagreement that suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the center of a halo matters, but also the mode of its deposition. It might prove impossible to predict the halo response without a detailed understanding of a galaxy's assembly history. (Abriged)

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arXiv2008

Phase-space structure in the local dark matter distribution and its signature in direct detection experiments

We study predictions for dark matter phase-space structure near the Sun based on high-resolution simulations of six galaxy halos taken from the Aquarius Project. The local DM density distribution is predicted to be remarkably smooth; the density at the Sun differs from the mean over a best-fit ellipsoidal equidensity contour by less than 15% at the 99.9% confidence level. The local velocity distribution is also very smooth, but it differs systematically from a (multivariate) Gaussian distribution. This is not due to the presence of individual clumps or streams, but to broad features in the velocity modulus and energy distributions that are stable both in space and time and reflect the detailed assembly history of each halo. These features have a significant impact on the signals predicted for WIMP and axion searches. For example, WIMP recoil rates can deviate by ~10% from those expected from the best-fit multivariate Gaussian models. The axion spectra in our simulations typically peak at lower frequencies than in the case of multivariate Gaussian velocity distributions. Also in this case, the spectra show significant imprints of the formation of the halo. This implies that once direct DM detection has become routine, features in the detector signal will allow us to study the dark matter assembly history of the Milky Way. A new field, "dark matter astronomy", will then emerge.

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arXiv2008

An Alternative Origin for Hypervelocity Stars

Halo stars with unusually high radial velocity ("hypervelocity" stars, or HVS) are thought to be stars unbound to the Milky Way that originate from the gravitational interaction of stellar systems with the supermassive black hole at the Galactic center. We examine the latest HVS compilation and find peculiarities that are unexpected in this black hole-ejection scenario. For example, a large fraction of HVS cluster around the constellation of Leo and share a common travel time of $\sim 100$-200 Myr. Furthermore, their velocities are not really extreme if, as suggested by recent galaxy formation models, the Milky Way is embedded within a $2.5\times 10^{12} h^{-1} M_{\odot}$ dark halo with virial velocity of $\sim 220$ km/s. In this case, the escape velocity at $\sim 50$ kpc would be $\sim 600$ km/s and very few HVS would be truly unbound. We use numerical simulations to show that disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system. These stars are arranged in a thinly-collimated outgoing ``tidal tail'' stripped from the dwarf during its latest pericentric passage. We speculate that some HVS may therefore be tidal debris from a dwarf recently disrupted near the center of the Galaxy. In this interpretation, the angular clustering of HVS results because from our perspective the tail is seen nearly ``end on'', whereas the common travel time simply reflects the fact that these stars were stripped simultaneously from the dwarf during a single pericentric passage. This proposal is eminently falsifiable, since it makes a number of predictions that are distinct from the black-hole ejection mechanism and that should be testable with improved HVS datasets.

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arXiv2008

The Unorthodox Orbits of Substructure Halos

(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.

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arXiv2008

A blueprint for detecting supersymmetric dark matter in the Galactic halo

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 which 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 halos of dwarf satellite galaxies. Here we show, using the largest ever simulation of the formation of a galactic halo, that such small-scale structure will, in fact, have a negligible impact on dark matter detectability. Rather, the dominant and likely 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 (CDM) model.

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arXiv2008

The Aquarius Project: the subhalos of galactic halos

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.

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arXiv2008

The redshift dependence of the structure of massive LCDM halos

(Abridged) We use two very large cosmological simulations to study how the density profiles of relaxed LCDM 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, $d\log ρ/d\log r \propto r^α$, 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 alpha ~ 0.16 for present-day galaxy halos to alpha~ 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 >=3e11Msun). Furthermore, in our biggest simulation, the average concentration of the most massive}, relaxed halos is constant at c200 ~3.5 to 4 for 0<=z<= 3. These results have important implications for currently planned surveys of distant clusters.

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