Showing papers by "Julio F. Navarro published in 2010"

The diversity and similarity of simulated cold dark matter haloes

Abstract: We study the structure ofCDM halos using a suite of N-body simulations of un- precedented numerical resolution. The Aquarius Project follows the formation of 6 different galaxy-sized halos simulated several times at varying numerical resolution, allowing numerical convergence to be assessed directly. The highest resolution sim- ulation represents a single dark matter halo using 4.4 billion particles, of which 1.1 billion end up within the virial radius. Our analysis confirms that the mass pro- file ofCDM halos deviates slightly but systematically from the form proposed by Navarro, Frenk & White. The spherically-averaged density profile becomes progres- sively shallower inwards and, at the innermost resolved radius, the logarithmic slope isdln�/dlnr < � 1, convincingly ruling out recent claims of a steep � / r 1.2 central cusp. The radial dependence of is well approximated by a power-law, / r � (the Einasto profile). The shape parameter, �, varies slightly but significantly from halo to halo, implying that the mass profiles ofCDM halos are not strictly universal: different halos cannot, in general, be rescaled to look exactly alike. 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, �/� 3 / r 1.875 . The index here is identical to that of Bertschinger's similarity solution for self-similar infall onto a point mass from an otherwise uniform Einstein-de Sitter Universe. The origin of this striking behaviour is unclear, but its robustness suggests that it reflects a fundamental structural property ofCDM halos. 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.

Galactic stellar haloes in the CDM model

Abstract: We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high-resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function (LF) and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1 < z < 7 from less than five significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (10^(7)–10^(8) M_⊙). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (~10^(8)–10^(9) M_⊙) and density/surface-brightness profiles of the stellar haloes (from 10 to 100 kpc) are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (<10 kpc) of our haloes are highly prolate (c/a ~ 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations.

Galaxy-induced transformation of dark matter haloes

Abstract: We use N-body/gasdynamical cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its surrounding dark matter halo. Two series of simulations are compared; one that follows only the evolution of the dark matter com- ponent of individual haloes in the propercold dark matter (� CDM) cosmological context, and a second series where a baryonic component is added and followed hydrodynamically. The simulations with baryons include radiative cooling but neglect the formation of stars and their feedback. The efficient, unimpeded cooling that results leads most baryons to collect at the halo centre in a centrifugally supported disc which, due to angular momentum losses, is too small and too massive when compared with typical spiral galaxies. This admittedly unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their surrounding dark haloes. We find, in agreement with earlier work, that the shape of the halo becomes more axisymmetric: post galaxy assembly, haloes are trans- formed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (� c/a �∼ 0.85). Haloes always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the traditional 'adiabatic-contraction' hypothesis. The reduced contraction helps to reconcileCDM haloes with constraints on the dark matter content inside the solar circle and should alleviate the longstanding difficulty of matching simultaneously the scaling properties of galaxy discs and the galaxy luminosity function. The halo contraction we report is also less pronounced than found in earlier simulations, a disagreement which 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 centre of a halo matters, but also the mode of its deposition. Although simple formulae might work in particular cases where galaxies form nearly adiabatically, in general it might prove impossible to predict the halo response to galaxy formation without a detailed understanding of a galaxy's detailed assembly history.

Feedback and the structure of simulated galaxies at redshift z=2

Abstract: 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 active galactic nucleus (AGN)-driven outflows. These different feedback models result in large variations in the abundance and structural properties of bright galaxies at z = 2. In agreement with earlier work, models with inefficient or no feedback lead to the formation of massive compact galaxies collecting a large fraction (upwards of 50 per cent) of all available baryons in each halo. Increasing the efficiency of feedback reduces the baryonic mass and increases the size of simulated galaxies. A model that includes supernova-driven gas outflows aided by the energetic output of AGNs reduces galaxy masses by roughly a factor of similar to 10 compared with the no-feedback case. Other models give results that straddle these two extremes. Despite the large differences in galaxy formation efficiency, the net specific angular momentum of a galaxy is, on average, roughly half that of its surrounding halo, independent of halo mass (in the range probed) and of the feedback scheme. Feedback thus affects the baryonic mass of a galaxy much more severely than its spin. 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 discs are common in moderate-feedback runs, making up typically similar to 50 per cent of all galaxies at the centres of haloes with virial mass exceeding similar to 1011 M-circle dot. The size, stellar masses and circular speeds of simulated galaxies formed in such runs have properties in between those of large star-forming discs 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.

Assembly History and Structure of Galactic Cold Dark Matter Halos

Abstract: We use the Aquarius simulation series to study the imprint of assembly history on the structure of Galaxy-mass cold dark matter halos. Our results confirm earlier work regarding the influence of mergers on the mass density profile and the inside-out growth of halos. The inner regions that contain the visible galaxies are stable since early times and are significantly affected only by major mergers. Particles accreted diffusely or in minor mergers are found predominantly in the outskirts of halos. Our analysis reveals trends that run counter to current perceptions of hierarchical halo assembly. For example, major mergers (i.e. those with progenitor mass ratios greater than 1:10) contribute little to the total mass growth of a halo, on average less than 20 per cent for our six Aquarius halos. The bulk is contributed roughly equally by minor mergers and by "diffuse" material which is not resolved into individual objects. This is consistent with modeling based on excursion-set theory which suggests that about half of this diffuse material should not be part of a halo of any scale. Interestingly, the simulations themselves suggest that a significantly fraction is not truly diffuse, since it was ejected from earlier halos by mergers prior to their joining the main system. The Aquarius simulations resolve halos to much lower mass scales than are expected to retain gas or form stars. These results thus confirm that most of the baryons from which visible galaxies form are accreted diffusely, rather than through mergers, and they suggest that only relatively rare major mergers will affect galaxy structure at later times.

Distance determination for RAVE stars using stellar models

Abstract: Aims. We develop a method for deriving distances from spectroscopic data and obtaining full 6D phase-space coordinates for the RAVE survey’s second data release. Methods. We used stellar models combined with atmospheric properties from RAVE (effective temperature, surface gravity and metallicity) and (J − Ks) photometry from archival sources to derive absolute magnitudes. In combination with apparent magnitudes, sky coordinates, proper motions from a variety of sources and radial velocities from RAVE, we are able to derive the full 6D phasespace coordinates for a large sample of RAVE stars. This method is tested with artificial data, Hipparcos trigonometric parallaxes and observations of the open cluster M 67. Results. When we applied our method to a set of 16 146 stars, we found that 25% (4037) of the stars have relative (statistical) distance errors of 4), 25% (1744) have relative distance errors <31%, while 50% (3488) and 75% (5231) have relative errors smaller than 36% and 42%, respectively. Our full dataset shows the expected decrease in the metallicity of stars as a function of distance from the Galactic plane. The known kinematic substructures in the U and V velocity components of nearby dwarf stars are apparent in our dataset, confirming the accuracy of our data and the reliability of our technique. We provide independent measurements of the orientation of the UV velocity ellipsoid and of the solar motion, and they are in very good agreement with previous work. Conclusions. The distance catalogue for the RAVE second data release is available at http://www.astro.rug.nl/~rave, and will be updated in the future to include new data releases.

Secondary infall and the pseudo-phase-space density profiles of cold dark matter haloes

Abstract: We use N-body simulations to investigate the radial dependence of the density, ρ, and velocity dispersion, σ, in cold dark matter (CDM) haloes. In particular, we explore how closely Q ≡ ρ/σ 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 s i (r') profiles follow closely a power law near the centre, 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 ∝ 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 haloes beyond their characteristic scale radius.

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

Abstract: 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.

A search for new members of the beta Pic, Tuc-Hor and epsilon Cha moving groups in the RAVE database

Abstract: We report on the discovery of new members of nearby young moving groups, exploiting the full power of combining the RAVE survey with several stellar age diagnostic methods and follow-up high-resolution optical spectroscopy. The results include the identification of one new and five likely members of the beta Pictoris moving group, ranging from spectral types F9 to M4 with the majority being M dwarfs, one K7 likely member of the epsilon Cha group and two stars in the Tuc-Hor association. Based on the positive identifications we foreshadow a great potential of the RAVE database in progressing toward a full census of young moving groups in the solar neighbourhood.

The Dawning of the Stream of Aquarius in RAVE

Abstract: 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 Besan\c{c}on 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.

The earliest stars and their relics in the Milky Way

Abstract: We have implemented a simple model to identify the likely sites of the first stars and galaxies in the high-resolution simulations of the formation of galactic dark matter haloes of the Aquarius Project. The first star in a galaxy like the Milky Way formed around redshift z = 35; by z = 10, the young galaxy contained up to ∼3 x 10 4 dark matter haloes capable of forming stars by molecular hydrogen cooling. These minihaloes were strongly clustered, and feedback may have severely limited the actual number of Population III stars that formed. By the present day, the remnants of the first stars would be strongly concentrated to the centre of the main halo. If a second generation of long-lived stars formed near the first (the first star relics), we would expect to find half of them within 30 h -1 kpc of the Galactic Centre and a significant fraction in satellites where they may be singled out by their anomalous metallicity patterns. The first halo in which gas could cool by atomic hydrogen line radiation formed at z = 25; by z = 10, the number of such 'first galaxies' had increased to ∼300. Feedback might have decreased the number of first galaxies at the time when they undergo significant star formation, but not the number that survive to the present because near neighbours merge. Half of all the 'first galaxies' that formed before z = 10 merge with the main halo before z ∼ 3 and most lose a significant fraction of their mass. However, today there should still be more than 20 remnants orbiting within the central ∼30 h -1 kpc of the Milky Way. These satellites have circular velocities of a few kilometres per second or more, comparable to those of known Milky Way dwarfs. They are a promising hunting ground for the remnants of the earliest epoch of star formation.

Double-lined spectroscopic binary stars in the rave survey

Abstract: We devise a new method for the detection of double-lined binary stars in a sample of the Radial Velocity Experiment (RAVE) survey spectra The method is both tested against extensive simulations based on synthetic spectra and compared to direct visual inspection of all RAVE spectra It is based on the properties and shape of the cross-correlation function, and is able to recover ~80% of all binaries with an orbital period of order 1 day Systems with periods up to 1 yr are still within the detection reach We have applied the method to 25,850 spectra of the RAVE second data release and found 123 double-lined binary candidates, only eight of which are already marked as binaries in the SIMBAD database Among the candidates, there are seven that show spectral features consistent with the RS CVn type (solar type with active chromosphere) and seven that might be of W UMa type (over-contact binaries) One star, HD 101167, seems to be a triple system composed of three nearly identical G-type dwarfs The tested classification method could also be applicable to the data of the upcoming Gaia mission

Double-lined Spectroscopic Binary Stars in the Radial Velocity Experiment Survey

Abstract: We devise a new method for the detection of double-lined binary stars in a sample of the Radial Velocity Experiment (RAVE) survey spectra. The method is both tested against extensive simulations based on synthetic spectra, and compared to direct visual inspection of all RAVE spectra. It is based on the properties and shape of the cross-correlation function, and is able to recover ~80% of all binaries with an orbital period of order 1 day. Systems with periods up to 1 year are still within the detection reach. We have applied the method to 25,850 spectra of the RAVE second data release and found 123 double-lined binary candidates, only eight of which are already marked as binaries in the SIMBAD database. Among the candidates, there are seven that show spectral features consistent with the RS CVn type (solar type with active chromosphere) and seven that might be of W UMa type (over-contact binaries). One star, HD 101167, seems to be a triple system composed of three nearly identical G-type dwarfs. The tested classification method could also be applicable to the data of the upcoming Gaia mission.

The RAVE Survey: Rich in Very Metal-poor Stars

Abstract: Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H]

Galactic Stellar Haloes in the CDM Model

Abstract: We present new high‐resolution models of the structure and composition of diffuse stellar haloes surrounding galaxies residing in Milky Way‐like dark haloes. We combine the Aquarius suite of high resolution halo simulations with the semi‐analytic model GALFORM. Our technique is fast and highly adaptable. It incorporates the full cosmological context and realistic potential evolution required for direct comparison with observations. We are using these models to examine the assembly of the Milky Way’s stellar halo, its relationship to the surviving dwarf satellite population, and the kinematics and chemical composition of halo substructure.

The RAVE Survey: Rich in Very Metal-Poor Stars

Abstract: Very metal-poor stars are of obvious importance for many problems in chemical evolution, star formation, and galaxy evolution. Finding complete samples of such stars which are also bright enough to allow high-precision individual analyses is of considerable interest. We demonstrate here that stars with iron abundances [Fe/H] < -2 dex, and down to below -4 dex, can be efficiently identified within the Radial Velocity Experiment (RAVE) survey of bright stars, without requiring additional confirmatory observations. We determine a calibration of the equivalent width of the Calcium triplet lines measured from the RAVE spectra onto true [Fe/H], using high spectral resolution data for a subset of the stars. These RAVE iron abundances are accurate enough to obviate the need for confirmatory higher-resolution spectroscopy. Our initial study has identified 631 stars with [Fe/H] <= -2, from a RAVE database containing approximately 200,000 stars. This RAVE-based sample is complete for stars with [Fe/H] < -2.5, allowing statistical sample analysis. We identify three stars with [Fe/H] <= -4. Of these, one was already known to be `ultra metal-poor', one is a known carbon-enhanced metal-poor star, but we obtain [Fe/H]= -4.0, rather than the published [Fe/H]=-3.3, and derive [C/Fe] = +0.9, and [N/Fe] = +3.2, and the third is at the limit of our S/N. RAVE observations are on-going and should prove to be a rich source of bright, easily studied, very metal-poor stars.

Distance determination for RAVE stars using stellar models

Abstract: (Abridged) Aims:We develop a method for deriving distances from spectroscopic data and obtaining full 6D phase-space coordinates for the RAVE survey's second data release. Methods: We used stellar models combined with atmospheric properties from RAVE (Teff, logg and [Fe/H]) and (J-Ks) photometry from archival sources to derive absolute magnitudes. We are able to derive the full 6D phase-space coordinates for a large sample of RAVE stars. This method is tested with artificial data, Hipparcos trigonometric parallaxes and observations of the open cluster M67. Results: When we applied our method to a set of 16 146 stars, we found that 25% (4 037) of the stars have relative (statistical) distance errors of 4), 25% (1 744) have relative distance errors < 31%, while 50% (3 488) and 75% (5 231) have relative errors smaller than 36% and 42%, respectively. Our full dataset shows the expected decrease in the metallicity of stars as a function of distance from the Galactic plane. The known kinematic substructures in the U and V velocity components of nearby dwarf stars are apparent in our dataset, confirming the accuracy of our data and the reliability of our technique. We provide independent measurements of the orientation of the UV velocity ellipsoid and of the solar motion, and they are in very good agreement with previous work. Conclusions: The distance catalogue for the RAVE second data release is available at this http URL

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

Abstract: 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.