Showing papers by "Julio F. Navarro published in 2012"
TL;DR: The major morphological features of a galaxy are thought to be determined by the assembly history and net spin of its surrounding dark halo as mentioned in this paper, which is very poorly correlated with their halo properties: discs form in haloes with high and low net spin, and mergers play a negligible role in the formation of spheroids.
Abstract: The major morphological features of a galaxy are thought to be determined by the assembly history and net spin of its surrounding dark halo. In the simplest scenario, disc galaxies form predominantly in haloes with high angular momentum and quiet recent assembly history, whereas spheroids are the slowly rotating remnants of repeated merging events. We explore these assumptions using 100 systems with halo masses similar to that of the Milky Way, identified in a series of cosmological gasdynamical simulations: the Galaxies–Intergalactic Medium Interaction Calculation (GIMIC). At z=0, the simulated galaxies exhibit a wide variety of morphologies, from dispersion-dominated spheroids to pure disc galaxies. Surprisingly, these morphological features are very poorly correlated with their halo properties: discs form in haloes with high and low net spin, and mergers play a negligible role in the formation of spheroids, whose stars form primarily in situ. With hindsight, this weak correlation between halo and galaxy properties is unsurprising given that a minority of the available baryons
(∼40 per cent) end up in galaxies.More important to morphology is the coherent alignment of the angular momentum of baryons that accrete over time to form a galaxy. Spheroids tend to form when the spin of newly accreted gas is misaligned with that of the extant galaxy, leading to the episodic formation of stars with different kinematics that cancel out the net rotation of the system. Discs, on the other hand, form out of gas that flows in with similar angular momentum to that of earlier accreted material. Gas accretion from a hot corona thus favours disc formation, whereas gas that flows ‘cold’, often along separate, misaligned filaments, favours the formation of spheroids. In this scenario, many spheroids consist of the superposition of stellar components with distinct kinematics, age and metallicity, an arrangement that might survive to the present day given the paucity of major mergers. Since angular momentum is acquired largely at turnaround, morphology depends on the early interplay between the tidal field and the shape of the material destined to form a galaxy.
247 citations
TL;DR: The Phoenix Project as discussed by the authors is a set of Λ cold dark matter (CDM) simulations of the dark matter component of nine rich galaxy clusters, each cluster is simulated at least at two different numerical resolutions.
Abstract: We introduce the Phoenix Project, a set of Λ cold dark matter (CDM) simulations of the dark matter component of nine rich galaxy clusters. Each cluster is simulated at least at two different numerical resolutions. For eight of them, the highest resolution corresponds to ∼130 million particles within the virial radius, while for one this number is over one billion. We study the structure and substructure of these systems and contrast them with six galaxy-sized dark matter haloes from the Aquarius Project, simulated at comparable resolution. This comparison highlights the approximate mass invariance of CDM halo structure and substructure. We find little difference in the spherically averaged mass, pseudo-phase-space density and velocity anisotropy profiles of Aquarius and Phoenix haloes. When scaled to the virial properties of the host halo, the abundance and radial distribution of subhaloes are also very similar, despite the fact that Aquarius and Phoenix haloes differ by roughly three decades in virial mass. The most notable difference is that cluster haloes have been assembled more recently and are thus significantly less relaxed than galaxy haloes, which leads to decreased regularity, increased halo-to-halo scatter and sizable deviations from the mean trends. This accentuates the effects of the strong asphericity of individual clusters on surface density profiles, which may vary by up to a factor of 3 at a given radius, depending on projection. The high apparent concentration reported for some strong-lensing clusters might very well reflect these effects. A more recent assembly also explains why substructure in some Phoenix haloes is slightly more abundant than in Aquarius, especially in the inner regions. Resolved subhaloes nevertheless contribute only 11 ± 3 per cent of the virial mass in Phoenix clusters. Together, the Phoenix and Aquarius simulation series provide a detailed and comprehensive prediction of the CDM distribution in galaxies and clusters when the effects of baryons can be neglected.
199 citations
TL;DR: In this article, the authors used rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints, and they found that almost one-half of the dwarfs in their sample with stellar mass in the range of 106 < Mgal/M⊙ < 107 are at odds with this restriction: either they live in haloes with masses substantially below 1010 M ⊙ or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs.
Abstract: The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in haloes less massive than ∼1010 M⊙. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one-half of the dwarfs in our sample with stellar mass in the range of 106 < Mgal/M⊙ < 107 are at odds with this restriction: either they live in haloes with masses substantially below 1010 M⊙ or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard Λ cold dark matter scenario. Extending galaxy formation to haloes well below 1010 M⊙ would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.
137 citations
TL;DR: In this article, the authors use the Millennium Simulation series to study how the dynamical state of dark matter haloes affects the relation between mass and concentration and find that a large fraction of massive systems are identified when they are substantially out of equilibrium and in a particular phase of their dynamical evolution: the more massive the halo, the more likely it is found at a transient stage of high concentration.
Abstract: We use the Millennium Simulation series to study how the dynamical state of dark matter haloes affects the relation between mass and concentration. We find that a large fraction of massive systems are identified when they are substantially out of equilibrium and in a particular phase of their dynamical evolution: the more massive the halo, the more likely it is found at a transient stage of high concentration. This state reflects the recent assembly of massive haloes and corresponds to the first pericentric passage of recently accreted material when, before virialization, the kinetic and potential energies reach maximum and minimum values, respectively. This result explains the puzzling upturn in the mass-concentration relation reported in recent work for massive haloes; indeed, the upturn disappears when only dynamically relaxed systems are considered in the analysis. Our results warn against applying simple equilibrium models to describe the structure of rare, massive galaxy clusters and urge caution when extrapolating scaling laws calibrated on lower mass systems, where such deviations from equilibrium are less common. The evolving dynamical state of galaxy clusters ought to be carefully taken into account if cluster studies are to provide precise cosmological constraints.
91 citations
TL;DR: In this paper, a cosmological simulation of the formation of the Local Group of Galaxies is used to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization.
Abstract: We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram-pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This "cosmic web stripping" may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in \Lambda CDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.
88 citations
University of Ljubljana1, University of Strasbourg2, University of Sydney3, Heidelberg University4, Australian National University5, University of Central Lancashire6, University of Cambridge7, King Abdulaziz University8, University of Padua9, University of Victoria10, ASTRON11, Macquarie University12, University College London13, Leibniz Association14, Johns Hopkins University15
TL;DR: In this paper, a morphological classification of ~350, 000 RAVE survey stellar spectra using locally linear embedding is presented, a dimensionality reduction method that enables representing the complex spectral morphology in a low-dimensional projected space while still preserving the properties of local neighborhoods of spectra.
Abstract: The RAdial Velocity Experiment (RAVE) is a medium-resolution (R ~ 7500) spectroscopic survey of the Milky Way that has already obtained over half a million stellar spectra. They present a randomly selected magnitude-limited sample, so it is important to use a reliable and automated classification scheme that identifies normal single stars and discovers different types of peculiar stars. To this end, we present a morphological classification of ~350, 000 RAVE survey stellar spectra using locally linear embedding, a dimensionality reduction method that enables representing the complex spectral morphology in a low-dimensional projected space while still preserving the properties of the local neighborhoods of spectra. We find that the majority of all spectra in the database (~ 90%-95%) belong to normal single stars, but there is also a significant population of several types of peculiars. Among them, the most populated groups are those of various types of spectroscopic binary and chromospherically active stars. Both of them include several thousands of spectra. Particularly the latter group offers significant further investigation opportunities since activity of stars is a known proxy of stellar ages. Applying the same classification procedure to the sample of normal single stars alone shows that the shape of the projected manifold in two-dimensional space correlates with stellar temperature, surface gravity, and metallicity.
59 citations
TL;DR: In this article, the color-magnitude diagram of! Centauri was investigated and it was shown 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.
Abstract: We have investigated the color-magnitude diagram of ! Centauri and nd 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] 0:7) appear to require both a high age ( 13 Gyr) and enhanced CNO abundances ([CNO/Fe] 0:9). Y 0:35 must also be assumed in order to counteract the eects of high CNO on turno colors, and thereby to obtain a good t to the relatively blue turno of this stellar population. This suggest a short chemical evolution period of time ( < 1 Gyr) for ! Cen. Our intermediate-mass (super-)AGB models are able to reproduce the high helium abundances, along with [N/Fe] 2 and substantial O depletions if uncertainties in the treatment of convection are fully taken into account. These abundance features distinguish the bMS stars from the dominant [Fe/H] 1:7 population. The most massive super-AGB stellar models (MZAMS 6:8 M , MHe;core 1:245 M ) predict too large N-enhancements, which limits their role in contributing to the extreme populations. In order to address the observed central concentration of stars with He-rich abundance we show here quantitatively that highly He- and N-enriched AGB ejecta have particularly ecient cooling properties. Based on these results and on the reconstruction of the orbit of ! Cen with respect to the Milky Way we propose the galactic plane passage gas purging scenario for the chemical evolution of this cluster. The bMS population formed shortly after the purging of most of the cluster gas as a result of the passage of ! Cen through the Galactic disk (which occurs today every 40Myrs for ! Cen) when the initial-mass function of the dominant population had \burned" through most of the Type II supernova mass range. AGB stars would eject most of their masses into the gas-depleted cluster through low-velocity winds that sink to the cluster core due to their favorable cooling properties and form the bMS population. In our discussion we follow our model 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. Subject headings: globular clusters: individual (! Cen) | Hertzsprung-Russell diagram | stars: AGB and post-AGB | stars: abundances | stars: evolution | stars: interiors
33 citations
TL;DR: In this article, the authors investigated the color-magnitude diagram of Omega Centauri and found 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.
Abstract: [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.
29 citations
TL;DR: The Phoenix Project as discussed by the authors is a set of CDM simulations of the dark matter component of nine rich galaxy clusters, each of which is simulated at least at two different numerical resolutions.
Abstract: [abridged] We introduce the Phoenix Project, a set of $\Lambda$CDM simulations of the dark matter component of nine rich galaxy clusters. Each cluster is simulated at least at two different numerical resolutions. For eight of them, the highest resolution corresponds to $\sim 130$ million particles within the virial radius, while for one this number is over one billion. We study the structure and substructure of these systems and contrast them with six galaxy-sized dark matter haloes from the Aquarius Project, simulated at comparable resolution. This comparison highlights the approximate mass invariance of CDM halo structure and substructure. We find little difference in the spherically-averaged mass, pseudo-phase-space density, and velocity anisotropy profiles of Aquarius and Phoenix haloes. When scaled to the virial properties of the host halo, the abundance and radial distribution of subhaloes are also very similar, despite the fact that Aquarius and Phoenix haloes differ by roughly three decades in virial mass. The most notable difference is that cluster haloes have been assembled more recently and are thus significantly less relaxed than galaxy haloes, which leads to decreased regularity, increased halo-to-halo scatter and sizable deviations from the mean trends. This accentuates the effects of the strong asphericity of individual clusters on surface density profiles, which may vary by up to a factor of three at a given radius, depending on projection. The high apparent concentration reported for some strong-lensing clusters might very well reflect these effects. A more recent assembly also explains why substructure in some Phoenix haloes is slightly more abundant than in Aquarius, especially in the inner regions. Resolved subhaloes nevertheless contribute only $11 \pm 3%$ of the virial mass in Phoenix clusters. .
19 citations
University of Ljubljana1, University of Strasbourg2, University of Sydney3, Heidelberg University4, Australian National University5, University of Central Lancashire6, University of Cambridge7, King Abdulaziz University8, University of Padua9, University of Victoria10, ASTRON11, Macquarie University12, University College London13, Leibniz Association14, Johns Hopkins University15
TL;DR: In this article, a morphological classification of 350,000 RAVE survey stellar spectra using locally linear embedding is presented, which enables representing the complex spectral morphology in a low dimensional projected space while still preserving the properties of the local neighborhoods of spectra.
Abstract: The RAdial Velocity Experiment (RAVE) is a medium resolution R~7500 spectroscopic survey of the Milky Way which already obtained over half a million stellar spectra. They present a randomly selected magnitude-limited sample, so it is important to use a reliable and automated classification scheme which identifies normal single stars and discovers different types of peculiar stars. To this end we present a morphological classification of 350,000 RAVE survey stellar spectra using locally linear embedding, a dimensionality reduction method which enables representing the complex spectral morphology in a low dimensional projected space while still preserving the properties of the local neighborhoods of spectra. We find that the majority of all spectra in the database ~90-95% belong to normal single stars, but there is also a significant population of several types of peculiars. Among them the most populated groups are those of various types of spectroscopic binary and chromospherically active stars. Both of them include several thousands of spectra. Particularly the latter group offers significant further investigation opportunities since activity of stars is a known proxy of stellar ages. Applying the same classification procedure to the sample of normal single stars alone shows that the shape of the projected manifold in two dimensional space correlates with stellar temperature, surface gravity and metallicity.