arXiv subjects

Julio F. Navarro

Publications and source records attributed to Julio F. Navarro.

228 records · Page 13Linked to original sources

arXiv1997

A Universal Density Profile from Hierarchical Clustering

We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of halo mass, of initial density fluctuation spectrum, and of the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fit over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation which reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results to previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, which allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.

astro-ph↗

arXiv1996

Cosmological Constraints from Rotation Curves of Disk Galaxies

High-resolution N--body simulations show a remarkable similarity in the structure of dark matter halos formed through dissipationless hierarchical clustering. Independent of halo mass, power spectrum, and cosmological parameters, the density profiles of dark halos can be accurately fit by scaling a simple two-parameter profile. The two parameters of the fit, which can be expressed as the mass and characteristic density of each halo, are strongly correlated. The characteristic density is directly proportional to the mean density of the universe at the time of assembly of each halo. Low mass halos have higher characteristic densities, reflecting their higher collapse redshift. Using this halo structure to fit the rotation curves of a large sample of disk galaxies, we show that halos formed in the standard cold dark matter (CDM) scenario are too centrally concentrated to be consistent with the rotation curves of low-surface brightness galaxies. Less concentrated halos, such as those formed in a low-density, flat CDM universe ($Ω_0=0.3$, $Λ=0.7$), are favored by these observations. Our fits also indicate that the dark halo is important at all radii, and that its total mass is strongly correlated with the luminosity of the galaxy. This model provides a natural explanation for the disk-halo conspiracy and for the lack of dependence of the Tully-Fisher relation on galaxy surface brightness.

astro-ph↗

arXiv1996

The Cores of Dwarf Galaxy Halos

We use N-body simulations to examine the effects of mass outflows on the density profiles of cold dark matter (CDM) halos surrounding dwarf galaxies. In particular, we investigate the consequences of supernova-driven winds that expel a large fraction of the baryonic component from a dwarf galaxy disk after a vigorous episode of star formation. We show that this sudden loss of mass leads to the formation of a core in the dark matter density profile, although the original halo is modeled by a coreless (Hernquist) profile. The core radius thus created is a sensitive function of the mass and radius of the baryonic disk being blown up. The loss of a disk with mass and size consistent with primordial nucleosynthesis constraints and angular momentum considerations imprints a core radius which is only a small fraction of the original scale-length of the halo. These small perturbations are, however, enough to reconcile the rotation curves of dwarf irregulars with the density profiles of haloes formed in the standard CDM scenario.

astro-ph↗

arXiv1996

Cluster correlation functions in N-body simulations

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

astro-ph↗

arXiv1995

Simulations of Galaxy Formation in Hierarchically Clustering Universes

I review the results of recent N--body/hydrodynamical simulations of the formation of galaxies in hierarchical universes, with special emphasis on the structure of dark matter halos, as well as on the mass and angular momentum of gaseous disks that assemble at the center of these halos.

astro-ph↗

arXiv1995

The Structure of CDM Halos

High resolution N-body simulations show that the density profiles of dark matter halos formed in the standard CDM cosmogony can be fit accurately by scaling a simple ``universal'' profile. Regardless of their mass, halos are nearly isothermal over a large range in radius, but significantly shallower than $r^{-2}$ near the center and steeper than $r^{-2}$ in the outer regions. The characteristic overdensity of a halo correlates strongly with halo mass in a manner consistent with the mass dependence of the epoch of halo formation. Matching the shape of the rotation curves of disk galaxies with this halo structure requires (i) disk mass-to-light ratios to increase systematically with luminosity, (ii) halo circular velocities to be systematically lower than the disk rotation speed, and (iii) that the masses of halos surrounding bright galaxies depend only weakly on galaxy luminosity. This offers an attractive explanation for the puzzling lack of correlation between luminosity and dynamics in observed samples of binary galaxies and of satellite companions of bright spiral galaxies, suggesting that the structure of dark matter halos surrounding bright spirals is similar to that of cold dark matter halos.

astro-ph↗

arXiv1995

Mass Estimates of X-Ray Clusters

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

astro-ph↗

arXiv1995

The Structure of Cold Dark Matter Halos

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

astro-ph↗

arXiv1994

Galaxy Formation in a Variety of Hierarchical Models

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

astro-ph↗

arXiv1994

The Assembly of Galaxies in a Hierarchically Clustering Universe

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

astro-ph↗

arXiv1994

Simulations of X-ray Clusters

We present simulations of the formation and evolution of galaxy clusters in the Cold Dark Matter cosmogony. Clusters with a wide range of mass were selected from previous N-body models, and were resimulated at higher resolution using a combined N-body/Smooth Particle Hydrodynamics code. The effects of radiative cooling on the gas are neglected. While many present-day clusters are predicted to be undergoing mergers, the density profiles of those that are approximately in equilibrium are all very similar, both for the gas and for the dark matter. These profiles show no sign of a uniform density core and steepen gradually from the centre outwards. The standard $β$-model is a reasonable fit over most of the radius range observable in real clusters. However, the value obtained for the slope parameter $β_f$ increases with the outermost radius of the fit. Temperature profiles of different simulated clusters are also similar. Typically the temperature is almost uniform in the regions which emit most of the X-ray flux but drops at larger radii. The gas temperature and dark matter velocity dispersion in equilibrium clusters give values of $β_T\equiv μm_pσ_{DM}^2/kT$ which are consistent with unity provided an X-ray emission-weighted temperature is used. Larger values of $β_T$ are found in merging objects where there is a transient boost in the velocity dispersion of the system. Thus $β_T >1$ may be an observational indicator of merging in real clusters. The similar structure of clusters of differing mass results in scaling relations between the X-ray and dynamical properties of clusters identified at

astro-ph↗

arXiv1994

A Recipe for Galaxy Formation Shaun Cole

We present a detailed prescription for how galaxy formation can be modelled in hierarchical theories of structure formation. Our model incorporates the formation and merging of dark matter halos, the shock heating and radiative cooling of baryonic gas gravitationally confined in these halos, the formation of stars regulated by the energy released by evolving stars and supernovae, the merging of galaxies within dark matter halos, and the spectral evolution of the stellar populations that are formed. The procedure that we describe is very flexible and can be applied to any hierarchical clustering theory. We explore the effects of varying the stellar initial mass function, star formation rates and galaxy merging. The results we compare with an extensive range of observational data, including the B and K galaxy luminosity functions, galaxy colours, the Tully-Fisher relation and galaxy number counts.These data strongly constrain the models and enable the relative importance of each of the physical process to be assessed. We present a broadly successful model defined by a plausible choice of parameters. This fiducial model produces a much more acceptable luminosity function than most previous studies. This is achieved through a modest rate of galaxy mergers and strong suppression of star formation in halos of low circular velocity. However, it fails to produce galaxies as red as many observed ellipticals and, compared with the observed Tully-Fisher relation, the model galaxies have circular velocities which are too large. ** uuencoded compressed postscript file containing all text and figures.**

astro-ph↗