Eduard Salvador-Sole

The origin of H I-deficiency in galaxies on the outskirts of the Virgo cluster. I. How far can galaxies bounce out of clusters?

Spiral galaxies that are deficient in neutral hydrogen are observed on the outskirts of the Virgo cluster. If their orbits have crossed the inner parts of the cluster, their interstellar gas may have been lost through ram pressure stripping by the hot X-ray emitting gas of the cluster. We estimate the maximum radius out to which galaxies can bounce out of a virialized system using analytical arguments and cosmological N-body simulations. In particular, we derive an expression for the turnaround radius in a flat cosmology with a cosmological constant that is simpler than previously derived expressions. We find that the maximum radius reached by infalling galaxies as they bounce out of their cluster is roughly between 1 and 2.5 virial radii. Comparing to the virial radius of the Virgo cluster, which we estimate from X-ray observations, these H I-deficient galaxies appear to lie significantly further away from the cluster center. Therefore, if their distances to the cluster core are correct, the H I-deficient spiral galaxies found outside of the Virgo cluster cannot have lost their gas by ram pressure from the hot intracluster gas.

Merger versus Accretion and the Structure of Dark Matter Halos

High-resolution N-body simulations of hierarchical clustering in a wide variety of cosmogonies show that the density profiles of dark matter halos are universal, with low-mass halos being denser than their more massive counterparts. This mass-density correlation is interpreted as reflecting the earlier typical formation time of less massive objects. We investigate this hypothesis in the light of formation times defined as the epoch at which halos experience their last major merger. Such halo formation times are calculated by means of a modification of the extended Press & Schechter formalism that includes a phenomenological frontier, Δm, between tiny and notable relative mass captures leading to the distinction between merger and accretion. For Δm ~ 0.6, we confirm that the characteristic density of halos is essentially proportional to the mean density of the universe at their time of formation. Yet, proportionality with respect to the critical density yields slightly better results for open universes. In addition, we find that the scale radius of halos is also essentially proportional to their virial radius at the time of formation. We show that these two relations are consistent with the following simple scenario. Violent relaxation caused by mergers rearranges the structure of halos leading to the same density profile with universal values of the dimensionless characteristic density and scale radius. Between mergers, halos grow gradually through the accretion of surrounding layers by keeping their central parts steady and expanding their virial radius as the critical density of the universe diminishes.

On the Origin of the Inner Structure of Halos

We calculate by means of the Press-Schechter formalism the density profile developed by dark matter halos during accretion, i.e., the continuous aggregation of small clumps. We find that the shape of the predicted profile is similar to that shown by halos in high-resolution cosmological simulations. Furthermore, the mass-concentration relation is correctly reproduced at any redshift in all the hierarchical cosmologies analyzed except for very large halo masses. The role of major mergers, which can cause the rearrangement of the halo structure through violent relaxation, is also investigated. We show that, as a result of the boundary conditions imposed by the matter continuously infalling into the halo during the violent relaxation process, the shape of the density profile emerging from major mergers is essentially identical to the shape the halo would have developed through pure accretion. This result explains why, according to high-resolution cosmological simulations, relaxed halos of a given mass have the same density profile regardless of whether they have had a recent merger and why both spherical infall and hierarchical assembly lead to very similar density profiles. Finally, we demonstrate that the density profile of relaxed halos is not affected by the capture of clumps of intermediate mass either.

The Nature of Dark Matter and the Density Profile and Central Behavior of Relaxed Halos

We show that the two basic assumptions of the model recently proposed by Manrique and coworkers for the universal density profile of cold dark matter (CDM) halos, namely, that these objects grow inside out during periods of smooth accretion and that their mass profile and its radial derivatives are all continuous functions, are both well understood in terms of the very nature of CDM. Those two assumptions allow one to derive the typical density profile of halos of a given mass from the accretion rate characteristic of the particular cosmology. This profile was shown by Manrique and coworkers to recover the results of numerical simulations. In the present paper, we investigate its behavior beyond the ranges covered by present-day N-body simulations. We find that the central asymptotic logarithmic slope depends crucially on the shape of the power spectrum of density perturbations: it is equal to a constant negative value for power-law spectra and has central cores for the standard CDM power spectrum. The predicted density profile in the CDM case is well fitted by the 3D Sersic profile over at least 10 decades in halo mass. The values of the Sersic parameters depend on the mass of the structure considered. A practical procedure is provided that allows one to infer the typical values of the best NFW or Sersic fitting law parameters for halos of any mass and redshift in any given standard CDM cosmology.

Galaxy And Mass Assembly (GAMA): refining the local galaxy merger rate using morphological information

We use the Galaxy And Mass Assembly (GAMA) survey to measure the local Universe mass-dependent merger fraction and merger rate using galaxy pairs and the CAS (concentration, asymmetry, and smoothness) structural method, which identifies highly asymmetric merger candidate galaxies. Our goals are to determine which types of mergers produce highly asymmetrical galaxies and to provide a new measurement of the local galaxy major merger rate. We examine galaxy pairs at stellar mass limits down to M* = 108 M⊙ with mass ratios of 4:1) the lower mass companion becomes highly asymmetric, whereas the larger galaxy is much less affected. The fraction of highly asymmetric paired galaxies which have a major merger companion is highest for the most massive galaxies and drops progressively with decreasing mass. We calculate that the mass-dependent major merger fraction is fairly constant at ∼1.3–2 per cent within 109.5 < M* < 1011.5 M⊙, and increases to ∼4 per cent at lower masses. When the observability time-scales are taken into consideration, the major merger rate is found to approximately triple over the mass range we consider. The total comoving volume major merger rate over the range 108.0 < M* < 1011.5 M⊙ is (1.2 ± 0.5) × 10−3 h370 Mpc−3 Gyr−1.

THE DYNAMICAL SURVIVAL OF SMALL-SCALE SUBSTRUCTURE IN RELAXED GALAXY CLUSTERS

We consider the dynamical evolution of small-scale substructure in clusters within two extreme alternate scenarios for their possible origin: 1) the accretion of groups (or small clusters) on quasi-radial orbits, and 2) the merger of clusters of similar masses, followed by the decoupling of their dense cores. Using simple analytical arguments and checking with numerical computations, we show that objects are destroyed by the tidal field of the global cluster potential if their mean density is small compared to the mean cluster density within the radius of closest approach of the group or detached core. Accreted groups and small clusters are thus tidally disrupted in one cluster crossing. Since the cores of clusters are much denser than groups, they are considerably more robust to tides, but the least massive are destroyed or severely stripped by tides, while the others are brought to the cluster center by dynamical friction (and subsequently merge) in less than one orbit. The longest lived substructures are detached cores, roughly ten times less massive than the cluster, starting in near-circular orbits beyond

Major mergers of haloes, the growth of massive black holes and the evolving luminosity function of quasars

1 \, h^{-1} \, \rm Mpc

Scaling Evolution of Universal Dark Matter Halo Density Profiles

from the cluster center.

Scale radii and aggregation histories of dark haloes

We construct a physically motivated analytical model for the quasar luminosity function, based on the joint star formation and feeding of massive black holes suggested by the observed correlation between the black hole mass and the stellar mass of the hosting spheroids. The parallel growth of massive black holes and host galaxies is assumed to be triggered by major mergers of haloes. The halo major merger rate is computed within the framework of the extended Press-Schechter model. The evolution of black holes on cosmological time-scales is achieved by the integration of the governing set of differential equations, established from a few reasonable assumptions that account for the distinct (Eddington-limited or supply-limited) accretion regimes. Finally, the typical light curves of the reactivated quasars are obtained under the assumption that, in such accretion episodes, the fall of matter on to the black hole is achieved in a self-regulated stationary way. The predicted quasar luminosity function is compared with the luminosity functions of the 2dF quasi-stellar object sample and other, higher-redshift data. We find good agreement in all cases, except for z < 1 where the basic assumption of our model is likely to break down.

The Effects of the Peak-Peak Correlation on the Peak Model of Hierarchical Clustering

Dark matter halos show a universal density profile with a scaling such that less massive systems are typically denser. This mass-density relation is well described by a proportionality between the characteristic density of halos and the mean cosmic density at halo formation time. It has recently been shown that this proportionality could be the result of the following simple evolutionary picture. Halos form in major mergers with essentially the same, cosmogony-dependent, dimensionless profile and then grow inside out as a consequence of accretion. Here we verify the consistency of this picture and show that it predicts the correct zero point of the mass-density relation.