R. Domínguez-Tenreiro

Double starbursts triggered by mergers in hierarchical clustering scenarios

We use cosmological smooth particle hydrodynamical (SPH) simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, owing to gas inflows driven as the satellite approaches, and the second one when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems that do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. The potential wells concentrate owing to the accumulation of baryons in the central regions and of dark matter as the result of the pulling in by baryons. The coupled evolution of the dark matter and baryons would lead to an evolutionary sequence during which systems with shallower total potential wells suffer early gas inflows during the orbital decay phase that help to feed their central mass concentration, pulling in dark matter and contributing to build up more stable systems. Within this scenario, starbursts triggered by early gas inflows are more likely to occur at early stages of evolution of the systems and to be an important contributor to the formation of stellar bulges. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50 per cent of the stars (in SCDM) being formed at high z.

Disk Formation in Hierarchical Hydrodynamical Simulations:A Way Out of the Angular Momentum Catastrophe

We report results on the formation of disklike structures in two cosmological hydrodynamical simulations, which share the same initial conditions, in a hierarchical clustering scenario. In the first simulation, a simple and generic implementation of star formation has allowed galaxy-like objects with stellar bulges and extended, populated disks to form. Gas in the disk comes both from particles that survive mergers, keeping in part their angular momentum content, and from new gas supplied by infall once the merger process is over, with global specific angular momentum conservation. The stellar bulge forms from gas that has lost most of its angular momentum. In the second simulation, no star formation has been included. In this case, objects consist of an overpopulated central gas concentration and an extended, underpopulated disk. The central concentration forms from particles that suffer an important angular momentum loss in violent events, and it often contains more than 70% of the objects baryonic mass. The external disk forms by late infall of gas that roughly conserves its specific angular momentum. The difference between these two simulations is likely to be due to the stabilizing character of the stellar bulge-like cores that form in the first simulation, which diminishes the inflow of gas triggered by mergers and interactions.

Bright and dark matter in elliptical galaxies : mass and velocity distributions from self-consistent hydrodynamical simulations

We have analysed the mass and velocity distributions of two samples of relaxed elliptical-like objects (ELOs) identified, at z = 0, in a set of self-consistent hydrodynamical simulations operating in the context of a concordance cosmological model. ELOs have been identified as those virtual galaxies having a prominent, dynamically relaxed stellar spheroidal component, with no extended discs and very low gas content. Our analysis shows that they are embedded in extended, massive dark matter haloes, and they also have an extended corona of hot diffuse gas. Dark matter haloes have experienced adiabatic contraction along their assembly process. The relative ELO dark- to bright-mass content and space distributions show broken homology, and they are consistent with observational results on the dark matter fraction at the central regions, as well as on the gradients of the mass-to-light ratio profiles for boxy ellipticals, as a function of their stellar masses. These results indicate that massive ellipticals miss stars (i.e. baryons) at their central regions, as compared to less massive ones. Our simulations indicate that these missing baryons could be found beyond the virial radii as a hot, diffuse plasma. This mass homology breaking could have important implications to explain the physical origin of the Fundamental Plane relation. The projected stellar mass profiles of our virtual ellipticals can be well fitted by the Sersic formula, with shape parameters n that agree, once a stellar mass-to-light ratio independent of position is assumed, with those obtained from surface brightness profiles of ellipticals. The agreement includes the empirical correlations of n with size, luminosity and velocity dispersion. The total mass density profiles show a power-law behaviour over a large r/r vir interval, consistent with data on massive lens ellipticals at shorter radii. The velocity dispersion profiles show kinematical segregation, with no systematic mass dependence (i.e. no dynamical homology breaking) and a positive anisotropy (i.e. radial orbits), roughly independent of the radial distance outside the central regions. The line-of-sight (LOS) velocity dispersion profiles are declining. These results give, for the first time from cosmological simulations, a rather detailed insight into the intrinsic mass and velocity distributions of the dark, stellar and gaseous components of virtual ellipticals. The consistency with observations strongly suggests that they could also describe important intrinsic characteristics of real ellipticals, as well as some of their properties recently inferred from observational data (e.g. downsizing, the appearance of blue cores, the increase of the stellar mass contributed by the elliptical population as z decreases).

Conservation Laws in Smooth Particle Hydrodynamics: The DEVA Code

We describe DEVA, a multistep AP3M-like SPH code particularly designed to study galaxy formation and evolution in connection with the global cosmological model. This code uses a formulation of SPH equations that ensures both energy and entropy conservation by including the so-called ∇h terms. Particular attention has also been paid to angular momentum conservation and to the accuracy of our code. We find that in order to avoid unphysical solutions, our code requires that cooling processes be implemented in a non-multistep way. We detail various cosmological simulations that have been performed to test DEVA and also to study the influence of the ∇h terms. Our results indicate that such correction terms have a nonnegligible effect on some cosmological simulations, especially on high-density regions associated with either shock fronts or central cores of collapsed objects. Moreover, they suggest that codes paying particular attention to the implementation of conservation laws of physics at the scales of interest can attain good accuracy levels in conservation laws with limited computational resources. Another important result of this work is that the combined effects of entropy violation and multistep cooling implementation in cosmological simulations can be particularly dramatic concerning the mass distribution in the objects they produce.

The main sequence and the fundamental metallicity relation in MaGICC Galaxies: evolution and scatter

Using cosmological galaxy simulations from the MaGICC project, we study the evolution of the stellar masses, star formation rates and gas-phase abundances of star-forming galaxies. We derive the stellar masses and star formation rates using observational relations based on spectral energy distributions by applying the new radiative transfer code grasil-3d to our simulated galaxies. The simulations match well the evolution of the stellar mass–halo mass relation, have a star-forming main sequence that maintains a constant slope out to redshift z ∼ 2, and populate projections of the stellar mass – star formation – metallicity plane, similar to observed star-forming disc galaxies. We discuss small differences between these projections in observational data and in simulations, and the possible causes for the discrepancies. The light-weighted stellar masses are in good agreement with the simulation values, the differences between the two varying between 0.06 and 0.20 dex. We also find good agreement between the star formation rate tracer and the true (time-averaged) simulation star formation rates. Regardless, if we use mass- or light-weighted quantities, our simulations indicate that bursty star formation cycles can account for the scatter in the star-forming main sequence.

Disc‐like objects in hierarchical hydrodynamical simulations: comparison with observations

We present results from a careful and detailed analysis of the structural and dynamical properties of a sample of 29 disc-like objects identified at z=0 in three AP3M–SPH fully consistent cosmological simulations. These simulations are realizations of a CDM hierarchical model, in which an inefficient Schmidt-law-like algorithm to model the stellar formation process has been implemented. We focus on properties that can be constrained with available data from observations of spiral galaxies, namely the bulge and disc structural parameters and the rotation curves. Comparison with data from Broeils, de Jong and Courteau gives satisfactory agreement, in contrast with previous findings using other codes. This suggests that the stellar formation implementation we have used has succeeded in forming compact bulges that stabilize disc-like structures in the violent phases of their assembly, while in the quiescent phases the gas has cooled and collapsed in accord with the Fall & Efstathiou standard model of disc formation.

The Lack of Structural and Dynamical Evolution of Elliptical Galaxies since z ~ 1.5: Clues from Self-Consistent Hydrodynamic Simulations

We present the results of a study of the evolution of the parameters that characterize the structure and dynamics of the relaxed elliptical-like objects (ELOs) identified at redshifts z = 0, z = 1, and z = 1.5 in a set of hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model. The values of the stellar mass M, the stellar half-mass radius r, and the mean square velocity for stars ? have been measured in each ELO and found to populate, at any z, a flattened ellipsoid close to a plane (the dynamical plane, DP). Our simulations indicate that at the intermediate zs considered, individual ELOs evolve, increasing their M, r, and ? parameters as a consequence of ongoing mass assembly, but nevertheless, their DP is roughly preserved within its scatter, in agreement with observations of the fundamental plane at different zs. We briefly discuss how this lack of significant dynamical and structural evolution in ELO samples arises, in terms of the two different phases operating in the mass aggregation history of their dark matter halos. According to our simulations, most dissipation involved in ELO formation takes place at the early violent phase, causing the M, r, and ? parameters to settle down to the DP and, moreover, the transformation of most of the available gas into stars. In the subsequent slow phase, ELO stellar mass growth preferentially occurs through nondissipative processes, so that the DP is preserved and the ELO star formation rate considerably decreases. These results hint, for the first time, at a possible way of explaining, in the context of cosmological simulations, different and apparently paradoxical observational results for elliptical galaxies.

Clues on the Physical Origin of the Fundamental Plane from Self-Consistent Hydrodynamical Simulations

We report on a study of the parameters characterizing the mass and velocity distributions of two samples of relaxed elliptical-like objects (ELOs) identified, at z = 0, in a set of self-consistent hydrodynamical simulations operating in the context of a concordance cosmological model. Star formation (SF) has been implemented in the simulations in the framework of the turbulent sequential scenario through a phenomenological parameterization that takes into account stellar physics processes implicitly through the values of a threshold gas density and an efficiency parameter. Each ELO sample is characterized by the values these parameters take. We have found that the (logarithms of the) ELO stellar masses, projected stellar half-mass radii, and stellar central line-of-sight (LOS) velocity dispersions define dynamical fundamental planes (FPs). Zero points depend on the particular values that the SF parameters take, while slopes do not change. The ELO samples have been found to show systematic trends with the mass scale in both the relative content and the relative distributions of the baryonic and the dark mass ELO components. The physical origin of these trends lies in the systematic decrease, with increasing ELO mass, of the relative dissipation experienced by the baryonic mass component along ELO mass assembly, resulting in a tilt of the dynamical FP relative to the virial plane. ELOs also show kinematical segregation, but it does not appreciably change with the mass scale. We have found that the dynamical FPs shown by the two ELO samples are consistent with that shown by the SDSS elliptical sample in the same variables, with no further need for any relevant contribution from stellar population effects to explain the observed tilt. These effects could, however, have contributed to the scatter of the observed FP, as the dynamical FPs have been found to be thinner than the observed one. The results we report on hint, for the first time, at a possible way to understand the tilt of the observed FP in a cosmological context.

Elliptical Galaxies at z = 0 from Self-consistent Hydrodynamical Simulations: Comparison with Sloan Digital Sky Survey Structural and Kinematical Data

We present results of an analysis of the structural and kinematical properties of a sample of elliptical-like objects (ELOs) identified in four hydrodynamical self-consistent simulations run with the DEVA code (Serna, Dominguez-Tenreiro, & Saiz). Star formation has been implemented in the code through a simple phenomenological parameterization, which takes into account stellar physics processes only implicitly through the values of a threshold gas density, ρg, thres, and an efficiency parameter, c*. The four simulations operate in the context of a Λ cold dark matter cosmological model consistent with observations, resolve ELO mass assembly at scales up to 2 kpc, and differ in the values of their star formation parameters. Stellar masses, projected half-mass radii, and central line-of-sight velocity dispersions, σlos, 0, have been measured on the ELO sample and their values compared with data from the Sloan Digital Sky Survey. For the first time in self-consistent simulations, a good degree of agreement has been shown, including the Faber-Jackson and the Dn-σlos, 0 relations, among others, but only when particular values of the ρg, thres and c* parameters are used. This demonstrates the effect that the star formation parameterization has on the ELO mass distribution. In addition, our results suggest that it is not strictly necessary, at the scales resolved in this work, to appeal to energy sources other than gravitational (as, for example, supernovae feedback effects) to account for the structure and kinematics of large elliptical galaxies.

Shape and kinematics of elliptical galaxies: evolution due to merging at z

Aims. We investigate the evolution in the shape and kinematics of elliptical galaxies in a cosmological framework. Methods. We identified relaxed, elliptical-like objects (ELOs) at redshifts