Search

We compare the measurements of the bispectrum and the estimate of its covariance obtained from a set of different methods for the efficient generation of approximate dark matter halo catalogues to the same quantities obtained from full N -body simulations. To this purpose we employ a large set of 300 realizations of the same cosmology for each method, run with matching initial conditions in order to reduce the contribution of cosmic variance to the comparison. In addition, we compare how the error on cosmological parameters such as linear and non-linear bias parameters depends on the approximate method used for the determination of the bispectrum variance. As general result, most methods provide errors within 10 per cent of the errors estimated from N -body simulations. Exceptions are those methods requiring calibration of the clustering amplitude but restrict this to 2-point statistics. Finally we test how our results are affected by being limited to a few hundreds measurements from N -body simulation by comparing with a larger set of several thousands of realizations performed with one approximate method. [ABSTRACT FROM AUTHOR]

We investigate the impact of galactic outflowmodelling on the formation and evolution of a disc galaxy, by performing a suite of cosmological simulations with zoomed-in initial conditions (ICs) of a Milky Way-sized halo. We verify how sensitive the general properties of the simulated galaxy are to the way in which stellar feedback triggered outflows are implemented, keeping ICs, simulation code and star formation (SF) model all fixed. We present simulations that are based on a version of the GADGET3 code where our sub-resolution model is coupled with an advanced implementation of smoothed particle hydrodynamics that ensures a more accurate fluid sampling and an improved description of gas mixing and hydrodynamical instabilities. We quantify the strong interplay between the adopted hydrodynamic scheme and the sub-resolution model describing SF and feedback. We consider four different galactic outflow models, including the one introduced by Dalla Vecchia & Schaye (2012) and a scheme that is inspired by the Springel & Hernquist (2003) model. We find that the sub-resolution prescriptions adopted to generate galactic outflows are the main shaping factor of the stellar disc component at low redshift. The key requirement that a feedback model must have to be successful in producing a disc-dominated galaxy is the ability to regulate the high-redshift SF (responsible for the formation of the bulge component), the cosmological infall of gas from the large-scale environment, and gas fall-back within the galactic radius at low redshift, in order to avoid a too high SF rate at z = 0. [ABSTRACT FROM AUTHOR]

We present a comparison of major methodologies of fast generating mock halo or galaxy catalogues. The comparison is done for two-point (power spectrum and two-point correlation function in real and redshift space), and the three-point clustering statistics (bispectrum and three-point correlation function). The reference catalogues are drawn from the BigMultiDark N-body simulation. Both friend-of-friends (including distinct haloes only) and spherical overdensity (including distinct haloes and subhalos) catalogues have been used with the typical number density of a large volume galaxy surveys. We demonstrate that a proper biasing model is essential for reproducing the power spectrum at quasi-linear and even smaller scales. With respect to various clustering statistics, a methodology based on perturbation theory and a realistic biasing model leads to very good agreement with N-body simulations. However, for the quadrupole of the correlation function or the power spectrum, only the method based on semi-N-body simulation could reach high accuracy (1 per cent level) at small scales, i.e. r < 25 h-1 Mpc or k > 0.15 h Mpc-1. Full N-body solutions will remain indispensable to produce reference catalogues. Nevertheless, we have demonstrated that the more efficient approximate solvers can reach a few per cent accuracy in terms of clustering statistics at the scales interesting for the large-scale structure analysis. This makes them useful for massive production aimed at covariance studies, to scan large parameter spaces, and to estimate uncertainties in data analysis techniques, such as baryon acoustic oscillation reconstruction, redshift distortion measurements, etc. [ABSTRACT FROM AUTHOR]

We present a quantification of the properties of bars in two N-body+ smoothed particle hydrodynamics cosmological simulations of spiral galaxies, named GA and AqC. The initial conditions were obtained using the zoom-in technique and represent two dark matter haloes of 2-3 x 1012 M⊙, available at two different resolutions. The resulting galaxies are presented in the companion paper of Murante et al. We find that the GA galaxy has a bar of length 8.8 kpc, present at the two resolution levels even though with a slightly different strength. Classical bar signatures (e.g. pattern of streaming motions, high m = 2 Fourier mode with roughly constant phase) are consistently found at both resolutions. Though a close encounter with a merging satellite at z ~ 0.6 (mass ratio 1: 50) causes a strong, transient spiral pattern and some heating of the disc, we find that bar instability is due to secular process, caused by a low Toomre parameter Q ≲ 1 due to accumulation of mass in the disc. The AqC galaxy has a slightly different history: it suffers a similar tidal disturbance due to a merging satellite at z ~ 0.5 but with a mass ratio of 1: 32, that triggers a bar in the high-resolution simulation, while at low resolution the merging is found to take place at a later time, so that both secular evolution and merging are plausible triggers for bar instability. [ABSTRACT FROM AUTHOR]

We measure and quantify properties of galactic outflows and diffuse gas at z ≥ 1 in cosmological hydrodynamical simulations. Our novel subresolution model, Multi-Phase Particle Integrator (MUPPI), implements supernova feedback using fully local gas properties, where the wind velocity and mass loading are not given as input. We find the following trends at z = 2 by analysing central galaxies having a stellar mass higher than 109M⊙. The outflow velocity and mass outflow rate (Ṁout) exhibit positive correlations with galaxy mass and with the star formation rate (SFR). However, most of the relations present a large scatter. The outflow mass loading factor (η) is between 0.2 and 10. The comparison effective model generates a constant outflow velocity, and a negative correlation of η with halo mass. The number fraction of galaxies where outflow is detected decreases at lower redshifts, but remains more than 80 per cent over z = 1-5. The outflow velocity correlation with SFR becomes flatter at z = 1, and η displays a negative correlation with halo mass in massive galaxies. Our study demonstrates that both the MUPPI and effective models produce significant outflows at ~1/10 of the virial radius; at the same time shows that the properties of outflows generated can be different from the input speed and mass loading in the effective model. Our MUPPI model, using local properties of gas in the subresolution recipe, is able to develop galactic outflows whose properties correlate with global galaxy properties, and consistent with observations. [ABSTRACT FROM AUTHOR]

We present results of cosmological simulations of disc galaxies carried out with the gadget-3 TreePM+SPH code, where star formation and stellar feedback are described using our MUlti Phase Particle Integrator model. This description is based on a simple multiphase model of the interstellar medium at unresolved scales, where mass and energy flows among the components are explicitly followed by solving a system of ordinary differential equations. Thermal energy from supernovae is injected into the local hot phase, so as to avoid that it is promptly radiated away. A kinetic feedback prescription generates the massive outflows needed to avoid the overproduction of stars. We use two sets of zoomed-in initial conditions of isolated cosmological haloes with masses (2-3) × 1012M⊙, both available at several resolution levels. In all cases we obtain spiral galaxies with small bulge-over-total stellar mass ratios (B/T ~ 0.2), extended stellar and gas discs, flat rotation curves and realistic values of stellar masses. Gas profiles are relatively flat, molecular gas is found to dominate at the centre of galaxies, with star formation rates following the observed Schmidt-Kennicutt relation. Stars kinematically belonging to the bulge form early, while disc stars show a clear inside-out formation pattern and mostly form after redshift z = 2. However, the baryon conversion efficiencies in our simulations differ from the relation given by Moster et al. at a 3σ level, thus indicating that our stellar discs are still too massive for the dark matter halo in which they reside. Results are found to be remarkably stable against resolution. This further demonstrates the feasibility of carrying out simulations producing a realistic population of galaxies within representative cosmological volumes, at a relatively modest resolution. [ABSTRACT FROM AUTHOR]

We explore new observationally constrained subresolution models of galactic outflows and investigate their impact on the circumgalactic medium (CGM) in the redshift range z = 2-4. We perform cosmological hydrodynamic simulations, including star formation, chemical enrichment and four cases of supernovae-driven outflows: no wind (NW), an energy-driven constant velocity wind (CW), a radially varying wind (RVWa) where the outflow velocity has a positive correlation with galactocentric distance (r) and a RVW with additional dependence on halo mass (RVWb). Overall, we find that the outflows expel metal-enriched gas away from galaxies, significantly quench the star formation, reduce the central galactic metallicity and enrich the CGM. At z = 2, the radial profiles of gas properties around galaxy centres are most sensitive to the choice of the wind model for halo masses in the range (109-1011)M&#664-. We infer that outflows in the RVWb model are least effective, with results similar to the NW case, except that the CGM is enriched more. Moreover, we find that the models CW and RVWa are similar, both showing the impact of effective winds, with the following notable differences. RVWa causes a greater suppression of star formation rate at z ≤ 5, and has a higher fraction of low-density (δ < 10), warm-hot (104-106 K) gas than in CW. Outflows in CW produce a higher and earlier enrichment of some intergalactic medium phases than in RVWa. By visual inspection, we note that the RVWa model shows galactic discs more pronounced than all the other wind models. We predict that some observational diagnostics are more promising to distinguish between different outflow driving mechanisms in galaxies: ZC of the CGM gas at r~ (30-300) h-1 kpc comoving, and C IV fraction of the inner gas at r < (4-5) h-1 kpc comoving. [ABSTRACT FROM AUTHOR]

ABSTRACT Dark matter halo merger trees are now routinely extracted from cosmological simulations of structure formation. These trees are frequently used as inputs to semi-analytic models of galaxy formation to provide the backbone within which galaxy formation takes place. By necessity, these merger trees are constructed from a finite set of discrete 'snapshots' of the N-body simulation and so have a limited temporal resolution. To date, there has been little consideration of how this temporal resolution affects the properties of galaxies formed within these trees. In particular, the question of how many snapshots are needed to achieve convergence in galaxy properties has not be answered. Therefore, we study the convergence in the stellar and total baryonic masses of galaxies, distribution of merger times, stellar mass functions and star formation rates in the G alacticus model of galaxy formation as a function of the number of 'snapshot' times used to represent dark matter halo merger trees. When utilizing snapshots between z= 20 and 0, we find that at least 128 snapshots are required to achieve convergence to within 5 per cent for galaxy masses, while 64 snapshots give convergence only to within 10 per cent for high-mass haloes. This convergence is obtained for mean quantities averaged over large samples of galaxies - significant variance for individual galaxies remains even when using very large numbers of snapshots. We find only weak dependence of the rate of convergence on the distribution of snapshots in time - snapshots spaced uniformly in the expansion factor, uniformly in the logarithm of expansion factor or uniformly in the logarithm of critical overdensity for collapse work equally well in almost all cases. We provide input parameters to G alacticus which allow this type of convergence study to be tuned to other simulations and to be carried out for other galaxy properties. [ABSTRACT FROM AUTHOR]

ABSTRACT We study the physical properties, formation histories and environment of galaxies without a significant 'classical' spheroidal component, as predicted by semi-analytical models of galaxy formation and evolution. This work is complementary to the analysis presented in De Lucia et al., where we focus on the relative contribution of various physical mechanisms responsible for bulge assembly in a Λ cold dark matter cosmology. We find that the fraction of bulgeless galaxies is a strong decreasing function of stellar mass: they represent a negligible fraction of the galaxy population with M★ > 1012 M⊙, but dominate at M★ < 1010 M⊙. We find a clear dichotomy in this galaxy population, between central galaxies of low-mass dark matter haloes and satellite galaxies in massive groups/clusters. We show that bulgeless galaxies are relatively young systems, that assemble most of their mass at low redshift, but they can also host very old stellar populations. Since galaxy-galaxy mergers are assumed to lead to the formation of a spheroidal component, in our models these galaxies form preferentially in low-mass haloes that host a small number of satellites galaxies. We show that the adopted modelling for galaxy mergers represents a key ingredient in determining the actual number of bulgeless galaxies. Our results show that these galaxies are not a rare population in theoretical models: at z ∼ 0, galaxies with no classical bulge (but often including galaxies with the equivalent of pseudo-bulges) account for up to 14 per cent of the galaxies with 1011 < M★/M⊙ < 1012. [ABSTRACT FROM AUTHOR]

It has recently been shown that galaxy formation models within the Λ cold dark matter cosmology predict that, compared to the observed population, small galaxies (with stellar masses ) form too early, are too passive since and host too old stellar populations at . We then expect an overproduction of small galaxies at that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use themorgana galaxy formation model andgrasil spectrophotometric radiative transfer code to generate mock catalogues of deep fields observed with Hubble Space Telescope Advanced Camera for Surveys. We add observational noise and the effect of Lyman α emission, and perform colour–colour selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i-dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of -dropouts at . We quantify the properties of these ‘excess’ galaxies and discuss the implications: these galaxies are hosted in dark matter haloes with circular velocities in excess of 100 km s−1, and their suppression may require a deep rethinking of stellar feedback processes taking place in galaxy formation. [ABSTRACT FROM AUTHOR]

We present a new (semi-)analytical model for feedback in galaxy formation. The interstellar medium (ISM) is modelled as a two-phase medium in pressure equilibrium, where the cold phase is fragmented into clouds with a given mass spectrum. Cold gas infalls from an external halo. Large clouds are continually formed by coagulation and destroyed by gravitational collapse. Stars form in the collapsing clouds; the remnants of exploding Type II supernovae (SNe) percolate into a single super-bubble (SB) that sweeps the ISM, heating the hot phase (if the SB is adiabatic) or cooling it (in the snowplough stage, when the interior gas of the SB has cooled). Different feedback regimes are obtained whenever SBs are stopped either in the adiabatic or in the snowplough stage, either by pressure confinement or by blowout. The resulting feedback regimes occur in well-defined regions of the space defined by vertical scalelength and surface density of the structure. In the adiabatic blowout regime, the efficiency of SNe in heating the ISM is rather low (∼5 per cent, with ∼80 per cent of the energy budget injected into the external halo), and the outcoming ISM is self-regulated to a state that, in conditions typical of our galaxy, is similar to that found in the Milky Way. Feedback is most efficient in the adiabatic confinement regime, where star formation is hampered by the very high thermal pressure and the resulting inefficient coagulation. In some significant regions of the parameter space, confinement takes place in the snowplough stage; in this case, the hot phase has a lower temperature and star formation is quicker. In some critical cases, found at different densities in several regions of the parameter space, the hot phase is strongly depleted and the cold phase percolates the whole volume, giving rise to a burst of star formation. While the hot phase is allowed to leak out of the star-forming region, and may give rise to a tenuous wind that escapes the potential well of a small galactic halo, strong galactic winds are predicted to occur only in critical cases or in the snowplough confinement regime whenever the SBs are able to percolate the volume. This model provides a starting point for constructing a realistic grid of feedback solutions to be used in galaxy formation codes, either semi-analytical or numerical. The predictive power of this model extends to many properties of the ISM, so that most parameters can be constrained by reproducing the main properties of the Milky Way. [ABSTRACT FROM AUTHOR]

ABSTRACT We apply theztracealgorithm to the optical NOG and infrared PSCz galaxy catalogues to reconstruct the pattern of primordial fluctuations that have generated our local Universe. We check that the density fields traced by the two catalogues are well correlated, and consistent with a linear relation [either in δ or in log (1 +δ) ] with relative bias (of NOG with respect to PSCz) b[sub rel]= 1.1 ± 0.1 . The relative bias relation is used to fill the optical zone of avoidance at |b| < 20° using the PSCz galaxy density field. We perform extensive testing on simulated galaxy catalogues to optimize the reconstruction. The quality of the reconstruction is predicted to be good at large scales, up to a limiting wavenumber k[sub lim]≃ 0.4 h Mpc[sup -1] beyond which all information is lost. We find that the improvement arising from the denser sampling of the optical catalogue is compensated by the uncertainties connected to the larger zone of avoidance. The initial conditions reconstructed from the NOG catalogue are found (analogously to those from the PSCz) to be consistent with a Gaussian paradigm. We use the reconstructions to produce sets of initial conditions ready to be used for constrained simulations of our local Universe. [ABSTRACT FROM AUTHOR]

We study the ability of PINOCCHIO (PINpointing Orbit-Crossing Collapsed Hierarchical Objects) to predict the merging histories of dark matter (DM) haloes, comparing the PINOCCHIO predictions with the results of two large N-body simulations run from the same set of initial conditions. We focus our attention on the quantities most relevant to galaxy formation and large-scale structure studies. PINOCCHIO is able to predict the statistics of merger trees with a typical accuracy of 20 per cent. Its validity extends to higher-order moments of the distribution of progenitors. The agreement is also valid at the object-byobject level, with 70-90 per cent of the progenitors cleanly recognized when the parent halo is cleanly recognized itself. Predictions are also presented for quantities that are usually not reproduced by semi-analytic codes, such as the two-point correlation function of the progenitors of massive haloes and the distribution of initial orbital parameters of merging haloes. For the accuracy of the prediction and for the facility with which merger histories are produced, PINOCCHIO provides a means to generate catalogues of DM haloes, which is extremely competitive with large-scale N-body simulations, making it a suitable tool for galaxy formation and large-scale structure studies. [ABSTRACT FROM AUTHOR]