arXiv subjects

C. Dalla Vecchia

Publications and source records attributed to C. Dalla Vecchia.

24 records · Page 2Linked to original sources

arXiv2015

Evolution of galaxy stellar masses and star formation rates in the EAGLE simulations

We investigate the evolution of galaxy masses and star formation rates in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These comprise a suite of hydrodynamical simulations in a $Λ$CDM cosmogony with subgrid models for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. The subgrid feedback was calibrated to reproduce the observed present-day galaxy stellar mass function and galaxy sizes. Here we demonstrate that the simulations reproduce the observed growth of the stellar mass density to within 20 per cent. The simulation also tracks the observed evolution of the galaxy stellar mass function out to redshift z = 7, with differences comparable to the plausible uncertainties in the interpretation of the data. Just as with observed galaxies, the specific star formation rates of simulated galaxies are bimodal, with distinct star forming and passive sequences. The specific star formation rates of star forming galaxies are typically 0.2 to 0.4 dex lower than observed, but the evolution of the rates track the observations closely. The unprecedented level of agreement between simulation and data makes EAGLE a powerful resource to understand the physical processes that govern galaxy formation.

astro-ph.GA↗

arXiv2014

The First Billion Years project: gamma-ray bursts at z>5

Long gamma-ray burst's (LGRB's) association to the death of massive stars suggest they could be used to probe the cosmic star formation history (CSFH) with high accuracy, due to their high luminosities. We utilise cosmological simulations from the First Billion Years project to investigate the biases between the CSFH and the LGRB rate at z>5, assuming various different models and constraints on the progenitors of LGRBs. We populate LGRBs using a selection based on environmental properties and demonstrate that the LGRB rate should trace the CSFH to high redshifts. The measured LGRB rate suggests that LGRBs have opening angles of theta_jet=0.1 deg, although the degeneracy with the progenitor model cannot rule out an underlying bias. We demonstrate that proxies that relate the LGRB rate with global LGRB host properties do not reflect the underlying LGRB environment, and are in fact a result of the host galaxy's spatial properties, such that LGRBs can exist in galaxies of solar metallicity. However, we find a class of host galaxies that have low stellar mass and are metal-rich, and that their metallicity dispersions would not allow low-metallicity environments. Detection of hosts with this set of properties would directly reflect the progenitor's environment. We predict that 10% of LGRBs per year are associated with this set of galaxies that would have forbidden line emission that could be detected by instruments on the James Webb Space Telescope. Such a discovery would place strong constraints on the collapsar model and suggest other avenues to be investigated.

astro-ph.HE↗

arXiv2013

The impact of baryons on the spins and shapes of dark matter haloes

We use numerical simulations to investigate how the statistical properties of dark matter (DM) haloes are affected by the baryonic processes associated with galaxy formation. We focus on how these processes influence the spin and shape of a large number of DM haloes covering a wide range of mass scales, from galaxies to clusters at redshifts zero and one, extending to dwarf galaxies at redshift two. The haloes are extracted from the OverWhelmingly Large Simulations, a suite of state-of-the-art high-resolution cosmological simulations run with a range of feedback prescriptions. We find that the median spin parameter in DM-only simulations is independent of mass, redshift and cosmology. At z=0 baryons increase the spin of the DM in the central region (<=0.25 r_200) by up to 30 per cent when feedback is weak or absent. This increase can be attributed to the transfer of angular momentum from baryons to the DM, but is no longer present at z=2. We also present fits to the mass dependence of the DM halo shape at both low and high redshift. At z=0 the sphericity (triaxiality) is negatively (positively) correlated with halo mass and both results are independent of cosmology. Interestingly, these mass-dependent trends are markedly weaker at z=2. While the cooling of baryons acts to make the overall DM halo more spherical, stronger feedback prescriptions tend to reduce the impact of baryons by reducing the central halo mass concentration. More generally, we demonstrate a strongly positive (negative) correlation between halo sphericity (triaxiality) and galaxy formation efficiency, with the latter measured using the central halo baryon fraction. In conclusion, our results suggest that the effects of baryons on the DM halo spin and shape are minor when the effects of cooling are mitigated, as required by realistic models of galaxy formation, although they remain significant for the inner halo.

astro-ph.CO↗

arXiv2012

Influence of baryons on the orbital structure of dark matter haloes

We explore the dynamical signatures imprinted by baryons on dark matter haloes during the formation process using the OverWhelmingly Large Simulations (OWLS), a set of state-of-the-art high-resolution cosmological hydrodynamical simulations. We present a detailed study of the effects of the implemented feedback prescriptions on the orbits of dark matter particles, stellar particles and subhaloes, analysing runs with no feedback, with stellar feedback and with feedback from supermassive black holes. We focus on the central regions (0.25 r_{200}) of haloes with virial masses ~ 6 x 10^{13} (~ 7 x 10^{11}) Msun/h at z = 0(2). We also investigate how the orbital content (relative fractions of the different orbital types) of these haloes depends on several key parameters such as their mass, redshift and dynamical state. The results of spectral analyses of the orbital content of these simulations are compared, and the change in fraction of box, tube and irregular orbits is quantified. Box orbits are found to dominate the orbital structure of dark matter haloes in cosmological simulations. There is a strong anticorrelation between the fraction of box orbits and the central baryon fraction. While radiative cooling acts to reduce the fraction of box orbits, strong feedback implementations result in a similar orbital distribution to that of the dark matter only case. The orbital content described by the stellar particles is found to be remarkably similar to that drawn from the orbits of dark matter particles, suggesting that either they have forgotten their dynamical history, or that subhaloes bringing in stars are not biased significantly with respect to the main distribution. The orbital content of the subhaloes is in broad agreement with that seen in the outer regions of the particle distributions.

astro-ph.CO↗

arXiv2012

The Aquila comparison Project: The Effects of Feedback and Numerical Methods on Simulations of Galaxy Formation

We compare the results of thirteen cosmological gasdynamical codes used to simulate the formation of a galaxy in the LCDM structure formation paradigm. The various runs differ in their hydrodynamical treatment (SPH, moving-mesh and AMR) but share the same initial conditions and adopt their latest published model of cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z=0, due mainly to the different implementations of feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas-rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disk is discernible in most simulations, though its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observation. Models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disk component. Our conclusions hold at two different numerical resolutions. Some differences can also be traced to the numerical techniques: more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high-angular momentum stellar disks remains a challenge.

astro-ph.GA↗

arXiv2005

Simulations of magnetic fields in filaments

The intergalactic magnetic field within filaments should be less polluted by magnetised outflows from active galaxies than magnetic fields in clusters. Therefore, filaments may be a better laboratory to study magnetic field amplification by structure formation than galaxy clusters which typically host many more active galaxies. We present highly resolved cosmological AMR simulations of magnetic fields in the cosmos and make predictions about the evolution and structure of magnetic fields in filaments. Comparing our results to observational evidence for magnetic fields in filaments suggests that amplification of seed fields by gravitational collapse is not sufficient to produce IGM fields. Finally, implications for cosmic ray transport are discussed.

astro-ph↗