Vlasov versus N-body: the H\'enon sphere

We perform a detailed comparison of the phase-space density traced by the particle distribution in Gadget simulations to the result obtained with a spherical Vlasov solver using the splitting algorithm. The systems considered are apodized H\'enon spheres with two values of the virial ratio, R ~ 0.1 and 0.5. After checking that spherical symmetry is well preserved by the N-body simulations, visual and quantitative comparisons are performed. In particular we introduce new statistics, correlators and entropic estimators, based on the likelihood of whether N-body simulations actually trace randomly the Vlasov phase-space density. When taking into account the limits of both the N-body and the Vlasov codes, namely collective effects due to the particle shot noise in the first case and diffusion and possible nonlinear instabilities due to finite resolution of the phase-space grid in the second case, we find a spectacular agreement between both methods, even in regions of phase-space where nontrivial physical instabilities develop. However, in the colder case, R=0.1, it was not possible to prove actual numerical convergence of the N-body results after a number of dynamical times, even with N=10$^8$ particles.Comment: 19 pages, 11 figures, MNRAS, in pres

Active Galactic Nuclei and Massive Galaxy Cores

Self-consistent N-body simulations have been performed in order to study the effects of a central active galactic nucleus (AGN) on the dark matter profile of a typical giant elliptical galaxy. In our analysis, we assume that periodic bipolar outbursts from a central AGN can induce harmonic oscillatory motions on both sides of the gas core. Using realistic AGN properties, we find that the motions of the gas core, driven by such feedback processes, can flatten the dark matter and/or stellar profiles after 4-5 Gyr. Such results are consistent with observational studies such as those of Kormendy et al. (2006) which suggest that most giant elliptical galaxies have cores or ``missing light'' in their inner part. Since stars behave as a ``collisionless'' fluid similar to dark matter, the density profile both of stars and dark matter should be affected in a similar way, leading to an effective reduction in the central brightness

On the coldness of the local Hubble flow: the role of baryons

(Abridged) Our aim is to investigate whether the presence of baryons can have any significant influence on the properties of the local Hubble flow which has proved to be "cold". We use two cosmological zoom simulations in the standard LCDM cosmology with the same set of initial conditions to study the formation of a local group-like system within a sphere of ~7 Mpc/h. The first one is a pure dark matter simulation (runDM) while a complete treatment of the physics of baryons is introduced in the second one (runB). We found that galaxies identified in runB and their corresponding dark matter haloes in runDM have very similar spatial distributions and dynamical properties on large scales. Then, when analyzing the velocity field and the deviation from a pure Hubble flow in both simulations, namely when computing the dispersion of peculiar velocities of galaxies \sigma*(R) and those of their corresponding dark matter haloes in runDM, we found no particular differences for distances R=1 to 8 Mpc from the local group mass center. The results indicate that the "true" \sigma*(R) values can be estimated from the pure dark matter simulation with a mean error of 3 km/s when dark matter haloes are selected with maximum circular velocities of Vc$\ge$30 km/s, corresponding to a population of dark matter haloes in runB that host galaxies. By investigating the properties of the Hubble flow at distances R~0.7 to 3 Mpc, we also found that the estimation of the total mass enclosed at the radius of the zero-velocity surface R0, using the spherical infall model adapted to LCDM, can be underestimated by at least 50%.Comment: Accepted for publication in MNRAS, 11 pages, 7 figure

Mass Determination of Groups of Galaxies: Effects of the Cosmological Constant

The spherical infall model first developed by Lema\^{i}tre and Tolman was modified in order to include the effects of a dark energy term. The resulting velocity-distance relation was evaluated numerically. This equation, when fitted to actual data, permits the simultaneous evaluation of the central mass and of the Hubble parameter. Application of this relation to the Local Group, when the dark energy is modeled by a cosmological constant, yields a total mass for the M31-Milky Way pair of (2.5 +/- 0.7) x 10^12 M\_sun, a Hubble parameter H\_0 = 74 +/- 4 km s^-1 Mpc^-1 and a 1-D velocity dispersion for the flow of about 39 km s^-1. The zero-velocity and the marginally bound surfaces of the Local Group are at about 1.0 and 2.3 Mpc respectively from the center of mass. A similar analysis for the Virgo cluster yields a mass of (1.10 +/- 0.12) x 10^15 M\_sun and H\_0 = 65 +/- 9 km s^-1 Mpc^-1. The zero-velocity is located at a distance of 8.6 +/- 0.8 Mpc from the center of the cluster. The predicted peculiar velocity of the Local Group towards Virgo is about 190 kms^-1, in agreement with other estimates. Slightly lower masses are derived if the dark energy is represented by a fluid with an equation of state P = w\epsilon with w = -2/3.Comment: 13 pages, 3 figures. Version to appear in New Astronomy. Typing errors corrected in relation (1) and in percentage value in page

Associations of Dwarf Galaxies

Hubble Space Telescope Advanced Cameras for Surveys has been used to determine accurate distances for 20 galaxies from measurements of the luminosity of the brightest red giant branch stars. Five associations of dwarf galaxies that had originally been identified based on strong correlations on the plane of the sky and in velocity are shown to be equally well correlated in distance. Two more associations with similar properties have been discovered. Another association is identified that is suggested to be unbound through tidal disruption. The associations have the spatial and kinematic properties expected of bound structures with 1 - 10 x 10^11 solar mass. However, these entities have little light with the consequence that mass-to-light ratios are in the range 100 - 1000 in solar units. Within a well surveyed volume extending to 3 Mpc, all but one known galaxy lies within one of the groups or associations that have been identified.Comment: 50 pages, 2 tables, 15 encapsulated figures, 1 (3 part) jpg figure. Submitted to Astronomical Journa

Galaxy merger histories and the role of merging in driving star formation at z>1

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1zR(4:1 3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at ztodays stellar mass was formed.Peer reviewe

Locally Cold Flows from Large-Scale Structure

We show that the "cold" Hubble flow observed for galaxies around the Milky Way does not represent a problem in cosmology but is due to the particular geometry and dynamics of our local wall. The behavior of the perturbed Hubble flow around the Milky Way is the result of two main factors: at small scales (R < 1 Mpc) the inflow is dominated by the gravitational influence of the Milky Way. At large scales (R > 1 Mpc) the out flow reflects the expansion of our local wall which "cools down" the peculiar velocities. This is an intrinsic property of walls and is independent of cosmology. We find the dispersion of the local Hubble flow (1 < R < 3 Mpc) around simulated "Milky Way" haloes located at the centre of low-density cosmological walls to be {\sigma}_H ~ 30 km/s, in excellent agreement with observations. The expansion of our local wall is also reflected in the value of the measured local Hubble constant. For "Milky Way" haloes inside walls, we find super-Hubble flows with h_local \simeq 0.77 - 1.13. The radius of equilibrium (R_0) depends not only on the mass of the central halo and the Hubble expansion but also on the dynamics given by the local LSS geometry. The super-Hubble flow inside our local wall has the effect of reducing the radius at which the local expansion balances the gravitational influence of the Milky Way. By ignoring the dynamical effect of the local wall, the mass of the Milky Way estimated from R_0 can be underestimated by as much as ~ 30%.Comment: 5 pages, 3 figures, Submitted to MNRA

Analytic solution of Chemical Evolution Models with Type Ia SNe

Context: In the last years, a significant number of works have focused on finding analytic solutions for the chemical enrichment models of galactic systems, including the Milky Way. Some of these solutions, however, cannot account for the enrichment produced by Type Ia SNe due to the presence of the delay time distributions (DTDs) in the models. Aims: We present a new analytic solution for the chemical evolution model of the Galaxy. This solution can be used with different prescriptions of the DTD, including the single and double degenerate scenarios, and allows the inclusion of an arbitrary number of pristine gas infalls. Methods: We integrate the chemical evolution model by extending the instantaneous recycling approximation with the contribution of Type Ia SNe. For those DTDs that lead to non-analytic integrals, we describe them as a superposition of Gaussian, exponential and 1/t functions using a restricted least-squares fitting method. Results: We obtain the exact solution for a chemical model with Type Ia SNe widely used in previous works. This solution can reproduce the expected chemical evolution of the alpha and iron-peak elements in less computing time than numerical integration methods. We compare the pattern in the [Si/Fe] vs. [Fe/H] plane observed by APOGEE DR17 with that predicted by the model. We find the low alpha sequence can be explained by a delayed gas infall. We exploit the applicability of our solution by modelling the chemical evolution of a simulated Milky Way-like galaxy from its star formation history. The implementation of our solution is released as a python package. Conclusions: Our solution constitutes a promising tool for the Galactic Archaeology and is able to model the observed trends in alpha element abundances versus [Fe/H] in the solar neighbourhood. We infer the chemical information of a simulated galaxy modelled without Chemistry.Comment: Accepted for publication in A&A. 22 pages, 16 figures. ChEAP code available at https://bitbucket.org/pedroap/cheap/src/master