48 research outputs found
Evolution of the Phase-Space Density of Dark Matter Halos and Mixing Effects in Merger Events
Cosmological N-body simulations were performed to study the evolution of the
phase-space density Q = rho/sigma^3 of dark matter halos. No significant
differences in the scale relations Q ~ sigma^(-2.1) or Q ~ M^(-0.82) are seen
for "cold" or "warm" dark matter models. The follow up of individual halos from
z = 10 up to the present time indicate the existence of two main evolutionary
phases: an early and fast one (10 > z > 6.5), in which Q decreases on the
average by a factor of 40 as a consequence of the randomization of bulk motions
and a late and long one (6.5 > z > 0), in which Q decreases by a factor of 20
because of mixing induced by merger events. The study of these halos has also
evidenced that rapid and positive variations of the velocity dispersion,
induced by merger episods, are related to a fast decrease of the phase density
Q.Comment: 6 pages, accepted by MNRA
Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization
[Abridged] Understanding how the intergalactic medium (IGM) was reionized at
z > 6 is one of the big challenges of current high redshift astronomy. It
requires modelling the collapse of the first astrophysical objects (Pop III
stars, first galaxies) and their interaction with the IGM, while at the same
time pushing current observational facilities to their limits. The
observational and theoretical progress of the last few years have led to the
emergence of a coherent picture in which the budget of hydrogen-ionizing
photons is dominated by low-mass star-forming galaxies, with little
contribution from Pop III stars and quasars. The reionization history of the
Universe therefore critically depends on the number density of low-mass
galaxies at high redshift. In this work, we explore how changes in the
statistical properties of initial density fluctuations affect the formation of
early galaxies. Following Habouzit et al. (2014), we run 5 N-body simulations
with Gaussian and (scale-dependent) non-Gaussian initial conditions, all
consistent with Planck constraints. By appealing to a galaxy formation model
and to a population synthesis code, we compute the far-UV galaxy luminosity
function down to M_UV = -14 at redshift 7 < z < 15. We find that models with
strong primordial non-Gaussianities on < Mpc scales show a far-UV luminosity
function significantly enhanced in low-mass galaxies. We adopt a reionization
model calibrated from state-of-the-art hydrodynamical simulations and show that
such non-Gaussianities leave a clear imprint on the Universe reionization
history and electron Thomson scattering optical depth tau_E. Although current
uncertainties in the physics of reionization and on the determination of tau_E
still dominate the signatures of non-Gaussianities, our results suggest that
tau_E could ultimately be used to constrain the statistical properties of
initial density fluctuations.Comment: 18 pages, 12 figures, accepted for publication in MNRA
Testing primordial non-Gaussianities on galactic scales at high redshift
Primordial non-Gaussianities provide an important test of inflationary
models. Although the Planck CMB experiment has produced strong limits on
non-Gaussianity on scales of clusters, there is still room for considerable
non-Gaussianity on galactic scales. We have tested the effect of local
non-Gaussianity on the high redshift galaxy population by running five
cosmological N-body simulations down to z=6.5. For these simulations, we adopt
the same initial phases, and either Gaussian or scale-dependent non-Gaussian
primordial fluctuations, all consistent with the constraints set by Planck on
clusters scales. We then assign stellar masses to each halo using the halo -
stellar mass empirical relation of Behroozi et al. (2013). Our simulations with
non-Gaussian initial conditions produce halo mass functions that show clear
departures from those obtained from the analogous simulations with Gaussian
initial conditions at z>~10. We observe a >0.3 dex enhancement of the low-end
of the halo mass function, which leads to a similar effect on the galaxy
stellar mass function, which should be testable with future galaxy surveys at
z>10. As cosmic reionization is thought to be driven by dwarf galaxies at high
redshift, our findings may have implications for the reionization history of
the Universe.Comment: 6 pages, 3 figures, 1 table, MNRAS (Letters) in pres
LyMAS: Predicting Large-Scale Lyman-alpha Forest Statistics from the Dark Matter Density Field
[abridged] We describe LyMAS (Ly-alpha Mass Association Scheme), a method of
predicting clustering statistics in the Ly-alpha forest on large scales from
moderate resolution simulations of the dark matter distribution, with
calibration from high-resolution hydrodynamic simulations of smaller volumes.
We use the "Horizon MareNostrum" simulation, a 50 Mpc/h comoving volume evolved
with the adaptive mesh hydrodynamic code RAMSES, to compute the conditional
probability distribution P(F_s|delta_s) of the transmitted flux F_s, smoothed
(1-dimensionally) over the spectral resolution scale, on the dark matter
density contrast delta_s, smoothed (3-dimensionally) over a similar scale. In
this study we adopt the spectral resolution of the SDSS-III BOSS at z=2.5, and
we find optimal results for a dark matter smoothing length sigma=0.3 Mpc/h
(comoving). In extended form, LyMAS exactly reproduces both the 1-dimensional
power spectrum and 1-point flux distribution of the hydro simulation spectra.
Applied to the MareNostrum dark matter field, LyMAS accurately predicts the
2-point conditional flux distribution and flux correlation function of the full
hydro simulation for transverse sightline separations as small as 1 Mpc/h,
including redshift-space distortion effects. It is substantially more accurate
than a deterministic density-flux mapping ("Fluctuating Gunn-Peterson
Approximation"), often used for large volume simulations of the forest. With
the MareNostrum calibration, we apply LyMAS to 1024^3 N-body simulations of a
300 Mpc/h and 1.0 Gpc/h cube to produce large, publicly available catalogs of
mock BOSS spectra that probe a large comoving volume. LyMAS will be a powerful
tool for interpreting 3-d Ly-alpha forest data, thereby transforming
measurements from BOSS and other massive quasar absorption surveys into
constraints on dark energy, dark matter, space geometry, and IGM physics.Comment: Accepted for publication in ApJ (minor corrections from the previous
version). Catalogs of mock BOSS spectra and relevant data can be found at:
http://www2.iap.fr/users/peirani/lymas/lymas.ht
Anisotropic q-Gaussian velocity distributions in LambdaCDM halos
The velocity distribution function (VDF) of dark matter (DM) halos in
CDM dissipationless cosmological simulations, which must be
non-separable in its radial and tangential components, is still poorly known.
We present the first single-parameter, non-separable, anisotropic model for the
VDF in CDM halos, built from an isotropic -Gaussian (Tsallis) VDF
of the isotropic set of dimensionless spherical velocity components (after
subtraction of streaming motions), normalized by the respective velocity
dispersions. We test our VDF on 90 cluster-mass halos of a dissipationless
cosmological simulation.
Beyond the virial radius, , our model VDF adequately reproduces
that measured in the simulated halos, but no -Gaussian model can adequately
represent the VDF within , as the speed distribution function is
then flatter-topped than any -Gaussian can allow. Nevertheless, our VDF fits
significantly better the simulations than the commonly used Maxwellian
(Gaussian) distribution, at virtually all radii within . Within
0.4 (1) , the non-Gaussianity index is (roughly) linearly
related to the slope of the density profile and also to the velocity anisotropy
profile. We provide a parametrization of the modulation of with radius for
both the median fits and the fit of the stacked halo. At radii of a few percent
of , corresponding to the Solar position in the Milky Way, our
best-fit VDF, although fitting better the simulations than the Gaussian one,
overproduces significantly the fraction of high velocity objects, indicating
that one should not blindly use these -Gaussian fits to make predictions on
the direct detection rate of DM particles.Comment: This version consolidates the published version and the Erratum
(changes in red
Black hole formation and growth with non-Gaussian primordial density perturbations
Quasars powered by massive black holes (BHs) with mass estimates above a
billion solar masses have been identified at redshift 6 and beyond. The
existence of such BHs requires almost continuous growth at the Eddington limit
for their whole lifetime, of order of one billion years. In this paper, we
explore the possibility that positively skewed scale-dependent non-Gaussian
primordial fluctuations may ease the assembly of massive BHs. In particular,
they produce more low-mass halos at high redshift, thus altering the production
of metals and ultra-violet flux, believed to be important factors in BH
formation. Additionally, a higher number of progenitors and of nearly
equal-mass halo mergers would boost the mass increase provided by BH-BH mergers
and merger-driven accretion. We use a set of two cosmological simulations, with
either Gaussian or scale-dependent non-Gaussian primordial fluctuations to
perform a proof-of-concept experiment to estimate how BH formation and growth
are altered. We estimate the BH number density and the fraction of halos where
BHs form, for both simulations and for two popular scenarios of BH formation
(remnants of the first generation of stars and direct collapse in the absence
of metals and molecular hydrogen). We find that the fractions of halos where
BHs form are almost identical, but that non-Gaussian primordial perturbations
increase the total number density of BHs for the both BH formation scenarios.
We also evolve BHs using merger trees extracted from the simulations and find
that non-Gaussianities increase both the BH masses and the number of the most
massive BHs.Comment: 11 pages, 10 figures, MNRAS accepte
Emergence and cosmic evolution of the Kennicutt-Schmidt relation driven by interstellar turbulence
© 2024 The Author(s). Published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/The scaling relations between the gas content and star formation rate of galaxies provide useful insights into the processes governing their formation and evolution. We investigated the emergence and the physical drivers of the global KennicuttâSchmidt (KS) relation at 0.25ââ€âzââ€â4 in the cosmological hydrodynamic simulation NEWHORIZON, capturing the evolution of a few hundred galaxies with a resolution down to 34 pc. The details of this relation vary strongly with the stellar mass of galaxies and the redshift. A power-law relation ÎŁSFRâââÎŁgasa with aâââ1.4, like that found empirically, emerges at zâââ2â
ââ
3 for the more massive half of the galaxy population. However, no such convergence is found in the lower-mass galaxies, for which the relation gets shallower with decreasing redshift. At galactic scales, the star formation activity correlates with the level of turbulence of the interstellar medium, quantified by the Mach number, rather than with the gas fraction (neutral or molecular), confirming the conclusions found in previous works. With decreasing redshift, the number of outliers with short depletion times diminishes, reducing the scatter of the KS relation, while the overall population of galaxies shifts toward low densities. Our results, from parsec-scale star formation models calibrated with local Universe physics, demonstrate that the cosmological evolution of the environmental (e.g., mergers) and internal conditions (e.g., gas fractions) conspire to shape the KS relation. This is an illustration of how the interplay of global and local processes leaves a detectable imprint on galactic-scale observables and scaling relations.Peer reviewe
Emergence and cosmic evolution of the Kennicutt-Schmidt relation driven by interstellar turbulence
The scaling relations between the gas content and star formation rate of
galaxies provide useful insights into processes governing their formation and
evolution. We investigate the emergence and the physical drivers of the global
Kennicutt-Schmidt (KS) relation at in the cosmological
hydrodynamic simulation NewHorizon capturing the evolution of a few hundred
galaxies with a resolution of 40 pc. The details of this relation vary
strongly with the stellar mass of galaxies and the redshift. A power-law
relation with ,
like that found empirically, emerges at for the most massive
half of the galaxy population. However, no such convergence is found in the
lower-mass galaxies, for which the relation gets shallower with decreasing
redshift. At the galactic scale, the star formation activity correlates with
the level of turbulence of the interstellar medium, quantified by the Mach
number, rather than with the gas fraction (neutral or molecular), confirming
previous works. With decreasing redshift, the number of outliers with short
depletion times diminishes, reducing the scatter of the KS relation, while the
overall population of galaxies shifts toward low densities. Using pc-scale star
formation models calibrated with local Universe physics, our results
demonstrate that the cosmological evolution of the environmental and intrinsic
conditions conspire to converge towards a significant and detectable imprint in
galactic-scale observables, in their scaling relations, and in their reduced
scatter.Comment: 26 pages, 22 figure
Dark Matter Accretion into Supermassive Black Holes
The relativistic accretion rate of dark matter by a black hole is revisited.
Under the assumption that the phase space density indicator,
, remains constant during the inflow, the
derived accretion rate can be higher up to five orders of magnitude than the
classical accretion formula, valid for non-relativistic and non-interacting
particles, when typical dark halo conditions are considered. For these typical
conditions, the critical point of the flow is located at distances of about
30-150 times the horizon radius. Application of our results to black hole seeds
hosted by halos issued from cosmological simulations indicate that dark matter
contributes to no more than ~10% of the total accreted mass, confirming that
the bolometric quasar luminosity is related to the baryonic accretion history
of the black hole.Comment: 7 pages, 6 figures. Accepted for publication in Phys.Rev.