44 research outputs found
The impact of baryonic physics on the subhalo mass function and implications for gravitational lensing
We investigate the impact of baryonic physics on the subhalo population by
analyzing the results of two recent hydrodynamical simulations (EAGLE and
Illustris), which have very similar configuration, but a different model of
baryonic physics. We concentrate on haloes with a mass between and
and redshift between 0.2 and 0.5, comparing with
observational results and subhalo detections in early-type galaxy lenses. We
compare the number and the spatial distribution of subhaloes in the fully hydro
runs and in their dark matter only counterparts, focusing on the differences
between the two simulations. We find that the presence of baryons reduces the
number of subhaloes, especially at the low mass end (), by different amounts depending on the model. The
variations in the subhalo mass function are strongly dependent on those in the
halo mass function, which is shifted by the effect of stellar and AGN feedback.
Finally, we search for analogues of the observed lenses (SLACS) in the
simulations, selecting them in velocity dispersion and dynamical properties. We
use the selected galaxies to quantify detection expectations based on the
subhalo populations in the different simulations, calculating the detection
probability and the predicted values for the projected dark matter fraction in
subhaloes and the slope of the mass function . We compare
these values with those derived from subhalo detections in observations and
conclude that the dark-matter-only and hydro EAGLE runs are both compatible
with observational results, while results from the hydro Illustris run do not
lie within the errors.Comment: 15 pages, 11 figures, accepted for publication in MNRA
Ellipsoidal halo finders and implications for models of triaxial halo formation
We describe an algorithm for identifying ellipsoidal haloes in numerical
simulations, and quantify how the resulting estimates of halo mass and shape
differ with respect to spherical halo finders. Haloes become more prolate when
fit with ellipsoids, the difference being most pronounced for the more
aspherical objects. Although the ellipsoidal mass is systematically larger,
this is less than 10% for most of the haloes. However, even this small
difference in mass corresponds to a significant difference in shape. We
quantify these effects also on the initial mass and deformation tensors, on
which most models of triaxial collapse are based. By studying the properties of
protohaloes in the initial conditions, we find that models in which protohaloes
are identified in Lagrangian space by three positive eigenvalues of the
deformation tensor are tenable only at the masses well-above . The
overdensity within almost any protohalo is larger than the critical
value associated with spherical collapse (increasing as mass decreases); this
is in good qualitative agreement with models which identify haloes requiring
that collapse have occured along all three principal axes, each axis having
turned around from the universal expansion at a different time. The
distributions of initial values are in agreement with the simplest predictions
associated with ellipsoidal collapse, assuming initially spherical protohaloes,
collapsed around random positions which were sufficiently overdense. However,
most protohaloes are not spherical and departures from sphericity increase as
protohalo mass decreases. [Abridged]Comment: 18 pages, 17 figures, accepted for publication in MNRA
A look to the inside of haloes: a characterisation of the halo shape as a function of overdensity in the Planck cosmology
In this paper we study the triaxial properties of dark matter haloes of a
wide range of masses extracted from a set of cosmological N-body simulations.
We measure the shape at different distances from the halo centre (characterised
by different overdensity thresholds), both in three and in two dimensions. We
discuss how halo triaxiality increases with mass, redshift and distance from
the halo centre. We also examine how the orientation of the different
ellipsoids are aligned with each other and what is the gradient in internal
shapes for halos with different virial configurations. Our findings highlight
that the internal part of the halo retains memory of the violent formation
process keeping the major axis oriented toward the preferential direction of
the in-falling material while the outer part becomes rounder due to continuous
isotropic merging events. This effect is clearly evident in high mass haloes -
which formed more recently - while it is more blurred in low mass haloes. We
present simple distributions that may be used as priors for various mass
reconstruction algorithms, operating in different wavelengths, in order to
recover a more complex and realistic dark matter distribution of isolated and
relaxed systems.Comment: accepted for publication by MNRAS (15 pag. and 14 fig.
Accretion of satellites onto central galaxies in clusters: merger mass ratios and orbital parameters
We study the statistical properties of mergers between central and satellite
galaxies in galaxy clusters in the redshift range , using a sample of
dark-matter only cosmological N-body simulations from Le SBARBINE dataset.
Using a spherical overdensity algorithm to identify dark-matter haloes, we
construct halo merger trees for different values of the over-density
. While the virial overdensity definition allows us to probe the
accretion of satellites at the cluster virial radius , higher
overdensities probe satellite mergers in the central region of the cluster,
down to , which can be considered a proxy for the
accretion of satellite galaxies onto central galaxies. We find that the
characteristic merger mass ratio increases for increasing values of :
more than of the mass accreted by central galaxies since
comes from major mergers. The orbits of satellites accreting onto central
galaxies tend to be more tangential and more bound than orbits of haloes
accreting at the virial radius. The obtained distributions of merger mass
ratios and orbital parameters are useful to model the evolution of the
high-mass end of the galaxy scaling relations without resorting to hydrodynamic
cosmological simulations.Comment: accepted by MNRAS (minor comments
Ellipsoidal collapse of dark matter haloes in cosmological simulations
Nowadays different observational campaigns agree on the standard cosmological model to explain and describe the formation and evolution of large scale structures in our Universe. In this scenario, almost 95% of the energy content of the Universe is in unknown forms of energy and matter, generally called dark energy and dark matter. The structures observed today are assumed to have grown gravitationally from small and initially Gaussian density fluctuations. As the universe expands, sufficiently overdense regions expand until they reach a maximum size and then collapse under the action of their own gravity: since dark matter is believed to be the dominant matter component of the universe, it leads the gravitational collapse process, forming structures called dark matter haloes. It is within the potential wells of these haloes that gas can shock, cool and eventually form stars and galaxies.
The main theoretical models on the gravitational collapse of dark matter haloes are the spherical collapse and the ellipsoidal collapse (EC) models. The former describes haloes as spherical overdense regions embedded in an uniform background, while the latter allows more possible shapes, defining haloes as homogeneous ellipsoids. Moreover, the ellipsoidal collapse model predicts that there is a direct connection between the evolution of an halo and the properties of the corresponding region in the initial conditions. Despite the fact that a triaxial modelling is obviously more realistic, the spherical approximation is still the most common choice.
In this work we analysed the results of several cosmological simulations (the GIF2 , Le SBARBINE - designed and run in Padova by our group - and the Millennium XXL simulations), with the aim of study the triaxiality of dark matter haloes in detail. In particular, we developed a new halo finder, called ``Ellipsoidal Overdensity Halo Finder'' (EO), which identifies dark matter haloes as triaxial ellipsoids at all times, thus following the prescription of the EC model. Using its results, we studied the properties of protohaloes in the initial conditions and their evolution through the whole history of the Universe: this is crucial to understand the role of the initial density peaks, which are believed to be the seeds of all the observed structures. Our results help to understand the dynamics of halo collapse, confirming many predictions of the EC model, but also provide hints for a more realistic modelling.
As the issue of halo triaxiality is still not completely solved in theory and simulations, it started to be considered very recently in observational studies. Galaxy clusters are the largest virialized systems in the Universe and, following hierarchical clustering, also the last to form; almost 80% of their mass is attributed to dark matter, while the rest to baryons. The estimate of mass of clusters is still an open problem and the uncertainties are also related to the triaxiality of the haloes that surrounds them. For example, the estimated mass is on average biased to be lower than the true one, due to the fact that the haloes are embedded are typically prolate and so the spherical modelling is not able to capture their real structure.
We studied the shape distributions of dark matter haloes at all times and for different cosmologies, using Le SBARBINE and the MXXL simulations. In this way, we derived some universal relations between the shape parameters and the mass of haloes, independent from the cosmological model and redshift. These results will be useful to generate mock halo catalogues with given triaxial properties and can be used in triaxial mass reconstruction methods that require priors for the axial ratio distributions. Then, we concentrated on very massive haloes to provide more accurate predictions for cluster-size haloes.
Finally, we studied the halo mass function and tested its universality. With this purpose, we identified dark matter haloes at six different density thresholds (the virial one and other multiples of the background and the critical densities, which are commonly used in literature). Our results confirm the universality of the halo mass function, when measured with virialized haloes, while it does not hold for other halo identifications. We provide the fitting formulae for all the overdensity, believing that they could be useful for observers, and a method to rescale from one to the others
Universality of dark matter haloes shape over six decades in mass: Insights from the Millennium XXL and SBARBINE simulations
For the last 30 years many observational and theoretical evidences have shown
that galaxy clusters are not spherical objects, and that their shape is much
better described by a triaxial geometry. With the advent of multi-wavelength
data of increasing quality, triaxial investigations of galaxy clusters is
gathering a growing interest from the community, especially in the time of
"precision cosmology". In this work, we aim to provide the first statistically
significant predictions in the unexplored mass range above 3x10^14 Mo/h, using
haloes from two redshifts (z=0 and z=1) of the Millennium XXL simulation. The
size of this cosmological dark matter only simulation (4.1 Gpc) allows the
formation of a statistically significant number of massive cluster scale haloes
(about 500 with M>2x10^15 Mo/h and 780000 with M>10^14 Mo/h). Besides, we aim
to extend this investigation to lower masses in order to look for universal
predictions across nearly six orders of magnitude in mass, from 10^10 to almost
10^16 Mo/h. For this purpose we use the SBARBINE simulations, allowing to model
haloes of masses starting from 10^10 Mo/h. We use an elliptical overdensity
method to select haloes and compute the shapes of the unimodal ones
(approximately 50%), while we discard the unrelaxed. The minor to major and
intermediate to major axis ratio are found to be well described by simple
functional forms. For a given mass we can fully characterize the shape of a
halo and give predictions about the distribution of axis ratios for a given
cosmology and redshift. Moreover, these results are in some disagreement with
the findings of Jing & Suto (2002) which are widely used in the community even
though they have to be extrapolated far beyond their original mass range. This
"recipe" is made available to the community in this paper and in a dedicated
web page.Comment: 13 pages, 16 figure
Flux-ratio anomalies from discs and other baryonic structures in the Illustris simulation
The flux ratios in the multiple images of gravitationally lensed quasars can
provide evidence for dark matter substructure in the halo of the lensing galaxy
if the flux ratios differ from those predicted by a smooth model of the lensing
galaxy mass distribution. However, it is also possible that baryonic structures
in the lensing galaxy, such as edge-on discs, can produce flux-ratio anomalies.
In this work, we present the first statistical analysis of flux-ratio anomalies
due to baryons from a numerical simulation perspective. We select galaxies with
various morphological types in the Illustris simulation and ray-trace through
the simulated halos, which include baryons in the main lensing galaxies but
exclude any substructures, in order to explore the pure baryonic effects. Our
ray-tracing results show that the baryonic components can be a major
contribution to the flux-ratio anomalies in lensed quasars and that edge-on
disc lenses induce the strongest anomalies. We find that the baryonic
components increase the probability of finding high flux-ratio anomalies in the
early-type lenses by about 8% and by about 10 - 20% in the disc lenses. The
baryonic effects also induce astrometric anomalies in 13% of the mock lenses.
Our results indicate that the morphology of the lens galaxy becomes important
in the analysis of flux-ratio anomalies when considering the effect of baryons,
and that the presence of baryons may also partially explain the discrepancy
between the observed (high) anomaly frequency and what is expected due to the
presence of subhalos as predicted by the CDM simulations.Comment: 16 pages, 11 figures, accepted by MNRA
Constraining the mass density of free-floating black holes using razor-thin lensing arcs
Strong lensing of active galactic nuclei in the radio can result in
razor-thin arcs, with a thickness of less than a milli-arcsecond, if observed
at the resolution achievable with very long baseline interferometry (VLBI).
Such razor-thin arcs provide a unique window on the coarseness of the matter
distribution between source and observer. In this paper, we investigate to what
extent such razor-thin arcs can constrain the number density and mass function
of `free-floating' black holes, defined as black holes that do not, or no
longer, reside at the centre of a galaxy. These can be either primordial in
origin or arise as by-products of the evolution of super-massive black holes in
galactic nuclei. When sufficiently close to the line of sight, free-floating
black holes cause kink-like distortions in the arcs, which are detectable by
eye in the VLBI images as long as the black hole mass exceeds Solar
masses. Using a crude estimate for the detectability of such distortions, we
analytically compute constraints on the matter density of free-floating black
holes resulting from null-detections of distortions along a realistic, fiducial
arc, and find them to be comparable to those from quasar milli-lensing. We also
use predictions from a large hydrodynamical simulation for the demographics of
free-floating black holes that are not primordial in origin, and show that
their predicted mass density is roughly four orders of magnitude below the
constraints achievable with a single razor-thin arc.Comment: 17 pages, 13 figures, 1 table, comments welcom
The interplay of Self-Interacting Dark Matter and baryons in shaping the halo evolution
We use high-resolution hydrodynamical simulation to test the difference of
halo properties in cold dark matter (CDM) and a self-interacting dark matter
(SIDM) scenario with a constant cross-section of
. We find that the
interplay between dark matter self-interaction and baryonic physics induces a
complex evolution of the halo properties, which depends on the halo mass and
morphological type, as well as on the halo mass accretion history. While high
mass haloes, selected as analogues of early-type galaxies, show cored profiles
in the SIDM run, systems of intermediate mass and with a significant disk
component can develop a profile that is similar or cuspier than in CDM. The
final properties of SIDM haloes - measured at z=0.2 - correlate with the halo
concentration and formation time, suggesting that the differences between
different systems are due to the fact that we are observing the impact
self-interaction. We also search for signatures of self-interacting dark matter
in the lensing signal of the main haloes and hints of potential differences in
the distribution of Einstein radii, which suggests that future wide-field
survey might be able to distinguish between CDM and SIDM models on this basis.
Finally, we find that the subhalo abundances are not altered in the adopted
SIDM model with respect to CDM.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Properties and observables of massive galaxies in self-interacting dark matter cosmologies
We use hydrodynamical cosmological simulations to test the differences
between cold and self-interacting dark matter models (CDM and SIDM) in the mass
range of massive galaxies
(). We consider two SIDM
models: one with constant cross section
and one where the cross section is velocity-dependent. We analyse the halo
density profiles and concentrations, comparing the predictions of
dark-matter-only and hydrodynamical simulations in all scenarios. We calculate
the best-fit Einasto profiles and compare the resulting parameters with
previous studies and define the best-fit concentration-mass relations. We find
that the inclusion of baryons reduces the differences between different dark
matter models with respect to the DM-only case. In SIDM hydro runs, deviations
from the CDM density profiles weakly depend on mass: the most massive systems
() show cored profiles, while the least massive ones
() have cuspier profiles. Finally, we compare the
predictions of our simulations to observational results, by looking at the dark
matter fractions and the distribution of strong lensing Einstein radii. We find
that in SIDM the DM-fractions decrease more rapidly with increasing stellar
mass than in CDM, leading to lower fractions at , a
distinctive signature of SIDM. At the same time, the distribution of Einstein
radii, derived from both CDM and SIDM hydro runs, is comparable to observed
samples of strong lenses with . We conclude that the
interplay between self-interaction and baryons can greatly reduce the expected
differences between CDM and SIDM models at this mass scale, and that techniques
able to separate the dark and luminous mass in the inner regions of galaxies
are needed to constrain self-interactions.Comment: 13 pages, 12 figure