N-body simulations

14 October, 202114 October, 2021

ΛCDM halo substructure properties revealed with high resolution and large volume cosmological simulations

Written by Angie Moliné.

Summary of the paper with the same title submitted to MNRAS.

arXiv: 2110.02097

In the current standard model of cosmology, ΛCDM, the structure of the Universe is formed via a hierarchical, bottom-up scenario with small primordial density perturbations growing to the point where they collapse into the filaments, walls and eventually dark matter (DM) haloes that form the underlying large-scale-structure filamentary web of the Universe. Galaxies are embedded in these massive, extended DM haloes teeming with self-bound substructure, the so-called subhaloes.

The study of the statistical and structural properties of the subhalo population is of prime importance because subhaloes represent important probes of the mass accretion history and dynamics of host haloes and accordingly, of the underlying cosmological model. In addition to representing a cosmological test by themselves, understanding both the statistical and structural properties of subhaloes plays a key role for many other diverse studies, such as gravitational lensing, stellar streams and indirect or direct DM detection experiments.

Studying the complicated dynamics of these subhaloes within their hosts requires numerical simulations, which have proven to be crucial for understanding structure formation in the Universe. By making use of data at different cosmic times from the Phi-4096 and Uchuu suite of high-resolution N-body cosmological simulations, in this work we improve upon previous studies aimed at characterize the subhalo population. More precisely, the superb numerical resolution and halo statistics of these simulations allow for a careful and dedicated study – for the first time consistently over more than seven decades in ratio of subhalo-to-host-halo mass – of the dependency of subhalo abundance with halo host mass as a function of subhalo mass, the maximum circular velocity of particles within the subhalo, Vmax, and distance to the host halo centre. We also dissect the evolution of these dependencies over cosmic time.

Subhalo structural properties are codified via a concentration parameter that does not depend on any specific, pre-defined density profile and relies only on Vmax. We derive such relation in the range 7-1500 km/s and find an important dependence on distance of the subhalo to the host halo centre, as already described in Moliné et al. (2017) for subhaloes in Milky-Way-like hosts. Interestingly, we also find subhaloes of the same mass to be significantly more concentrated when they reside inside more massive hosts. We provide accurate fits that take into account all mentioned dependencies. In addition, the study of the evolution of subhalo concentrations with cosmic time is very scarce in the literature as of today. We investigate such redshift evolution of concentrations and provide an accurate fit.

Our results offer an unprecedented detailed characterization of the subhalo population, consistent over a wide range of subhalo and host halo masses, as well as cosmic times. Our analyses enables precision work in any future research involving dark matter halo substructure.

29 October, 202029 October, 2020

Predicting the dark matter velocity distribution in galactic structures: tests against hydrodynamic cosmological simulations

Written by Thomas Lacroix.

Summary of the paper with the same title published in JCAP.

arXiv: 2005.03955

In this work, we have quantified the level of predictivity and the relevance of some isotropic models of velocity distribution functions, by comparing their predictions for several observables with direct measurements in highly resolved cosmological simulations, providing realistic test galaxies where both the dark matter and the baryons are dynamically linked through their mutual gravitational interactions. The main question we addressed is the following: can a reliable, though simplified, galactic mass model be translated into reliable predictions for the speed distribution and related moments? Answering this question and further quantifying the reliability of the procedure is important in a context in which (i) dark matter searches have been intensifying on galactic scales, and (ii) observational data have been accumulating which can better constrain the dark matter content of target objects or structures. Moreover, discovery prospects as well as exclusion limits on specific dark matter scenarios would certainly benefit from better estimates or control of theoretical uncertainties.

In particular, we have tested a complete model ’the Eddington inversion model’ encapsulating a full phase-space description of the dark matter lying in a self-gravitating object, built from first principles while based on several simplifying assumptions: dynamical equilibrium, spherical symmetry, and isotropy. This model, a generic solution to the collisionless Boltzmann equation, allows one to derive the phase-space distribution function of dark matter from the knowledge of its mass density profile and of the full gravitational potential of the system (both required to be spherically symmetric). Therefore, it can be fully derived from a galactic mass model, where the mass density distributions of all components are specified. We have compared this full phase-space distribution model with more ad hoc models for the velocity distribution only, based on declensions of the Maxwell-Boltzmann approximation; one inspired from the isothermal sphere where the peak velocity is set to the circular velocity, and another one in which the peak velocity derives from the velocity dispersion calculated by consistently solving the Jeans equation. These models were used to predict the speed distribution function of a system and several relevant speed moments, as well as relative speed moments.

Galactic mass models including a dark matter profile and several components for baryons, were fitted on three different highly resolved zoom-in cosmological simulations. These simulations were used in both their dark matter-only and their hydrodynamical configurations, the former resembling a would-be giant isolated dwarf spheroidal galaxies, and the latter resembling spiral galaxies similar to the Milky Way (the level of ’Milky Way-likeness’ is not essential in this work).

We compared the model predictions for several velocity-dependent observables directly with the simulation data. This allowed us to estimate that the Eddington model provides a fairly good description of the phase-space distribution function of dark matter in galactic structures, reaching a precision of ~10-20% for velocity or relative velocity moments of order n = +-1, 2. It may perform better in describing dark matter-only systems than those with baryonic domination at their centers, with a precision degrading by ~ 10% for the latter — although this is not generic, as one of our galaxies was still very well described. It is rather surprising, and even remarkable, that such a simple model can capture the dark matter dynamics so well, especially when one considers the strong assumptions it is built upon. Indeed, none of our simulated objects exhibits perfect dynamical relaxation, spherical symmetry, nor isotropy. Still, the model is able to capture their main dynamical features. We emphasize that the Eddington model, in this context, provides a better description of realistic systems than typical declensions of the Maxwell-Boltzmann approximation used in the literature (in the sense of models, not Gaussian fitting functions). Considering the latter are lacking in terms of solid theoretical grounds in this particular context, this is rather satisfactory from the theoretical point of view.

This work provides a quantitative estimate of the theoretical uncertainties affecting the Eddington inversion in the context of dark matter searches, both in dark matter-dominated objects and in spiral galaxies similar to the Milky Way. We stress that these uncertainties do account for departures from local equilibrium, which are at play in our virtual galaxies.

24 September, 201924 September, 2019

Properties of subhalos in the interacting dark matter scenario

[Moliné, Ángeles; Schewtschenko, Jascha A.; Sánchez-Conde, Miguel A.; Aguirre-Santaella, Alejandra; Cora, Sofía A.; Abadi, Mario]

Written by A. Moliné

Summary of the paper with the same title published in Galaxies.

arXiv: 1907.12531

The current standard model of cosmology is based on a cosmological constant to explain the late-time accelerated expansion of the Universe and a cold dark matter (CDM) component to account for the required additional gravitational attraction to form and support the galaxies and larger structures we observe today. In this framework, the structure of the Universe is formed via a hierarchical, bottom-up scenario with small primordial density perturbations growing to the point where they collapse into the filaments, walls and eventually dark matter (DM) halos that form the underlying large-scale-structure filamentary web of the Universe. The galaxies are embedded in these massive, extended DM halos teeming with self-bound substructure. Any viable cosmological model has to successfully predict both the abundance and internal properties of these structures and their substructures, and match the observational data on a wide range of scales.

One natural deviation from the collisionless CDM in the standard model is the assumption of the existence of interactions between DM and the standard model (SM) particles we know about, in particular, photons or neutrinos. This does not only affect the formation of DM structures on small scales, but also provides an explanation for the exact relic abundance of DM found in the Universe today. Such possible interacting dark matter (IDM) model would imply a suppression of small-scale structures due to a large collisional damping effect, even though the weakly interacting massive particle (WIMP) can still be the DM candidate. Because of this, IDM models can help alleviate alleged tensions between standard CDM predictions and observations at small mass scales.

Using a high-resolution cosmological N-body simulation specifically run within this alternative model, we investigate the properties of DM halo substructure or subhalos formed in. We also run its CDM counterpart, which allowed us to compare subhalo properties in both cosmologies. We show that, in the lower mass range covered by our simulation runs, both subhalo concentrations and abundances as a function of the distance to the host halo center and subhalo mass (or, alternatively maximum circular velocity) are systematically lower in IDM compared to the CDM scenario. Yet, as in CDM, we find that median IDM subhalo concentration values increase towards the innermost regions of their hosts for same mass subhalos. Also similarly to CDM, we find IDM subhalos to be more concentrated than field halos of the same mass.

Our work has a direct application on studies aimed at the indirect detection of DM where subhalos are expected to boost the DM signal of their host halos significantly. From our results, we conclude that the role of halo substructure in DM searches will be less important in interacting scenarios than in CDM, but is nevertheless far from being negligible.

28 June, 20182 July, 2019

Unidentified gamma-ray sources as targets for indirect dark matter detection with the Fermi Large Area Telescope

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title accepted by JCAP.

arXiv: 1906.11896

If the dark matter (DM) is made of WIMP particles, they should yield a gamma-ray annihilation flux. Provided the DM clumps, which are predicted by the bottom-up collapse scenario for structure formation within LCDM, are light enough, these are not expected to retain any baryons and, therefore to remain completely dark to standard telescopes.

Only in gamma rays we should detect this emission, with a highly curved and distinctive spectrum. Fermi-LAT, a space-based gamma-ray telescope, has roughly one third of detected sources of unknown nature, the so-called unidentified sources (unIDs). Are there subhalos hidden in the Fermi-LAT catalogs, just waiting to be properly classified? To answer this question we propose a ‘filtering’ of the unIDs, according to the expected characteristics of a DM subhalo. For example, a DM subhalo should be detected as a steady gamma-ray source, so if an unID is reported as variable, is removed from our pool of candidates. With these ‘filters’, we are able to reduce the DM candidates from 1235 to just 44 unIDs.

A correct computation of the expected annihilation fluxes (encoded in the so-called J-factor) requires a good understanding of the DM subhalo population in our Galaxy. To do so, we use a state-of-the-art N-body cosmological simulation, Via Lactea II (VL-II). Unfortunately, the resolution of this, and any, simulation is limited, meaning we are unable to resolve the smallest members of the subhalo population (which can be as light as the Earth). Nevertheless, some of these subhalos may be close enough to us and still be bright enough to be detected by Fermi-LAT. To take them into account, we ‘repopulate’ the parent simulation with subhalos, reaching 3 orders of magnitude better resolution in mass. As the Earth position is arbitrary, to ensure a proper statistical treatment of the J-factors, 1000 realizations are done.

Also, we must compute the sensitivity of the Fermi-LAT to this kind of objects. The reported instrumental sensitivity is for a power-law source, i.e., with a non-curved spectrum, while we deal with highly-curved spectra, which also vary with the annihilation channel and mass of the WIMP. Furthermore, taking into account the diffuse gamma-ray emission and the considered catalog, this sensitivity also varies with the sky position (e.g. a source close to the Galactic plane, where the diffuse emission is more intense, will be harder to detect than a high-latitude source) and observation parameters (such as the energy threshold, total exposure time, instrumental response functions, etc.). We compute a huge grid of configurations, changing all the above mentioned parameters and generating a sensitivity skymap for each annihilation channel, WIMP mass, and catalog.

Finally, we are able to set constraints in the cross section vs. WIMP mass parameter space. We do so comparing the predictions of our repopulated simulations with the actual gamma-ray candidates. The obtained constraints are competitive with other targets such as the CMB or the dSphs, but they can still be improved via new rejections of candidates.