Field Halos Research Articles

Hermeian haloes: Extreme objects with two interactions in the past

Recent studies based on numerical models of the Local Group predict the existence of field haloes and galaxies that have visited both the Milky Way and M31 in the past, called Hermeian haloes. We extend this analysis beyond the Local Group using two high-resolution dark matter-only N-body simulations from the MultiDark suite. We define Hermeian haloes as field haloes which had close interactions with two other more massive field haloes in the past, called targets. We find that Hermeian haloes are a more extreme example of field haloes with interactions in the past than the well-known backsplash haloes that experienced only one interaction. Compared to backsplashers, Hermeians have more concentrated density profiles and tend to occupy more overdense regions. They also have higher velocities relative to their target haloes and relative to their neighbours within 1 h−1Mpc. Hermeian haloes can be found around every halo in the simulation (if the resolution is sufficient) and make up 0.4 to 2.3 per cent of the total number of field haloes (for haloes more massive than 1010h−1M⊙ and 3.3×107h−1M⊙, respectively), increasing to 10 per cent in overdense regions. They tend to be distributed close to the line connecting their targets, which may help to identify Hermeian haloes in observations. We also identify Local Group analogues in the simulation and find that about one-third (15 out of 49) of them contain Hermeian haloes if the distance between the two main haloes is below 1 h−1Mpc.

ZFIRE – The gas inflow inequality for satellite galaxies in cluster and field haloes at z = 2

ABSTRACT Gas inflow into galaxies should affect the star formation and hence the evolution of galaxies across cosmic time. In this work, we use TNG100 of the IllustrisTNG simulations to understand the role of environment on gas inflow rates in massive galaxies at z ≥ 2. We divide our galaxies (log (M⋆/M⊙) ≥ 10.5) into cluster ($\log \rm {M_{halo}/M_\odot }\ge 13$) and field ($\log \rm {M_{halo}/M_\odot } < 13$) galaxies at z = 2 and further divide into centrals and satellites. We track their gas inflow rates from z = 6 to 2, and find that the total gas inflow rates of satellite galaxies rapidly decline after their infall into cluster haloes as they reach the cluster centre. At z = 2, the gas inflow rate of cluster satellite galaxies is correlated with the cluster-centric radii and not the host halo mass. In contrast, the gas inflow rate in centre is strongly correlated with the host halo mass at z ≥ 2. Our study indicates that between redshifts 6 to 2, the gas that is normally accreted by the satellite galaxies, is redirected to the centre of the cluster halo as inflows to the cluster centrals and forming the intracluster medium. Our analysis suggest that the inequality of gas accretion between massive satellite and central galaxies is responsible for the starvation of cluster satellite galaxies that evolve into the massive quenched cluster galaxies observed at z < 0.5.

Galactic satellite systems in CDM, WDM and SIDM

ABSTRACT We investigate the population of bright satellites ($M_{*} \ge 10^{5} \, \mathrm{M}_{\odot }$) of haloes of mass comparable to that of the Milky Way in cosmological simulations in which the dark matter (DM) is either cold, warm, or self-interacting (CDM, WDM, and SIDM, respectively). The nature of the DM gives rise to differences in the abundance and structural properties of field haloes. In WDM, the main feature is a reduction in the total number of galaxies that form, reflecting a suppression of low-mass DM haloes and lower galaxy formation efficiency compared to CDM. For SIDM, the changes are structural, restricted to the central regions of haloes and dependent on the assumed self-interaction cross-section. We also consider different baryonic subgrid physics models for galaxy formation, in which supernova gas blowouts can or cannot induce the formation of a core in dwarf galaxies. Overall, the inclusion of baryons lessen the differences in the halo properties in the different DM models compared to DM-only simulations. This affects the satellite properties at infall and therefore their subsequent tidal stripping and survival rates. None the less, we find slightly less concentrated satellite radial distributions as the SIDM cross-section increases. Unfortunately, we also find that the satellite populations in simulations with baryon-induced cores in CDM and WDM can mimic the results found in SIDM, making the satellite stellar mass and maximum circular velocity functions heavily degenerate on the assumed nature of the DM and the adopted subgrid modelling. These degeneracies preclude using the brightest satellites of the Milky Way to constrain the nature of DM.

Hermeian haloes: Field haloes that interacted with both the Milky Way and M31

ABSTRACT The Local Group is a unique environment in which to study the astrophysics of galaxy formation. The proximity of the Milky Way and M31 enhances the frequency of interactions of the low-mass halo population with more massive dark matter haloes, which increases their concentrations and strips them of gas and other material. Some low-mass haloes pass through the haloes of the Milky Way or M31 and are either ejected into the field or exchanged between the two primary hosts. We use high resolution gas-dynamical simulations to describe a new class of field haloes that passed through the haloes of both the Milky Way and M31 at early times and are almost twice as concentrated as field haloes that do not interact with the primary pair. These ‘Hermeian’ haloes are distributed anisotropically at larger distances from the Local Group barycentre than the primary haloes and appear to cluster along the line connecting the Milky Way and M31. Hermeian haloes facilitate the exchange of dark matter, gas, and stars between the Milky Way and M31 and can enhance the star formation rates of the gas in the primary haloes during their interactions with them. We also show that some Hermeian haloes can host galaxies that, because they are embedded in haloes that are more concentrated than regular field haloes, are promising targets for indirect dark matter searches beyond the Milky Way virial radius and can produce signals that are competitive with those of some dwarf galaxies. Hermeian galaxies in the Local Group should be detectable by forthcoming wide-field imaging surveys.

Pre-processing, group accretion, and the orbital trajectories of associated subhaloes

ABSTRACT We use a high-resolution cosmological dark matter-only simulation to study the orbital trajectories of haloes and subhaloes in the environs of isolated hosts. We carefully tally all apsis points and use them to distinguish haloes that are infalling for the first time from those that occupy more evolved orbits. We find that roughly 21 per cent of resolved subhaloes within a host’s virial radius are currently on first infall, and have not yet reached their first orbital pericentre; roughly 44 per cent are still approaching their first apocentre after infall. For the range of host masses studied, roughly half of all accreted systems were pre-processed prior to infall, and about 20 per cent were accreted in groups. We confirm that the entire population of accreted subhaloes – often referred to as ‘associated’ subhaloes – extends far beyond the virial radii of their hosts, with roughly half currently residing at distances that exceed ≈1.2 × r200. Many of these backsplash haloes have gained orbital energy since infall, and occupy extreme orbits that carry them well past their initial turnaround radii. Such extreme orbits are created during the initial accretion and dissolution of loosely bound groups, but also through penetrating encounters between subhaloes on subsequent orbits. The same processes may also give rise to unexpectedly abrupt losses of orbital energy. These effects combine, giving rise to a large variation in the ratio of sequent apocentres for accreted systems. We find that, within two virial radii from host centres, the concentrations of first-infall haloes are remarkably similar to those of isolated field haloes, whereas backsplash haloes, as well as systems that were pre-processed, are considerably more concentrated.

The abundance and structure of subhaloes near the free streaming scale and their impact on indirect dark matter searches

ABSTRACT The free streaming motion of dark matter particles imprints a cutoff in the matter power spectrum and set the scale of the smallest dark matter halo. Recent cosmological N-body simulations have shown that the central density cusp is much steeper in haloes near the free streaming scale than in more massive haloes. Here, we study the abundance and structure of subhaloes near the free streaming scale at very high redshift using a suite of unprecedentedly large cosmological N-body simulations, over a wide range of the host halo mass. The subhalo abundance is suppressed strongly below the free streaming scale, but the ratio between the subhalo mass function in the cutoff and no cutoff simulations is well fitted by a single correction function regardless of the host halo mass and the redshift. In subhaloes, the central slopes are considerably shallower than in field haloes, however, are still steeper than that of the NFW profile. Contrary, the concentrations are significantly larger in subhaloes than haloes and depend on the subhalo mass. We compare two methods to extrapolate the mass–concentration relation of haloes and subhaloes to z = 0 and provide a new simple fitting function for subhaloes, based on a suite of large cosmological N-body simulations. Finally, we estimate the annihilation boost factor of a Milky-Way-sized halo to be between 1.8 and 6.2.

Properties of Subhalos in the Interacting Dark Matter Scenario

One possible and natural derivation from the collisionless cold dark matter (CDM) standard cosmological framework is the assumption of the existence of interactions between dark matter (DM) and photons or neutrinos. Such a 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. In this work, we investigate the properties of the DM halo substructure or subhalos formed in a high-resolution cosmological N-body simulation specifically run within these alternative models. 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 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 the same mass subhalos. Similarly to CDM, we find IDM subhalos to be more concentrated than field halos of the same mass. Our work has a direct application to 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 the halo substructure in DM searches will be less important in interacting scenarios than in CDM, but is nevertheless far from being negligible.

Impact of filaments on galaxy formation in their residing dark matter haloes

We use a high-resolution zoom-in hydrodynamical simulation to investigate the impact of filaments on galaxy formation in their residing dark matter haloes. A method based on the density field and the Hoshen–Kopelman algorithm is developed to identify filaments. We show that cold and dense gas pre-processed by dark matter filaments can be further accreted into residing individual low-mass haloes in directions along the filaments. Consequently, comparing with field haloes, gas accretion is very anisotropic for filament haloes. About 30 per cent of the accreted gas of a residing filament halo was pre-processed by filaments, leading to two different thermal histories for the gas in filament haloes. Filament haloes have higher baryon and stellar fractions when compared with their field counterparts. Without including stellar feedback, our results suggest that filaments assist gas cooling and enhance star formation in their residing dark matter haloes at high redshifts (i.e. |$z$| = 4.0 and 2.5).

The Hydrangea simulations: galaxy formation in and around massive clusters

We introduce the Hydrangea simulations, a suite of 24 cosmological hydrodynamic zoom-in simulations of massive galaxy clusters (M_200c = 10^14-10^15 M_Sun) with baryon particle masses of ~10^6 M_Sun. Designed to study the impact of the cluster environment on galaxy formation, they are a key part of the `Cluster-EAGLE' project (Barnes et al. 2017). They use a galaxy formation model developed for the EAGLE project, which has been shown to yield both realistic field galaxies and hot gas fractions of galaxy groups consistent with observations. The total stellar mass content of the simulated clusters agrees with observations, but central cluster galaxies are too massive, by up to 0.6 dex. Passive satellite fractions are higher than in the field, and at stellar masses Mstar > 10^10 M_Sun this environmental effect is quantitatively consistent with observations. The predicted satellite stellar mass function matches data from local cluster surveys. Normalized to total mass, there are fewer low-mass (Mstar < 10^10 M_Sun) galaxies within the virial radius of clusters than in the field, primarily due to star formation quenching. Conversely, the simulations predict an overabundance of massive galaxies in clusters compared to the field that persists to their far outskirts (> 5r_200c). This is caused by a significantly increased stellar mass fraction of (sub-)haloes in the cluster environment, by up to ~0.3 dex even well beyond r_200c. Haloes near clusters are also more concentrated than equally massive field haloes, but these two effects are largely uncorrelated.

Characterization of subhalo structural properties and implications for dark matter annihilation signals

A prediction of the standard LCDM cosmology is that dark matter (DM) halos are teeming with numerous self-bound substructure, or subhalos. The precise properties of these subhalos represent important probes of the underlying cosmological model. We use data from Via Lactea II and ELVIS N-body simulations to learn about the structure of subhalos with masses 1e6 - 1e11 Msun/h. Thanks to a superb subhalo statistics, we study subhalo properties as a function of distance to host halo center and subhalo mass, and provide a set of fits that accurately describe the subhalo structure. We also investigate the role of subhalos on the search for DM annihilation. Previous work has shown that subhalos are expected to boost the DM signal of their host halos significantly. Yet, these works traditionally assumed that subhalos exhibit similar structural properties than those of field halos, while it is known that subhalos are more concentrated. Building upon our N-body data analysis, we refine the substructure boost model of Sanchez-Conde & Prada (2014), and find boosts that are a factor 2-3 higher. We further refine the model to include unavoidable tidal stripping effects on the subhalo population. For field halos, this introduces a moderate 20-30% suppression. Yet, for subhalos like those hosting dwarf galaxy satellites, tidal stripping plays a critical role, the boost being at the level of a few tens of percent at most. We provide a parametrization of the boost for field halos that can be safely applied over a wide halo mass range.

The chosen few: the low-mass haloes that host faint galaxies

Since reionization prevents star formation in most halos below 3 x 10^9 solar masses, dwarf galaxies only populate a fraction of existing dark matter halos. We use hydrodynamic cosmological simulations of the Local Group to study the discriminating factors for galaxy formation in the early Universe and connect them to the present-day properties of galaxies and halos. A combination of selection effects related to reionization, and the subsequent evolution of halos in different environments, introduces strong biases between the population of halos that host dwarf galaxies, and the total halo population. Halos that host galaxies formed earlier and are more concentrated. In addition, halos more affected by tidal stripping are more likely to host a galaxy for a given mass or maximum circular velocity, vmax, today. Consequently, satellite halos are populated more frequently than field halos, and satellite halos of 10^8 - 10^9 solar masses or vmax of 12 - 20 km/s, similar to the Local Group dwarf spheroidals, have experienced a greater than average reduction in both mass and vmax after infall. They are on closer, more radial orbits with higher infall velocities and earlier infall times. Together, these effects make dwarf galaxies highly biased tracers of the underlying dark matter distribution.

Boosting the annihilation boost: Tidal effects on dark matter subhalos and consistent luminosity modeling

In the cold dark matter paradigm, structures form hierarchically, implying that large structures contain smaller substructures. These subhalos will enhance signatures of dark matter annihilation such as gamma rays. In the literature, typical estimates of this boost factor assume a concentration-mass relation for field halos, to calculate the luminosity of subhalos. However, since subhalos accreted in the gravitational potential of their host loose mass through tidal stripping and dynamical friction, they have a quite characteristic density profile, different from that of the field halos of the same mass. In this work, we quantify the effect of tidal stripping on the boost factor, by developing a semi-analytic model that combines mass-accretion history of both the host and subhalos as well as subhalo accretion rates. We find that when subhalo luminosities are treated consistently, the boost factor increases by a factor 2-5, compared to the typical calculation assuming a field-halo concentration. This holds for host halos ranging from sub-galaxy to cluster masses and is independent of the subhalo mass function or specific concentration-mass relation. The results are particularly relevant for indirect dark matter searches in the extragalactic gamma-ray sky.

All about baryons: revisiting SIDM predictions at small halo masses

We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both $\Lambda$ dominated, Cold (CDM) and Self--Interacting (SIDM) dark matter models. The SIDM model adopts a constant cross section of 2 $cm^{2}/g$, a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter cores. Remarkably, SIDM fails to significantly lower the central dark matter density at halo peak velocities V$_{max}$ $<$ 30 Km/s. This is due to the fact that the central regions of very low--mass field halos have relatively low central velocity dispersion and densities, leading to time scales for SIDM collisions greater than a Hubble time. CDM halos with V$_{max}$ $<$ 30 km/s have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm2/g SIDM cross section, the DM content of very low mass CDM and SIDM halos differs by no more than a factor of two within 100-200pc. At larger halo masses ($\sim$ 10$^{10}$ solar masses), the introduction of baryonic processes creates field dwarf galaxies with dark matter cores and central DM$+$baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity dependent cross section that rapidly increases to values larger than 2 $cm^{2}/g$ for halos with V$_{max}$ < 25-30 Km/s needs to be introduced.

SOLVING THE PUZZLE OF SUBHALO SPINS

Investigating the spin parameter distribution of subhaloes in two high resolution isolated halo simulations, re- cent work by Onions et al. suggested that typical subhalo spins are consistently lower than the spin distribution found for field haloes. To further examine this puzzle, we have analyzed simulations of a cosmological volume with sufficient resolution to resolve a significant subhalo population. We confirm the result of Onions et al. and show that the typical spin of a subhalo decreases with decreasing mass and increasing proximity to the host halo center. We interpret this as the growing influence of tidal stripping in removing the outer layers, and hence the higher angular momentum particles, of the subhaloes as they move within the host potential. Investigating the redshift dependence of this effect, we find that the typical subhalo spin is smaller with decreasing redshift. This indicates a temporal evolution as expected in the tidal stripping scenario.

The shape of dark matter subhaloes in the Aquarius simulations

We analyze the Aquarius simulations to characterize the shape of dark matter halos with peak circular velocity in the range 8<Vmax<200 km/s, and perform a convergence study using the various Aquarius resolution levels. For the converged objects, we determine the principal axis (a<b<c) of the normalized inertia tensor as a function of radius. We find that the triaxiality of field halos is an increasing function of halo mass, so that the smallest halos in our sample are ~40-50% rounder than Milky Way-like objects at the radius where the circular velocity peaks, rmax. We find that the distribution of subhalo axis ratios is consistent with that of field halos of comparable Vmax. Inner and outer contours within each object are well aligned, with the major axis preferentially pointing in the radial direction for subhalos closest to the center of their host halo. We also analyze the dynamical structure of subhalos likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group. These halos have axis ratios that increase with radius, and which are mildly triaxial with <b/a>~0.75 and <c/a>~0.60 at r~1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.

Subhaloes gone Notts: spin across subhaloes and finders

We present a study of a comparison of spin distributions of subhaloes found associated with a host halo. The subhaloes are found within two cosmological simulation families of Milky Way-like galaxies, namely the Aquarius and GHALO simulations. These two simulations use different gravity codes and cosmologies. We employ ten different substructure finders, which span a wide range of methodologies from simple overdensity in configuration space to full 6-d phase space analysis of particles.We subject the results to a common post-processing pipeline to analyse the results in a consistent manner, recovering the dimensionless spin parameter. We find that spin distribution is an excellent indicator of how well the removal of background particles (unbinding) has been carried out. We also find that the spin distribution decreases for substructure the nearer they are to the host halo's, and that the value of the spin parameter rises with enclosed mass towards the edge of the substructure. Finally subhaloes are less rotationally supported than field haloes, with the peak of the spin distribution having a lower spin parameter.

Convolution Lagrangian perturbation theory for biased tracers

We present a new formulation of Lagrangian perturbation theory which allows accurate predictions of the real- and redshift-space correlation functions of the mass field and dark matter halos. Our formulation involves a non-perturbative resummation of Lagrangian perturbation theory and indeed can be viewed as a partial resummation of the formalism of Matsubara (2008a,b) in which we keep exponentiated all of the terms which tend to a constant at large separation. One of the key features of our method is that we naturally recover the Zel'dovich approximation as the lowest order of our expansion for the matter correlation function. We compare our results against a suite of N-body simulations and obtain good agreement for the correlation functions in real-space and for the monopole correlation function in redshift space. The agreement becomes worse for higher multipole moments of the redshift-space, halo correlation function. Our formalism naturally includes non-linear bias and explains the strong bias-dependence of the multipole moments of the redshift-space correlation function seen in N-body simulations.

Galaxies going MAD: the Galaxy-Finder Comparison Project

With the ever increasing size and complexity of fully self-consistent simulations of galaxy formation within the framework of the cosmic web, the demands upon object finders for these simulations has simultaneously grown. To this extent we initiated the Halo Finder Comparison Project that gathered together all the experts in the field and has so far led to two comparison papers, one for dark matter field haloes (Knebe et al. 2011), and one for dark matter subhaloes (Onions et al. 2012). However, as state-of-the-art simulation codes are perfectly capable of not only following the formation and evolution of dark matter but also account for baryonic physics (e.g. hydrodynamics, star formation, feedback) object finders should also be capable of taking these additional processes into consideration. Here we report on a comparison of codes as applied to the Constrained Local UniversE Simulation (CLUES) of the formation of the Local Group which incorporates much of the physics relevant for galaxy formation. We compare both the properties of the three main galaxies in the simulation (representing the MW, M31, and M33) as well as their satellite populations for a variety of halo finders ranging from phase-space to velocity-space to spherical overdensity based codes, including also a mere baryonic object finder. We obtain agreement amongst codes comparable to (if not better than) our previous comparisons, at least for the total, dark, and stellar components of the objects. However, the diffuse gas content of the haloes shows great disparity, especially for low-mass satellite galaxies. This is primarily due to differences in the treatment of the thermal energy during the unbinding procedure. We acknowledge that the handling of gas in halo finders is something that needs to be dealt with carefully, and the precise treatment may depend sensitively upon the scientific problem being studied.

Gravitational waves from scalar field accretion

Our aim in this work is to outline some physical consequences of the interaction between black holes and scalar field halos in terms of gravitational waves. In doing so, the black hole is taken as a static and spherically symmetric gravitational source, i.e. the Schwarzschild black hole, and we work within the test field approximation, considering that the scalar field lives in the curved space-time outside the black hole. We focused on the emission of gravitational waves when the black hole is perturbed by the surrounding scalar field matter. The symmetries of the space-time and the simplicity of the matter source allow, by means of a spherical harmonic decomposition, to study the problem by means of a one-dimensional description. Some properties of such gravitational waves are discussed as a function of the parameters of the infalling scalar field, and allow us to make the conjecture that the gravitational waves carry information on the type of matter that generated them.

The Aquarius Project: the subhaloes of galactic haloes

We have performed the largest ever particle simulation of a Milky Way-sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far. We have also simulated a sample of 6 ultra-highly resolved Milky-way sized halos, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300,000 gravitationally bound subhalos within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors up to 1800 accurately reproduce the largest subhalos with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subhalos within subhalos, but contrary to recent claims, we find less substructure in subhalos than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3% within 100 kpc, and to be below 0.1% within the Solar Circle. The inner density profiles of subhalos show no sign of converging to a fixed asymptotic slope and are well fit by gently curving profiles of Einasto form. The mean concentrations of isolated halos are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km/s, an extrapolation over some 5 orders of magnitude in mass. However, at equal maximum circular velocity, subhalos are more concentrated than field halos, with a characteristic density that is typically ~2.6 times larger and increases towards the halo centre.