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Numerous metrics exist to quantify the dynamical state of galaxy clusters, both observationally and within simulations. Many of these correlate strongly with one another, but it is not clear whether all of these measures probe the same intrinsic properties. In this work, we use two different statistical approaches – principal component analysis (PCA) and uniform manifold approximation and projection (UMAP) – to investigate which dynamical properties of a cluster are in fact the best descriptors of its dynamical state. We use measurements taken directly from The Three Hundred suite of galaxy cluster simulations, as well as morphological properties calculated using mock X-ray and SZ maps of the same simulated clusters. We find that four descriptions of dynamical state naturally arise, and although correlations exist between these, a given cluster can be 'dynamically relaxed' according to all, none, or some of these four descriptions. These results demonstrate that it is highly important for future observational and theoretical studies to consider in which sense clusters are dynamically relaxed. Cluster dynamical states are complex and multidimensional, and so it is not meaningful to classify them simply as 'relaxed' and 'unrelaxed' based on a single linear scale. [ABSTRACT FROM AUTHOR]

Dark matter haloes grow at a rate that depends on the value of the cosmological parameters σ8 and Ωm through the initial power spectrum and the linear growth factor. While halo abundance is routinely used to constrain these parameters, through cluster abundance studies, the halo growth rate is not. In recent work, we proposed constraining the cosmological parameters using observational estimates of the overall dynamical 'age' of clusters, expressed, for instance, by their half-mass assembly redshift z 50. Here, we explore the prospects for using the instantaneous growth rate, as estimated from the halo merger rate, from the average growth rate over the last dynamical time, or from the fraction of systems with recent episodes of major growth. We show that the merger rate is mainly sensitive to the amplitude of fluctuations σ8, while the rates of recent growth provide constraints in the Ωm–σ8 plane that are almost orthogonal to those provided by abundance studies. Data collected for forthcoming cluster abundance studies, or studies of the galaxy merger rate in current and future galaxy surveys, may thus provide additional constraints on the cosmological parameters complementary to those already derived from halo abundance. [ABSTRACT FROM AUTHOR]

The faint satellites of the local Universe provide an important benchmark for our understanding of structure formation and galaxy formation, but satellite populations are hard to identify beyond the Local Group. We recently developed an iterative method to quantify satellite abundance using galaxy clustering and tested it on a local sample in the COSMOS field, where accurate photometric redshifts are available for a large number of faint objects. In this paper, we consider the properties of these satellite populations in more detail, studying the satellite stellar mass function (SSMF), the satellite-central connection, and quenching as a function of satellite and central mass and colour. Despite the limited sample size, our results show good consistency with those from much larger surveys and constrain the SSMF down to some of the lowest primary masses considered to date. We reproduce several known trends in satellite abundance and quenching, and find evidence for one new one, a dependence of the quiescent fraction on the primary-to-secondary halo mass ratio. We discuss the prospects for the clustering method in current and forthcoming surveys. [ABSTRACT FROM AUTHOR]

Accurate models of the structural evolution of dark matter subhaloes, as they orbit within larger systems, are fundamental to understanding the detailed distribution of dark matter at the present day. Numerical simulations of subhalo evolution support the idea that the mass-loss associated with tidal stripping is most naturally understood in energy space, with the particles that are the least bound being removed first. Starting from this premise, we recently proposed a zero-parameter 'energy-truncation model' for subhalo evolution. We tested this model with simulations of tidal stripping of satellites with initial NFW profiles, and showed that the energy-truncation model accurately predicts both the mass-loss and density profiles. In this work, we apply the model to a variety of Hernquist, Einasto, and King profiles. We show that it matches the simulation results quite closely in all cases, indicating that it may serve as a universal model to describe tidally stripped collision-less systems. A key prediction of the energy-truncation model is that the central density of dark matter subhaloes is conserved as they lose mass; this has important implications for dark matter annihilation calculations, and for other observational tests of dark matter. [ABSTRACT FROM AUTHOR]

We present a new method for quantifying the abundance of satellites around field galaxies and in groups. The method is designed to work with samples such as photometric redshift catalogues, that do not have full spectroscopic coverage, but for which some redshift or distance information is available. It consists of identifying the galaxies most likely to be centrals, and using the clustering signal around them as a template to iteratively decompose the full population into satellite and central populations. Thus, it is similar to performing crowded-field photometry by using isolated stars to determine the point spread function of the image. The method does not identify individual satellites or centrals conclusively, but assigns a probability to each galaxy of being one or the other. Averaged over a large sample, it provides a statistical estimate of satellite abundance, even in crowded fields with large redshift uncertainties. We test the method using data from the COSMOS field, which includes a large set of local objects with accurate photometric redshifts. We measure satellite abundance as a function of central stellar or halo mass, as well as the satellite luminosity function, and find results consistent with previous studies, but extending over a broader range of central masses. We also consider a number of possible systematic uncertainties in the method, and show that they are generally smaller than our random errors. Having presented the method in this paper, we will use it to study the properties of the satellite populations in a forthcoming one. [ABSTRACT FROM AUTHOR]

The orbital parameters of dark matter (DM) subhaloes play an essential role in determining their mass-loss rates and overall spatial distribution within a host halo. Haloes in cosmological simulations grow by a combination of relatively smooth accretion and more violent mergers, and both processes will modify subhalo orbits. To isolate the impact of the smooth growth of the host halo from other relevant mechanisms, we study subhalo orbital evolution using numerical calculations in which subhaloes are modelled as massless particles orbiting in a time-varying spherical potential. We find that the radial action of the subhalo orbit decreases over the first few orbits, indicating that the response to the growth of the host halo is not adiabatic during this phase. The subhalo orbits can shrink by a factor of ∼1.5 in this phase. Subsequently, the radial action is well conserved and orbital contraction slows down. We propose a model accurately describing the orbital evolution. Given these results, we consider the spatial distribution of the population of subhaloes identified in high-resolution cosmological simulations. We find that it is consistent with this population having been accreted at |$z \lesssim 3$|⁠ , indicating that any subhaloes accreted earlier are unresolved in the simulations. We also discuss tidal stripping as a formation scenario for NGC 1052-DF2, an ultra diffuse galaxy significantly lacking DM, and find that its expected DM mass could be consistent with observational constraints if its progenitor was accreted early enough, |$z \gtrsim 1.5$|⁠ , although it should still be a relatively rare object. [ABSTRACT FROM AUTHOR]

The ability to accurately predict the evolution of tidally stripped haloes is important for understanding galaxy formation and testing the properties of dark matter. Most studies of substructure evolution make predictions based on empirical models of tidal mass-loss that are calibrated using numerical simulations. This approach can be accurate in the cases considered, but lacks generality and does not provide a physical understanding of the processes involved. Recently, we demonstrated that truncating NFW distribution functions sharply in energy results in density profiles that resemble those of tidally stripped systems, offering a path to constructing physically motivated models of tidal mass-loss. In this work, we review calculations of mass-loss based on energy truncation alone, and then consider what secondary effects may modulate mass-loss beyond this. We find that a combination of dependence on additional orbital parameters and variations in individual particle energies over an orbit results in a less abrupt truncation in energy space as a subhalo loses mass. Combining the energy truncation approach with a simple prediction for the mass-loss rate, we construct a full model of mass-loss that can accurately predict the evolution of a subhalo in terms of a single parameter ηeff. This parameter can be fully determined from the initial orbital and halo properties, and does not require calibration with numerical simulations. [ABSTRACT FROM AUTHOR]

Kinematic studies have produced accurate measurements of the total dark matter mass and mean dark matter density within the optical extent of galaxies for large samples of objects. Here we consider theoretical predictions for the latter quantity, |$\bar{\rho }_{\rm dm}$|⁠ , measured within the isophotal radius R 23.5, for isolated haloes with universal density profiles. Through a combination of empirical scaling relations, we show that |$\bar{\rho }_{\rm dm}$| is expected to depend weakly on halo mass and strongly on redshift. When galaxy haloes fall into larger groups or clusters, they become tidally stripped, reducing their total dark matter mass, but this process is expected to preserve central density until an object is close to disruption. We confirm this with collisonless simulations of cluster formation, finding that subhaloes have values of |$\bar{\rho }_{\rm dm}$| close to the analytic predictions. This suggests that |$\bar{\rho }_{\rm dm}$| may be a useful indicator of infall redshift on to the cluster. We test this hypothesis with data from the SHIVir survey, which covers a reasonable fraction of the Virgo cluster. We find that galaxies with high |$\bar{\rho }_{\rm dm}$| do indeed trace the densest regions of the cluster, with a few notable exceptions. Samples selected by environment have higher densities at a significance of 3.5–4σ, while samples selected by density are more clustered at 3–3.5σ significance. We conclude that halo density can be a powerful tracer of the assembly history of clusters and their member galaxies. [ABSTRACT FROM AUTHOR]

We propose a new method for generating equilibrium models of spherical systems of collisionless particles that are finite in extent, but whose central regions resemble dark matter haloes from cosmological simulations. This method involves iteratively removing unbound particles from a Navarro--Frenk--White (NFW) profile truncated sharply at some radius. The resulting models are extremely stable, and thus provide a good starting point for N-body simulations of isolated haloes. We provide a code to generate such models for NFW and a variety of other common density profiles. We then develop an analytic approximation to this truncated distribution function. Our method proceeds by analogy with the King model, truncating and shifting the original distribution function of an infinitely extended NFW profile in energy space. We show that the density profiles of our models closely resemble the tidally truncated density profiles seen previously in studies of satellite evolution. Pursuing this analogy further with a series of simulations of tidal mass-loss, we find that our models provide a good approximation to the full distribution function of tidally stripped systems, thus allowing theoretically motivated phase-space calculations for such systems. [ABSTRACT FROM AUTHOR]

We present a weak lensing mass map covering ∼124 deg2 of the Canada–France–Hawaii Telescope Stripe 82 Survey (CS82). We study the statistics of rare peaks in the map, including peak abundance, the peak–peak correlation functions and the tangential-shear profiles around peaks. We find that the abundance of peaks detected in CS82 is consistent with predictions from a Λ cold dark matter cosmological model, once noise effects are properly included. The correlation functions of peaks with different signal-to-noise ratio (SNR) are well described by power laws, and there is a clear cross-correlation between the Sloan Digital Sky Survey III/Constant Mass galaxies and high SNR peaks. The tangential-shear profiles around peaks increase with peak SNR. We fit analytical models to the tangential-shear profiles, including a projected singular isothermal sphere (SIS) model and a projected Navarro, Frenk & White (NFW) model, plus a two-halo term. For the high SNR peaks, the SIS model is rejected at ∼3σ. The NFW model plus a two-halo term gives more acceptable fits to the data. Some peaks match the positions of optically detected clusters, while others are relatively dark. Comparing dark and matched peaks, we find a difference in lensing signal of a factor of 2, suggesting that about half of the dark peaks are false detections. [ABSTRACT FROM AUTHOR]

The balance of evidence indicates that individual galaxies and groups or clusters of galaxies are embedded in enormous distributions of cold, weakly interacting dark matter. These dark matter "halos" provide the scaffolding for all luminous structures in the universe, and their properties comprise an essential part of the current cosmological model. I review the internal properties of dark matter halos, focussing on the simple universal trends predicted by numerical simulations of structure formation. Simulations indicate that halos should all have roughly the same spherically averaged density profile and kinematic structure and predict simple distributions of shape, formation history, and substructure in density and kinematics, over an enormous range of halo mass and for all common variants of the concordance cosmology. I describe observational progress towards testing these predictions by measuring masses, shapes, profiles, and substructure in real halos using baryonic tracers or gravitational lensing. An important property of simulated halos (possibly the most important property) is their dynamical "age", or degree of internal relaxation. I review recent gravitational lensing studies of galaxy clusters which will measure substructure and relaxation in a large sample of individual cluster halos, producing quantitative measures of age that are well matched to theoretical predictions. [ABSTRACT FROM AUTHOR]

Diffuse 511-keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 yr. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ∼70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the Wilkinson Microwave Anisotropy Probe‘haze’) and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511-keV emission with the brightest X-ray binaries. [ABSTRACT FROM AUTHOR]

We measure the distribution of velocities for prograde and retrograde satellite galaxies using a combination of published data and new observations for 78 satellites of 63 extremely isolated disc galaxies (169 satellites total). We find that the velocity distribution is non-Gaussian (>99.9 per cent confidence), but that it can be described as the sum of two Gaussians, one of which is broad , has a mean prograde velocity of , and contains ∼55 per cent of the satellites, while the other is slightly retrograde with a mean velocity of and and contains ∼45 per cent of the satellites. Both of these components are present over all projected radii and found in the sample regardless of cuts on primary inclination or satellite disc angle. The double-Gaussian shape, however, becomes more pronounced among satellites of more luminous primaries. We remove the potential dependence of satellite velocity on primary luminosity using the Tully–Fisher relation and still find the velocity distribution to be asymmetric and even more significantly non-Gaussian. The asymmetric velocity distribution demonstrates a connection between the inner, visible disc galaxy, and the kinematics of the outer, dark halo. The reach of this connection, extending even beyond the virial radii, suggests that it is imprinted by the satellite infall pattern and large-scale effects, rather than by higher level dynamical processes in the formation of the central galaxy or late-term evolution of the satellites. [ABSTRACT FROM AUTHOR]

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of dark matter halo formation based on merger trees. In this paper, we present the basic predictions of our full model of halo formation. We first describe the overall properties of substructure in galaxy, group or cluster haloes at the present day. We then discuss the evolution of substructure, and the effect of the mass-accretion history of an individual halo on the mass function and orbital grouping of its subhalo population. We show, in particular, that the shape of the subhalo mass function is strongly correlated with the formation epoch of the halo. In a third paper in this series, we will compare the results of our semi-analytic method with the results of self-consistent numerical simulations of halo formation. [ABSTRACT FROM AUTHOR]

In a previous paper, we described a new method for including detailed information about substructure in semi-analytic models of halo formation based on merger trees. In this paper, we compare the predictions of our model with results from self-consistent numerical simulations. We find that in general the two methods agree extremely well, particularly once numerical effects and selection effects in the choice of haloes are taken into account. As expected from the original analyses of the simulations, we see some evidence for artificial overmerging in the innermost regions of the simulated haloes, either because substructure is being disrupted artificially or because the group-finding algorithms used to identify substructure are not detecting all the bound clumps in the highest-density regions. Our analytic results suggest that greater mass and force resolution may be required before numerical overmerging becomes negligible in all current applications. We discuss the implications of this result for observational and experimental tests of halo substructure, such as the analysis of discrepant magnification ratios in strongly lensed systems, terrestrial experiments to detect dark matter particles directly or indirect detection experiments searching for positrons, gamma-rays, neutrinos or other dark matter decay or annihilation products. [ABSTRACT FROM AUTHOR]

We investigate the possibility that present-day galaxies and their dark matter haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the supermassive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic discs, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host haloes. We predict the mass and abundance of MBHs in present-day galactic haloes, and also estimate the MBH mass accretion rates as well as bolometric luminosities for two accretion scenarios. MBHs that have not undergone recent merging will retain associated dark matter cusps that were enhanced by black hole accretion growth, and may be possible sources of gamma-rays via neutralino annihilations. [ABSTRACT FROM AUTHOR]

The hierarchical mergers that form the haloes of dark matter surrounding galaxies, groups and clusters are inefficient, leaving substantial amounts of dense substructure, in the form of stripped halo cores or ‘subhaloes’, orbiting within these systems. Using a semi-analytic model of satellite dynamics, we study the evolution of haloes as they merge hierarchically, to determine how much substructure survives merging and how the properties of individual subhaloes change over time. We find that subhaloes evolve, owing to mass loss, orbital decay and tidal disruption, on a characteristic time-scale equal to the period of radial oscillations at the virial radius of the system. Subhaloes with realistic densities and density profiles lose 25–45 per cent of their mass per pericentric passage, depending on their concentration and on the circularity of their orbit. As the halo grows, the subhalo orbits also grow in size and become less bound. Based on these general patterns, we suggest a method for including realistic amounts of substructure in semi-analytic models based on merger trees. We show that the parameters in the resulting model can be fixed by requiring self-consistency between different levels of the merger hierarchy. In a companion paper, we will compare the results of our model with numerical simulations of halo formation. [ABSTRACT FROM AUTHOR]

Hot gas trapped in a dark matter halo will produce a decrement in the surface brightness of the microwave background, the Sunyaev–Zel'dovich (SZ) effect. While massive clusters produce the strongest central SZ decrements, we point out that a local galaxy halo, specifically the halo of M31, may be one of the brightest integrated SZ sources in the sky. For various realistic gas distributions consistent with current X-ray limits, we show that the integrated SZ decrement from M31 will be comparable to decrements already detected in more distant sources, provided its halo contains an appreciable quantity of hot gas. A measurement of this decrement would provide direct information on the mass, spatial distribution and thermodynamic state of hot gas in a low-mass halo, and could place important constraints on current models of galaxy formation. Detecting such an extended (∼10°), low-amplitude signal will be challenging, but should be possible with all-sky SZ maps from satellite missions such as the Wilkinson Microwave Anisotropy Probe or the Planck Surveyor. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine the expected signal from annihilation events in realistic cold dark matter haloes. If the weakly interacting massive particle is a neutralino, with an annihilation cross-section predicted in minimal supersymmetry models for the lightest stable relic particle, the central cusps and dense substructure seen in simulated haloes may produce a substantial flux of energetic gamma-rays. We derive expressions for the relative flux from such events in simple haloes with various density profiles, and use these to calculate the relative flux produced within a large volume as a function of redshift. This flux peaks when the first haloes collapse, but then declines as small haloes merge into larger systems of lower density. Simulations show that haloes contain a substantial amount of dense substructure, left over from the incomplete disruption of smaller haloes as they merge together. We calculate the additional contribution to the flux caused by this substructure, and show that it can increase the annihilation signal substantially. Overall, the present-day flux from annihilation events may be an order of magnitude larger than predicted by previous calculations. We discuss the implications of these results for current and future gamma-ray experiments. [ABSTRACT FROM AUTHOR]

The number and distribution of dwarf satellite galaxies remain a critical test of cold dark matter-dominated structure formation on small scales. Until recently, observational information about galaxy formation on these scales has been limited mainly to the Local Group. We have searched for faint analogues of Local Group dwarfs around nearby bright galaxies, using a spatial clustering analysis of the photometric catalog of the Sloan Digital Sky Survey (SDSS) Data Release 8. Several other recent searches of SDSS have detected clustered satellite populations down to Δmr ≡ (mr, sat – mr, main) ∼ 6-8, using photometric redshifts to reduce background contamination. SDSS photometric redshifts are relatively imprecise, however, for faint and nearby galaxies. Instead, we use angular size to select potential nearby dwarfs and consider only the nearest isolated bright galaxies as primaries. As a result, we are able to detect an excess clustering signal from companions down to Δmr = 12, 4 mag fainter than most recent studies. We detect an overdensity of objects at separations <400 kpc, corresponding to about 4.6 ± 0.5 satellites per central galaxy, consistent with the satellite abundance expected from the Local Group, given our selection function. Although the sample of satellites detected is incomplete by construction, since it excludes the least and most compact dwarfs, this detection provides a lower bound on the average satellite luminosity function, down to luminosities corresponding to the faintest “classical” dwarfs of the Local Group. [ABSTRACT FROM AUTHOR]

We have performed a search for faint companions around the nearest massive galaxies. We see a clear signature of clustering of faint objects, both in projected separation and in velocity offset. The inferred satellite luminosity functions confirm that the abundance of faint satellites seen in the Local Group is typical of other nearby systems. [ABSTRACT FROM PUBLISHER]

Individual dark matter halos in cosmological simulations vary widely in their detailed structural properties, properties such as concentration, shape, spin, and degree of internal relaxation. Recent non-parametric (principal component) analyses suggest that a few principal components explain a large fraction of the scatter in these structural properties. The main principal component is closely aligned with concentration, which in turn is known to be related to the mass accretion history (MAH) of the halo, as described by its merger tree. Here, we examine more generally the connection between the MAH and structural parameters. The space of mass accretion histories has principal components of its own. The strongest, accounting for almost 60% of the scatter between individual histories, can be interpreted as the age of the system. We give an analytic fit for this first component, which provides a rigorous way of defining the dynamical age of a halo. The second strongest component, representing acceleration or deceleration of growth at late times, accounts for 25% of the scatter. Relating structural parameters to formation history, we find that concentration correlates strongly with the early history of the halo, while shape and degree of relaxation or dynamical equilibrium correlate with the later history. We examine the inferences about formation history that can be drawn by splitting halos into sub-samples based on observable properties such as concentration and shape. Applications include the definition young and old samples of galaxy clusters in a quantitative way, or empirical tests of environmental processing rates in clusters. [ABSTRACT FROM AUTHOR]