The cold dark matter halo mass function is much steeper than the galaxy stellar mass function on galactic and subgalactic scales. This difference is usually reconciled by assuming that the galaxy formation efficiency drops sharply with decreasing halo mass, so that virtually no dwarf galaxies form in halos less massive than ~ 10^10 M_sun. In turn, this implies that, at any given radius, the dark mass enclosed by a galaxy must exceed a certain minimum. We use rotation curves of dwarf galaxies compiled from the literature to explore whether their enclosed mass is consistent with these constraints. We find that almost one half of the dwarfs in our sample with stellar mass between 10^6-10^7 M_sun are at odds with this restriction: either they live in halos with masses substantially below 10^10 M_sun or there is a mechanism capable of reducing the dark mass enclosed by some of the faintest dwarfs. Neither possibility is easily accommodated within the standard LCDM scenario. Extending galaxy formation to halos well below 10^10 M_sun would lead to large numbers of dwarf galaxies in excess of current estimates; at the same time, the extremely low stellar mass of the systems involved makes it unlikely that baryonic effects can reduce their dark matter content. Resolving this challenge seems to require new insights into dwarf galaxy formation, or perhaps a radical revision of the prevailing paradigm.
We study the distribution of intragroup dark matter in small groups of dark matter halos of galaxy-like size in a LCDM cosmology. These groups are identified using a physical criterion and may be an appropriate representation of small galaxy groups. We quantify the amount of intragroup dark matter and characterize its distribution. We find that compact associations of halos, as well as those intermediate and loose groups, have rather flat intragroup dark matter profiles with logarithmic slopes of gamma ~0 and ~-0.2$, respectively. Hence, the intragroup dark matter of these halo systems does no…
$Λ$CDM-model predicts an excess of dark halos compared to the observations. The excess is seen from the estimates of the virialized mass inside the Local Supercluster and its surroundings. It is shown that account for cosmological velocity fluctuations in the process of formation of the population of dark halos it is possible to eliminate this contradiction remaining within the framework of the $Λ$CDM cosmology. Based on the formalism of Press and Schechter, we suggest a model of formation of dark halo population which takes into account the kinematic effects in the dark matter. The model allo…
In this paper, we examine in detail the key structural properties of high redshift dark matter haloes as a function of their spin parameter. We perform and analyze high resolution cosmological simulations of the formation of structure in a LCDM Universe. We study the mass function, ellipticities, shapes, density profiles, rotation curves and virialization for a large sample of dark matter haloes from z = 15 - 6. We also present detailed convergence tests for individual haloes. We find that high spin haloes have stronger clustering strengths (up to 25%) at all mass and redshift ranges at these …
We present an adaptation of the standard scenario of disk-galaxy formation to the concordant LCDM cosmology aimed to derive analytical expressions for the scale length and rotation speed of present-day disks that form within four different, cosmologically motivated protogalactic dark matter halo-density profiles. We invoke a standard galaxy-formation model that includes virial equilibrium of spherical dark halos, specific angular momentum conservation during gas cooling, and adiabatic halo response to the gas inflow. The mean mass-fraction and mass-to-light ratio of the central stellar disk ar…
The emptiness of the Local Void has been put forward as a serious challenge to the current standard paradigm of structure formation in LCDM. We use a high resolution cosmological N-body simulation, the Millennium-II run, combined with a sophisticated semi-analytic galaxy formation model, to explore statistically whether the local void is allowed within our current knowledge of galaxy formation in LCDM. We find that about 15 percent of the Local Group analogue systems (11 of 77) in our simulation are associated with nearby low density regions having size and 'emptiness' similar to those of the …
We study the mass and redshift dependence of the concentration parameter in Nbody simulations spanning masses from $10^{10} \hMsun$ to $10^{15} \hMsun$ and redshifts from 0 to 2. We present a series of fitting formulas that accurately describe the time evolution of the concentration-mass relation since z=2. Using arguments based on the spherical collapse model we study the behaviour of the scale length of the density profile during the assembly history of haloes, obtaining physical insights on the origin of the observed time evolution of the concentration mass relation. We present preliminary …
We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in th…
As part of our current programme to test LCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity …
We study the concentration of dark matter halos and its evolution in N-body simulations of the standard LCDM cosmology. The results presented in this paper are based on 4 large N-body simulations with about 10 billion particles each: the Millennium-I and II, Bolshoi, and MultiDark simulations. The MultiDark (or BigBolshoi) simulation is introduced in this paper. This suite of simulations with high mass resolution over a large volume allows us to compute with unprecedented accuracy the concentration over a large range of scales (about six orders of magnitude in mass), which constitutes the stat…
New statistical properties of dark matter halos in Lagrangian space are presented. Tracing back the dark matter particles constituting bound halos resolved in a series of N-body simulations, we measure quantitatively the correlations of the proto-halo's inertia tensors with the local tidal tensors and investigate how the correlation strength depends on the proto-halo's sphericity, local density and filtering scale. It is shown that the majority of the proto-halos exhibit strong correlations between the two tensors provided that the tidal field is smoothed on the proto-halo's mass scale. The co…
We present a detailed study of the redshift evolution of dark matter halo structural parameters in a LambdaCDM cosmology. We study the mass and redshift dependence of the concentration, shape and spin parameter in Nbody simulations spanning masses from 10^{10} Msun/h to 10^{15} Msun/h and redshifts from 0 to 2. We present a series of fitting formulas that accurately describe the time evolution of the concentration-mass relation since z=2. Using arguments based on the spherical collapse model we study the behaviour of the scale length of the density profile during the assembly history of haloes…
We study some effects the inclusion of the radiation energy component in the universe, Omega_r, can have on several quantities of interest for the large-scale structure of the universe in a LCDM cosmological simulation; started at a very high redshift (z=500). In particular we compute the power spectrum density, the halo mass function, and the concentration-mass relation for haloes. We find that Omega_r has an important contribution in the long-term nonlinear evolution of structures in the universe. For instance, a lower matter density power, by approx 50%, in all scales is obtained when compa…
The mass aggregation and merger histories of present-day distinct haloes selected from the cosmological Millennium Simulations I and II are mapped into stellar mass aggregation and galaxy merger histories of central galaxies by using empirical stellar-to-halo and stellar-to-gas mass relations. The growth of bulges driven by the galaxy mergers/interactions is calculated using dynamical prescriptions. The predicted bulge demographics at redshift z~0 is consistent with observations (Zavala+2012). Here we present the evolution of the morphological mix (traced by the bulge-to-total mass ratio, B/T)…
We present the first and so far the only simulations to follow the fine-grained phase-space structure of galaxy haloes formed from generic LCDM initial conditions. We integrate the geodesic deviation equation in tandem with the N-body equations of motion, demonstrating that this can produce numerically converged results for the properties of fine-grained phase-space streams and their associated caustics, even in the inner regions of haloes. Our effective resolution for such structures is many orders of magnitude better than achieved by conventional techniques on even the largest simulations. W…
The growth of structure probes the re-ionization history and quasar abundance in the Universe, constituting an important probe of the cosmological predictions. Halos are not directly observable, however, so their mass and evolution must be inferred indirectly. Studies based on the assumption of a constant halo to stellar mass ratio M_h/M_* (extrapolated from z<4) reveal significant tension with LCDM---a failure known as "The Impossibly Early Galaxy Problem". But whether this ratio evolves or remains constant through redshift 4<z<10 is still being debated. To eliminate the tension with LCDM, it…
We compare the half-light circular velocities, V_{1/2}, of dwarf galaxies in the Local Group to the predicted circular velocity curves of galaxies in the NIHAO suite of LCDM simulations. We use a subset of 34 simulations in which the central galaxy has a stellar luminosity in the range 0.5 x 10^5 < L_V < 2 x 10^8 L_{sun}. The NIHAO galaxy simulations reproduce the relation between stellar mass and halo mass from abundance matching, as well as the observed half-light size vs luminosity relation. The corresponding dissipationless simulations over-predict the V_{1/2}, recovering the problem known…
The flux-ratio anomalies observed in multiply-lensed quasar images are most plausibly explained as the result of perturbing structures superposed on the underlying smooth matter distribution of the primary lens. The cold dark matter cosmological model predicts that a large number of substructures should survive inside larger halos but, surprisingly, this population alone has been shown to be insufficient to explain the observed distribution of the flux ratios of quasar's multiple images. Other halos (and their own subhalos) projected along the line of sight to the primary lens have been consid…
In this second paper we present the first Nbody cosmological simulations of strongly coupled Dark Energy models (SCDEW), a class of models that alleviates theoretical issues related to the nature of dark energy. SCDEW models assume a strong coupling between Dark Energy (DE) and an ancillary Cold Dark Matter (CDM) component together with the presence of an uncoupled Warm Dark Matter component. The strong coupling between CDM and DE allows us to preserve small scale fluctuations even if the warm particle is quite light ($\approx 100$ eV). Our large scale simulations show that, for $10^{11}<M/M_\…
Galaxies and their dark-matter halos have posed several challenges to the Dark Energy plus Cold Dark Matter (LCDM) cosmological model. These discrepancies between observations and theory intensify for the lowest-mass (`dwarf') galaxies. LCDM predictions for the number, spatial distribution, and internal structure of low-mass dark-matter halos have historically been at odds with observed dwarf galaxies, but this is partially expected, because many predictions modeled only the dark-matter component. Any robust LCDM prediction must include, hand-in-hand, a model for galaxy formation to understand…
It has long been regarded as difficult for a cosmological model to account simultaneously for the galaxy luminosity, mass, and velocity distributions. We revisit this issue using a modern compilation of observational data along with the best available large-scale cosmological simulation of dark matter. We find that the standard cosmological model, used in conjunction with halo abundance matching (HAM) and simple dynamical corrections, fits all basic statistics of galaxies with circular velocities Vcirc > 80 km/s. Our observational constraint is the luminosity-velocity relation which allows all…
The Lambda Cold Dark Matter (LCDM) paradigm makes specific predictions for the abundance, structure, substructure and clustering of dark matter halos, the sites of galaxy formation. These predictions can be directly tested, in the low-mass halo regime, by dark matter-dominated dwarf galaxies. A number of potential challenges to LCDM have been identified when confronting the expected properties of dwarfs with observation. I review our understanding of a few of these issues, including the `missing satellites' and the `too-big-to-fail' problems, and argue that neither poses an insurmountable chal…
In this paper, we analysed the halos and voids properties of a GR-based N-body simulation carried out at redshifts z= 0.0 and z= 0.8 as differences between dynamical dark energy models (namely PL and CPL) with respect to LCDM. Analysing the halos demonstrates that both models, PL and CPL, behave like LCDM, despite the velocity dispersion of halos was more sensitive to the dynamical dark energy model. In addition, a void finder was developed to extract the properties of voids from simulated data. Further statistical model on voids confirms that the PL model produces larger voids. In summary, ou…
We obtained ESI/Keck rotation curves of 10 MgII absorption selected galaxies (0.3 < z < 1.0) for which we have WFPC-2/HST images and high resolution HIRES/Keck and UVES/VLT quasar spectra of the MgII absorption profiles. We perform a kinematic comparison of these galaxies and their associated halo MgII absorption. For all 10 galaxies, the majority of the absorption velocities lie in the range of the observed galaxy rotation velocities. In 7/10 cases, the absorption velocities reside fully to one side of the galaxy systemic velocity and usually align with one arm of the rotation curve. In all c…
We present a new proxy for the overdensity peak height for which the large-scale clustering of haloes of a given mass does not vary significantly with the assembly history. The peak height, usually taken to be well represented by the virial mass, can instead be approximated by the mass inside spheres of different radii, which in some cases can be larger than the virial radius and therefore include mass outside the individual host halo. The sphere radii are defined as r = $a$ delta_t + $b$ log_10(M_vir/M_nl), where delta_t is the age relative to the typical age of galaxies hosted by haloes with…
We use a volume-limited spectroscopic sample of isolated galaxies in the Sloan Digital Sky Survey (SDSS) to investigate the frequency and radial distribution of luminous (M_r <~ -18.3) satellites like the Large Magellanic Cloud (LMC) around ~L* Milky Way analogs and compare our results object-by-object to LCDM predictions based on abundance matching in simulations. We show that 12% of Milky Way-like galaxies host an LMC-like satellite within 75 kpc (projected), and 42 % within 250 kpc (projected). This implies ~10% have a satellite within the distance of the LMC, and ~40% of L* galaxies host a…
We map the neutral atomic gas content of M33 using high resolution VLA and GBT observations and fit a tilted ring model to determine the orientation of the extended gaseous disk and its rotation curve. The disk of M33 warps from 8 kpc outwards without substantial change of its inclination with respect to the line of sight. Rotational velocities rise steeply with radius in the inner disk, reaching 100 km/s in 4 kpc, then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130 km/s out to 23 kpc. We derive the stellar mass surface density map of M33's optical dis…
We use the Millennium I and II cosmological simulations to revisit the impact of mergers in the growth of bulges in central galaxies in the LCDM scenario. We seed galaxies within the growing CDM haloes using semi-empirical relations to assign stellar and gaseous masses, and an analytic treatment to estimate the transfer of stellar mass to the bulge of the remnant after a galaxy merger. We find that this model roughly reproduces the observed correlation between the bulge-to-total (B/T) mass ratio and stellar mass in present-day central galaxies as well as their observed demographics, although l…
The standard cosmological paradigm currently lacks a detailed account of physics in the dark sector, the dark matter and energy that dominate cosmic evolution. In this paper, we consider the distinguishing factors between three alternative models - warm dark matter, quintessence and coupled dark matter-energy - and \lcdm{} through numerical simulations of cosmological structure formation. Key halo statistics - halo spin/velocity alignment between large-scale structure and neighboring haloes, halo formation time and migration - were compared across cosmologies within the redshift range 0$\leq$$…
The emerging empirical picture of galaxy stellar mass (Ms) assembly shows that galaxy population buildup proceeds from top to down in Ms. By connecting galaxies to LCDM halos and their histories, individual (average) Ms growth tracks can be inferred. These tracks show that massive galaxies assembled their Ms the earlier the more massive the halo, and that less massive galaxies are yet actively growing in Ms, the more active the less massive is the halo. The predicted star formation rates as a function of mass and the downsizing of the typical mass that separate active galaxies from the passive…
We investigate the structural properties, distribution and abundance of LCDM dark matter subhaloes using the Phi-4096 and Uchuu suite of N-body cosmological simulations. Thanks to the combination of their large volume, high mass resolution and superb statistics, we are able to quantify -- for the first time consistently over more than seven decades in ratio of subhalo-to-host-halo mass -- dependencies of subhalo properties with mass, maximum circular velocity, Vmax, host halo mass and distance to host halo centre. We also dissect the evolution of these dependencies over cosmic time. We provide…
We investigate the internal structure and density profiles of halos of mass $10^{10}-10^{14}~M_\odot$ in the Evolution and Assembly of Galaxies and their Environment (EAGLE) simulations. These follow the formation of galaxies in a $Λ$CDM Universe and include a treatment of the baryon physics thought to be relevant. The EAGLE simulations reproduce the observed present-day galaxy stellar mass function, as well as many other properties of the galaxy population as a function of time. We find significant differences between the masses of halos in the EAGLE simulations and in simulations that follow…
We study the abundance of satellite galaxies as a function of primary stellar mass using the SDSS/DR7 spectroscopic catalogue. In contrast with previous studies, which focussed mainly on bright primaries, our central galaxies span a wide range of stellar mass, 10^7.5 < M_*^pri/M_sun < 10^11, from dwarfs to central cluster galaxies. Our analysis confirms that the average number of satellites around bright primaries, when expressed in terms of satellite-to-primary stellar mass ratio (m_*^sat/M_*^pri), is a strong function of M_*^pri. On the other hand, satellite abundance is largely independent …
We study the $z=0$ properties of clusters (and large groups) of galaxies within the context of interacting and non-interacting quintessence cosmological models, using a series of adiabatic SPH simulations. Initially, we examine the average properties of groups and clusters, quantifying their differences in LCDM, uncoupled Dark Energy (\ude) and coupled Dark Energy (\cde) cosmologies. In particular, we focus upon radial profiles of the gas density, temperature and pressure, and we also investigate how the standard hydrodynamic equilibrium hypothesis holds in quintessence cosmologies. While we a…
Can the standard cosmological model be questioned on the basis of a single observed extreme galaxy cluster? Usually, the word extreme refers directly to cluster mass, which is not a direct observable and thus subject to substantial uncertainty. Hence, it is desirable to extend studies of extreme clusters to direct observables, such as the Einstein radius (ER). We aim to evaluate the occurrence probability of the large observed ER of MACS J0717.5 within the standard LCDM cosmology. In particular, we want to model the distribution function of the single largest ER in a given cosmological volume …
We compare the kinematics of M31's satellite galaxies to the mass profiles of the subhaloes they are expected to inhabit in LCDM. We consider the most massive subhaloes of an approximately M31-sized halo, following the assumption of a monotonic galaxy luminosity-to-subhalo mass mapping. While this abundance matching relation is consistent with the kinematic data for galaxies down to the luminosity of the bright satellites of the Milky Way and M31, it is not consistent with kinematic data for fainter dwarf galaxies (those with L <~ 10^8 Lsun). Comparing the kinematics of M31's dwarf Spheroidal …
We estimate the expected distribution of displacements between the two dominant dark matter (DM) peaks (DM-DM displacements) and between DM and gaseous baryon peak (DM-gas displacements) in dark matter halos with masses larger than $10^{13}$ Msun/h. We use as a benchmark the observation of SL2S J08544-0121, which is the lowest mass system ($1.0\times 10^{14}$ Msun/h) observed so far featuring a bi-modal dark matter distribution with a dislocated gas component. We find that $(50 \pm 10)$% of the dark matter halos with circular velocities in the range 300 km/s to 700 km/s (groups) show DM-DM dis…
We examine the present-day total stellar-to-halo mass (SHM) ratio as a function of halo mass for a new sample of simulated field galaxies using fully cosmological, LCDM, high resolution SPH + N-Body simulations.These simulations include an explicit treatment of metal line cooling, dust and self-shielding, H2 based star formation and supernova driven gas outflows. The 18 simulated halos have masses ranging from a few times 10^8 to nearly 10^12 solar masses. At z=0 our simulated galaxies have a baryon content and morphology typical of field galaxies. Over a stellar mass range of 2.2 x 10^3 to 4.…
We analyse the impact of galaxy--halo misalignment on the ability of weak lensing studies to constrain the shape of dark matter haloes, using a combination of the Millennium dark matter N-body simulation and different semi-analytic galaxy formation models, as well as simpler Monte Carlo tests. Since the distribution of galaxy--halo alignments is not known in detail, we test various alignment models, together with different methods of determining the halo shape. In addition to alignment, we examine the interplay of halo mass and shape, and galaxy colour and morphology with the resulting stacked…
We present mass models for the dark matter component of seven dwarf galaxies taken from "The HI Nearby Galaxy Survey" (THINGS) and compare these with those from numerical Lambda Cold Dark Matter (LCDM) simulations. The THINGS high-resolution data significantly reduce observational uncertainties and thus allow us to derive accurate dark matter distributions in these systems. We here use the bulk velocity fields when deriving the rotation curves of the galaxies. Compared to other types of velocity fields, the bulk velocity field minimizes the effect of small-scale random motions more effectively…
The velocity function derived from large scale surveys can be compared with the predictions of LCDM cosmology, by matching the measured rotation velocities Vrot of galaxies to the maximum circular velocity of dark matter (DM) halos Vmax. For Vrot<50km/s, a major discrepancy arises between the observed and LCDM velocity functions. However, the manner in which different observational measures of Vrot are associated with Vmax is not straight forward in dwarf galaxies. We instead relate galaxies to DM halos using the empirical baryon- mass to halo-mass relation, and show that different observation…
LCDM cosmological models with Early Dark Energy (EDE) have been proposed to resolve tensions between the Hubble constant H0 = 100h km/s/Mpc measured locally, giving h ~ 0.73, and H0 deduced from Planck cosmic microwave background (CMB) and other early universe measurements plus LCDM, giving h ~ 0.67. EDE models do this by adding a scalar field that temporarily adds dark energy equal to about 10% of the cosmological energy density at the end of the radiation-dominated era at redshift z ~ 3500. Here we compare linear and nonlinear predictions of a Planck-normalized LCDM model including EDE givin…
Simulations of the formation of late-type spiral galaxies in a cold dark matter LCDM universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/SPH simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Termed Eris, the simulation follows the assembly of a galaxy halo of mass Mvir=7.9x10^11 Msun with a total of N=18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supe…
The statistics of high speed satellite galaxies, as reported in the recent literature, can be a powerful diagnosis of the depth of the potential well of the host halo, and hence discriminate between competing gravitational theories. Naively one expects that high speed satellites are more common in Modified Newtonian Dynamics (MOND) than in cold dark matter (CDM) since an isolated MONDian system has an infinite potential well, while CDM halos have finite potential wells. In this \textit{Letter} we report on an initial test of this hypothesis in the context of the first generation of cosmologica…
We apply a semi-analytic galaxy formation model to two high resolution cosmological N-body simulations to investigate analogues of the Milky Way system. We select these according to observed properties of the Milky Way rather than by halo mass as in most previous work. For disk-dominated central galaxies with stellar mass (5--7) x 10d10Msun, the median host halo mass is 1.4 x 10d12Msun, with 1 sigma dispersion in the range [0.86, 3.1] x 10d12Msun, consistent with dynamical measurements of the Milky Way halo mass. For any given halo mass, the probability of hosting a Milky Way system is low, wi…
We investigate the spatial distribution of galactic satellites in high resolution simulations of structure formation in the LCDM model: the Aquarius dark matter simulations of individual halos and the Millennium II simulation of a large cosmological volume. To relate the simulations to observations of the Milky Way we use two alternative models to populate dark halos with "visible" galaxies: a semi-analytic model of galaxy formation and an abundance matching technique. We find that the radial density profile of massive satellites roughly follows that of the dark matter halo (unlike the distrib…
The mass distribution of halos, as specified by the halo mass function, is a key input for several cosmological probes. The sizes of $N$-body simulations are now such that, for the most part, results need no longer be statistics-limited, but are still subject to various systematic uncertainties. We investigate and discuss some of the reasons for these differences. Quantifying error sources and compensating for them as appropriate, we carry out a high-statistics study of dark matter halos from 67 $N$-body simulations to investigate the mass function and its evolution for a reference $Λ$CDM cosm…
We study the density profile of a dwarf halo in the decaying dark matter (DDM) cosmology, using a new algorithm that resolves halo density profiles down to the innermost $700$ pc robustly with high efficiency. Following Schwarzschild's orbit-based method, we have also developed a simplified model to calculate the DDM halo density profiles, which agree remarkably well with those from N-body simulations. Both zoom-in simulations and the simplified model reveal that dark matter decays lead to the flattening of central density and overall reduction of density in dwarf halos, and the underlying phy…
The arrangement of dwarf galaxies in a thin plane surrounding the Milky Way has been thought to contradict the prevailing cosmological model of cold dark matter in the Universe. New work suggests that this arrangement may just be a temporary alignment, bringing our galaxy back into agreement with theoretical expectations once the radial distribution of satellites is taken into account.
We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II Simulation in order to place the orbits of the Magellanic Clouds in a cosmological context. Our analysis shows that satellites massive enough to be LMC analogs are typically accreted at late times. Moreover, those that are accreted at early times and survive to the present have orbital properties that are discrepant with those observed for the LMC. The high velocity of the LMC, coupled with the dearth of unbound orbits seen in the simulation, argues that the mass of the MW's halo is unlikely to be less than 2 x 10^12…
We apply updated semi-analytic galaxy formation models simultaneously to the stored halo/subhalo merger trees of the Millennium and Millennium-II simulations. These differ by a factor of 125 in mass resolution, allowing explicit testing of resolution effects on predicted galaxy properties. We have revised the treatments of the transition between the rapid infall and cooling flow regimes of gas accretion, of the sizes of bulges and of gaseous and stellar disks, of supernova feedback, of the transition between central and satellite status as galaxies fall into larger systems, and of gas and star…
Recent studies suggest that only three of the twelve brightest satellites of the Milky Way (MW) inhabit dark matter halos with maximum circular velocity, V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM simulations of the Aquarius Project, which suggest that MW-sized halos should have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites in such massive subhalos is thus puzzling and may present a challenge to the LCDM paradigm. We note, however, that the number of massive subhalos depends sensitively on the (poorly-known) virial mass of the Milky Way…
We study the kinematics of satellites around isolated galaxies selected from the Sloan Digital Sky Survey (SDSS) spectroscopic catalog. Using a model of the phase-space density previously measured for the halos of LCDM dark matter cosmological simulations, we determine the properties of the halo mass distribution and the orbital anisotropy of the satellites as a function of the colour-based morphological type and the stellar mass of the central host galaxy. We place constraints on the halo mass and the concentration parameter of dark matter and the satellite number density profiles. We obtain …
Previous studies indicate that assembly bias effects are stronger for lower mass dark matter haloes. Here we make use of high resolution re-simulations of rich clusters and their surroundings from the Phoenix Project and a large volume cosmological simulation, the Millennium-II run, to quantify assembly bias effects on dwarf-sized dark matter haloes. We find that, in the regions around massive clusters, dwarf-sized haloes ($[10^9,10^{11}]\ms$) form earlier ($Δz \sim 2$ in redshift) and possess larger $V_{\rm max}$ ($\sim20%$) than the field galaxies. We find that this environmental dependence …
Recent studies have shown that massive elliptical galaxies have total mass density profiles within an effective radius that can be approximated as ρ_{tot}\propto r^{-γ'}, with mean slope <γ'>=2.08 \pm 0.03 and scatter σ_γ'=0.16 \pm 0.02. The small scatter of the slope (known as the bulge-halo conspiracy) is not generic in LCDM based models and therefore contains information about the galaxy formation process. We compute the distribution of γ' for LCDM-based models that reproduce the observed correlations between stellar mass, velocity dispersion, and effective radius of early-type galaxies in …
We study the formation of disk-dominated galaxies in a LCDM universe. Their existence is considered to be a challenge for the LCDM cosmology, because galaxy mergers isotropize stellar disks and trigger angular momentum transport in gas disks, thus fostering the formation of central stellar spheroids. Here, we postulate that the formation of stellar spheroids from gasrich disks is controlled by two parameters that characterize galaxy mergers, the mass ratio of merging dark matter halos, and the virial velocity of the larger merging halo. We utilize merger histories generated from realizations o…
The Baryonic Tully-Fisher Relation (BTFR) is an empirical relation between baryonic mass and rotation velocity in disk galaxies. It provides tests of galaxy formation models in LCDM and of alternative theories like MOND. Observations of gas rich galaxies provide a measure of the slope and normalization of the BTFR that is more accurate (if less precise) than that provided by star dominated spirals, as their masses are insensitive to the details of stellar population modeling. Recent independent data for such galaxies are consistent with Mb = AVf^4 with A = 47+/-6 Msun (km/s)^-4. This is equiva…
In this article we compare the halo mass function predicted by the excursion set theory with a drifting diffusive barrier against the results of N-body simulations for several cosmological models. This includes the standard LCDM case for a large range of halo masses, models with different types of primordial non-Gaussianity, and the Ratra-Peebles quintessence model of Dark Energy. We show that in all those cosmological scenarios, the abundance of dark matter halos can be described by a drifting diffusive barrier, where the two parameters describing the barrier have physical content. In the cas…
We study the imprint of non-standard dark energy (DE) and dark matter (DM) models on the 21cm intensity map power spectra from high-redshift neutral hydrogen (HI) gas. To this purpose we use halo catalogs from N-body simulations of dynamical DE models and DM scenarios which are statistically indistinguishable from the standard Cold Dark Matter model with Cosmological Constant (LCDM) using currently available cosmological observations. We limit our analysis to halo catalogs at redshift z = 1 and 2.3 which are common to all simulations. For each catalog we model the HI distribution by using a si…
