Halo Angular Momentum Research Articles

Importance of Initial Condition on Bar Secular Evolution: Role of Halo Angular Momentum Distribution Discontinuity

The dark matter halo properties, for example, mass, spin, and concentration, play a significant role in the formation and evolution of bars in disk galaxies. This study highlights the importance of a new parameter: the dark matter halo angular momentum distribution in the disk’s central region. We experiment with N-body galaxy models having a disk and dark matter similar to Milky Way–type galaxies. In these models, we vary the discontinuity of the angular momentum distribution of the halo (the total spin is the same for all models). Our N-body experiments suggest that bar forms in all models after a few Gyr of disk evolution. However, in the secular evolution of the bar, as we evolve these models until 9.78 Gyr, the bar gains its strength in the model with the most continuous halo angular momentum distribution, and the bar loses strength for the most discontinuous halo angular momentum distribution. The secular evolution of the bar suggests that box/peanut/x-shaped bulges similar to those found in the Milky Way disk should be more pronounced in halos with continuous halo angular momentum distributions. This study demonstrates the importance of the initial condition setup of galaxy systems, namely the discontinuity in the dark matter halo angular momentum distribution for a given density distribution, on the bar secular evolution in the disk galaxy simulations. Further, this study helps reconcile the conflicting results of bar secular evolution in a high-spinning halo of the recent literature.

Stellar mass dependence of galaxy size–dark matter halo radius relation probed by Subaru-HSC survey weak lensing measurements

ABSTRACT We investigate the stellar mass dependence of the galaxy size–dark matter halo radius relation for low-redshift galaxies using weak gravitational lensing measurements. Our sample consists of ∼38 000 galaxies more massive than $10^{8}\, {\rm M}_{\odot }\, h^{-2}$ and within z < 0.3 drawn from the overlap of Galaxy And Mass Assembly survey data release 4 and HSC-SSP PDR2. We divide our sample into a number of stellar mass bins and measure stacked weak lensing signals. We model the signals using a conditional stellar mass function to infer the stellar mass–halo mass relation. We fit a single Sérsic model to HSC i-band images of our galaxies and obtain their three-dimensional half-light radii. We use these measurements to construct a median galaxy size–mass relation. We then combine these relations to infer the galaxy size–halo radius relation. We confirm that this relation appears linear, given the statistical errors, i.e. the ratio of galaxy size to halo radius remains constant over two orders of magnitudes in stellar mass above $\sim 10^{9} \, {\rm M}_{\odot }\, h^{-2}$. Extrapolating the stellar mass–halo mass relation below this limit, we see an indication of a decreasing galaxy size–halo radius ratio with the decline in stellar mass. At stellar mass $\sim 10^{8} \, {\rm M}_{\odot }\, h^{-2}$, the ratio becomes 30 per cent smaller than its value in linear regime. The possible existence of a such trend in dwarf galaxy sectors calls for either modification in models employing a constant fraction of halo angular momentum transferred to explain sizes of dwarfs or else points towards our lack of knowledge about dark matter haloes of low-mass galaxies.

The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The specific angular momentum of ETGs

Context. Mass and angular momentum are key parameters of galaxies. Their coevolution establishes an empirical relation between the specific stellar angular momentum j* and the stellar mass M* that depends on morphology. Aims. For this work, we measured j* in a sample of 32 early-type galaxies (ETGs) from the ePN.S survey, using the full two-dimensional kinematic information. We present local λ profiles and projected j* profiles in apertures. We derived the distribution of these galaxies on the total j* − M* plane and determined the ratio between the stellar j* and the specific angular momentum of the host dark matter halo. Methods. We used integral-field-spectroscopic data in the central regions (one to two effective radii, Re) and planetary nebula (PN) kinematics in the outskirts (out to a mean 6Re). In the j* determination, we accounted for misaligned rotation and for the differences between light-weighted j* and mass-weighted j*, estimating also the effects of gradients in the mass-to-light ratio driven by variations in the initial mass function. We used simulated ETGs from the IllustrisTNG simulation TNG100 to correct for the limited radial coverage of the PN data and to account for projection effects on j*. Results. The radially extended, two-dimensional kinematic data show that the stellar halos of ETGs do not contain large stellar mass fractions of high j*. The j* profiles of fast-rotator ETGs are largely converged within the range of the data. For slow rotators, j* is still rising and is estimated to increase beyond 6Re by up to 40%, using simulated galaxies from TNG100. More than 60% of their stellar halo angular momentum is in misaligned rotation. We find that the ePN.S ETG sample displays the well-known correlation between j*, M*, and morphology: elliptical galaxies have systematically lower j* than similar mass S0 galaxies. However, fast and slow rotators lie on the same relation within errors with the slow rotators falling at the high M* end. A power-law fit to the mass-weighted j* − M* relation gives a slope of 0.55 ± 0.17 for the S0s and 0.76 ± 0.23 for the ellipticals, with normalization about four and nine times lower than spirals, respectively. The estimated retained fraction of angular momentum at 1010 ≤ M* ≤ 1010.5 M⊙ is ∼25% for S0s and > 10% for ellipticals, and decreases by ∼1.5 orders of magnitude at M* ∼ 1012 M⊙. Conclusions. Our results show that ETGs have substantially lower j* than spiral galaxies with similar M*. Their j* must be lost during their evolution, and/or retained in the hot gas component and the satellite galaxies that have not yet merged with the central galaxy.

Effects of Inner Halo Angular Momentum on the Peanut/X Shapes of Bars

Cosmological simulations show that dark matter halos surrounding baryonic disks have a wide range of angular momenta, which is measured by the spin parameter (λ). In this study, we bring out the importance of inner angular momentum (<30 kpc), measured in terms of the halo spin parameter, on the secular evolution of the bar using N-body simulations. We have varied the halo spin parameter λ from 0 to 0.1, for corotating (prograde) spinning halos and one counterrotating (retrograde) halo spin (λ = −0.1) with respect to the disk. We report that as the halo spin increases the buckling is also triggered earlier and is followed by a second buckling phase in high-spin halo models. The timescale for the second buckling is significantly longer than the first buckling. We find that bar strength does not reduce significantly after the buckling in all of our models, which provides new insights about the role of inner halo angular momentum unlike previous studies. Also, the buckled bar can still transfer significant angular momentum to the halo in the secular evolution phase, but it reduces with increasing halo spin. In the secular evolution phase, the bar strength increases and saturates to nearly equal values for all the models irrespective of halo spin and the sense of rotation with respect to the disk. The final boxy/peanut shape is more pronounced (∼20%) in high-spin halos having higher angular momentum in the inner region compared to nonrotating halos. We explain our results with angular momentum exchanges between the disk and halo.

The galaxy–halo size relation of low-mass galaxies in FIRE

ABSTRACT Galaxy sizes correlate closely with the sizes of their parent dark matter haloes, suggesting a link between halo formation and galaxy growth. However, the precise nature of this relation and its scatter remains to be understood fully, especially for low-mass galaxies. We analyse the galaxy–halo size relation (GHSR) for low-mass ($M_\star \sim 10^{7-9}\, {\rm M}_\odot$) central galaxies over the past 12.5 billion years with the help of cosmological volume simulations (FIREbox) from the Feedback in Realistic Environments (FIRE) project. We find a nearly linear relationship between the half-stellar mass galaxy size R1/2 and the parent dark matter halo virial radius Rvir. This relation evolves only weakly since redshift z = 5: $R_{1/2}\, [{\rm kpc}] = (0.053\pm 0.002)(R_{\rm vir}/35\, {\rm kpc})^{0.934\pm 0.054}$, with a nearly constant scatter $\langle \sigma \rangle = 0.084\, [{\rm dex}]$. While this ratio is similar to what is expected from models where galaxy disc sizes are set by halo angular momentum, the low-mass galaxies in our sample are not angular momentum supported, with stellar rotational to circular velocity ratios vrot/vcirc ∼ 0.15. Introducing redshift as another parameter to the GHSR does not decrease the scatter. Furthermore, this scatter does not correlate with any of the halo properties we investigate – including spin and concentration – suggesting that baryonic processes and feedback physics are instead critical in setting the scatter in the GHSR. Given the relatively small scatter and the weak dependence of the GHSR on redshift and halo properties for these low-mass central galaxies, we propose using galaxy sizes as an independent method from stellar masses to infer halo masses.

A sparse regression approach to modelling the relation between galaxy stellar masses and their host haloes

ABSTRACT Sparse regression algorithms have been proposed as the appropriate framework to model the governing equations of a system from data, without needing prior knowledge of the underlying physics. In this work, we use sparse regression to build an accurate and explainable model of the stellar mass of central galaxies given properties of their host dark matter (DM) halo. Our data set comprises 9521 central galaxies from the EAGLE hydrodynamic simulation. By matching the host haloes to a DM-only simulation, we collect the halo mass and specific angular momentum at present time and for their main progenitors in 10 redshift bins from z = 0 to z = 4. The principal component of our governing equation is a third-order polynomial of the host halo mass, which models the stellar-mass–halo-mass relation. The scatter about this relation is driven by the halo mass evolution and is captured by second- and third-order correlations of the halo mass evolution with the present halo mass. An advantage of sparse regression approaches is that unnecessary terms are removed. Although we include information on halo specific angular momentum, these parameters are discarded by our methodology. This suggests that halo angular momentum has little connection to galaxy formation efficiency. Our model has a root mean square error (RMSE) of 0.167log10(M*/M⊙), and accurately reproduces both the stellar mass function and central galaxy correlation function of EAGLE. The methodology appears to be an encouraging approach for populating the haloes of DM-only simulations with galaxies, and we discuss the next steps that are required.

The Effect of Massive Neutrinos on the Halo Spin Flip Phenomenon

Abstract The halo spin flip refers to the phenomenon that the spin axes of dark matter halos with masses above a certain threshold tend to be preferentially aligned perpendicular to the hosting large-scale filaments, while low-mass halos tend to have their spin axes aligned parallel to such structures. Extensive work has so far been conducted to understand this phenomenon under the assumption of cold dark matter and suggested that its origin should be closely related to the nonlinear evolution of the halo angular momentum in the anisotropic cosmic web. We present, for the first time, a numerical examination of this phenomenon assuming the presence of massive neutrinos, finding a clear and robust dependence of the threshold mass for the spin flip on the total neutrino mass. Our physical explanation is that the presence of more massive neutrinos retard the nonlinear evolution of the cosmic web, which in turn allows the halo spin vectors to better retain their memories of the initial tidal interactions in the nonlinear regime. Our finding implies that the statistical alignment of halo spins with the large-scale structures can be in principle used as a probe of the total neutrino mass.

The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals

In a Λ CDM Universe, the specific stellar angular momentum (j*) and stellar mass (M*) of a galaxy are correlated as a consequence of the scaling existing for dark matter haloes (jh ∝2∕3). The shape of this law is crucial to test galaxy formation models, which are currently discrepant especially at the lowest masses, allowing to constrain fundamental parameters, such as, for example, the retained fraction of angular momentum. In this study, we accurately determine the empirical j*−M* relation (Fall relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the unprecedented mass range 7 ≲ log M*∕M⊙≲ 11.5. We significantly improve all previous estimates of the Fall relation by determining j* profiles homogeneously for all galaxies, using extended HI rotation curves, and selecting only galaxies for which a robust j* could be measured (converged j*(&lt;R) radial profile). We find the relation to be well described by a single, unbroken power-law j* α M*α over the entire mass range, with α = 0.55 ± 0.02 and orthogonal intrinsic scatter of 0.17 ± 0.01 dex. We finally discuss some implications of this fundamental scaling law for galaxy formation models and, in particular, the fact that it excludes models in which discs of all masses retain the same fraction of the halo angular momentum.

Angular Momentum Evolution of Stellar Disks at High Redshifts

Abstract The stellar disk size of a galaxy depends on the ratio of the disk stellar mass to the halo mass, m ⋆ ≡ M ⋆/M dh, and the fraction of the dark halo angular momentum transferred to the stellar disk, j ⋆ ≡ J ⋆/J dh. Since m ⋆ and j ⋆ are determined by many star formation–related processes, measuring j ⋆ and m ⋆ at various redshifts is essential to understanding the formation history of disk galaxies. We use the 3D-HST GOODS-S, COSMOS, and AEGIS imaging data and photo-z catalog to examine j ⋆ and m ⋆ for star-forming galaxies at z ∼ 2–4, when disks are actively forming. We find that the j ⋆/m ⋆ ratio is ≃0.77 ± 0.06 for all three redshifts over the entire mass range examined, 8 × 1010 &lt; M dh/h −1 M ⊙ &lt; 2 × 1012, with a possible (&lt;30%) decrease with mass. This high ratio is close to those of local disk galaxies, descendants of our galaxies in terms of M dh growth, implying a nearly constant j ⋆/m ⋆ over the past 12 Gyr. These results are remarkable because mechanisms controlling angular momentum transfer to disks such as inflows and feedback depend on both cosmic time and halo mass, and, indeed, theoretical studies tend to predict j ⋆/m ⋆ changing with redshift and mass. It is found that some of the recent theoretical galaxy formation simulations predict a smaller j ⋆/m ⋆ than our values. We also find that a significant fraction of our galaxies appears to be unstable against bar formation.

Spin flips – II. Evolution of dark matter halo spin orientation, and its correlation with major mergers

We expand our previous study on the relationship between changes in the orientation of the angular momentum vector of dark matter haloes ("spin flips") and changes in their mass (Bett & Frenk 2012), to cover the full range of halo masses in a simulation cube of length 100 $h^{-1}$ Mpc. Since strong disturbances to a halo (such as might be indicated by a large change in the spin direction) are likely also to disturb the galaxy evolving within, spin flips could be a mechanism for galaxy morphological transformation without involving major mergers. We find that 35% of haloes have, at some point in their lifetimes, had a spin flip of at least $45\deg$ that does not coincide with a major merger. Over 75% of large spin flips coincide with non-major mergers; only a quarter coincide with major mergers. We find a similar picture for changes to the inner-halo spin orientation, although here there is an increased likelihood of a flip occurring. Changes in halo angular momentum orientation, and other such measures of halo perturbation, are therefore very important quantities to consider, in addition to halo mergers, when modelling the formation and evolution of galaxies and confronting such models with observations.

Morphology of galaxies with quiescent recent assembly history in a Λ-CDM universe

The standard disc formation scenario postulates that disc forms as the gas cools and flows into the centre of the dark matter halo, conserving the specific angular momentum. Major mergers have been shown to be able to destroy or highly perturb the disc components. More recently, the alignment of the material that is accreted to form the galaxy has been pointed out as a key ingredient to determine galaxy morphology. However, in a hierarchical scenario galaxy formation is a complex process that combines these processes and others in a non-linear way so that the origin of galaxy morphology remains to be fully understood. We aim at exploring the differences in the formation histories of galaxies with a variety of morphology, but quite recent merger histories, to identify which mechanisms are playing a major role. We analyse when minor mergers can be considered relevant to determine galaxy morphology. We also study the specific angular momentum content of the disc and central spheroidal components separately. We used cosmological hydrodynamical simulations that include an effective, physically motivated supernova feedback that is able to regulate the star formation in haloes of different masses. We analysed the morphology and formation history of a sample of 15 galaxies of a cosmological simulation. We performed a spheroid-disc decomposition of the selected galaxies and their progenitor systems. The angular momentum orientation of the merging systems as well as their relative masses were estimated to analyse the role played by orientation and by minor mergers in the determination of the morphology. We found the discs to be formed by conserving the specific angular momentum in accordance with the classical disc formation model. The specific angular momentum of the stellar central spheroid correlates with the dark matter halo angular momentum and determines a power law. Abridged

Weak gravitational lensing of intrinsically aligned galaxies

Subject of this paper is the weak lensing effect on galaxies that show intrinsically correlated ellipticities. In our model, we investigate the distortion of the ellipticity field if the galaxies experience an apparent shift in their position by weak lensing deflection and compare this effect to the shearing effect induced by tidal fields. Starting with a derivation of intrinsic ellipticity spectra by employing a tidal torquing model generating galactic angular momenta, we model the galaxy ellipticity by assuming that the galactic disk forms perpendicularly to the host halo angular momentum direction and derive intrinsic ellipticity E-mode and B-mode spectra from the angular momentum statistics. The lensing effect on the ellipticity field is modeled by employing the methodology developed in the framework of lensing of the cosmic microwave background polarisation. For EUCLID, ellipticity correlations are altered by lensing deflection on multipoles l>1000 by ~5% for the ellipticity E-modes and by ~30% for the B-modes, while a shallower survey would exhibit larger changes on larger angular scales. In addition to the convolving effect of lensing on the ellipticity spectra we investigate the E/B-mode conversion, and discuss the possibility of measuring correlations between different multipoles which is evoked by the homogeneity breaking effect of the lensing displacement. Our conclusion is that although shape correlations generated by weak gravitational shear is dominant, the shifting effect due to lensing is shaping the ellipticity spectra on small angular scales and causes a number of interesting phenomena, which might be observable by future surveys.

COSMIC VORTICITY AND THE ORIGIN HALO SPINS

In the standard model of cosmology, structure emerges out of non-rotational flow and the angular momentum of collapsing halos is induced by tidal torques. The growth of halo angular momentum in the linear and quasi-linear phases is associated with a shear, curl-free, flow and it is well described within the linear framework of tidal torque theory (TTT). However, TTT is rendered irrelevant as haloes approach turn around and virialization. At that stage the flow field around halos has non-zero vorticity. Using a cosmological simulation, we have examined the importance of the curl of the velocity field (vorticity) in determining halo spin, finding a strong alignment between the two. We have also examined the alignment of vorticity with the principle axes of the shear tensor, finding that it tends to be perpendicular to the axis along which material is collapsing fastest (e1). This behavior is independent of halo masses and cosmic web environment. Our results agree with previous findings on the tendency of halo spin to be perpendicular to e1, and of the spin of (simulated) halos and (observed) galaxies to be aligned with the large-scale structure. Our results imply that angular momentum growth proceeds in two distinct phases. In the first phase angular momentum emerges out of a shear, curl-free, potential flow, as described by TTT. In the second phase, in which haloes approach virialization, the angular momentum emerges out of a vortical flow and halo spin becomes strongly aligned with the vorticity of the ambient flow field.

THE SIZE-VIRIAL RADIUS RELATION OF GALAXIES

Sizes of galaxies are an important diagnostic for galaxy formation models. In this study I use the abundance matching ansatz, which has proven to be successful in reproducing galaxy clustering and other statistics, to derive estimates of the virial radius, R200, for galaxies of different morphological types and wide range of stellar mass. I show that over eight of orders of magnitude in stellar mass galaxies of all morphological types follow an approximately linear relation between 3D half-mass radius of their stellar distribution, rhalf and virial radius, rhalf~0.015R200 with a scatter of ~0.2 dex. Such scaling is in remarkable agreement with expectation of models which assume that galaxy sizes are controlled by halo angular momentum, which implies rhalf\propto lambda R200, where lambda is the spin of galaxy parent halo. The scatter about the relation is comparable with the scatter expected from the distribution of $\lambda$ and normalization of the relation agrees with that predicted by the model of Mo, Mao & White (1998), if galaxy sizes were set on average at z~1-2. Moreover, I show that when stellar and gas surface density profiles of galaxies of different morphological types are rescaled using radius r_n= 0.015 R200, the rescaled surface density profiles follow approximately universal exponential (for late types) and de Vaucouleurs (for early types) profiles with scatter of only 30-50% at R~1-3r_n. Remarkably, both late and early type galaxies have similar mean stellar surface density profiles at R>r_n. The main difference between their stellar distributions is thus at R<r_n. The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halo and that sizes of both late type disks and early type spheroids are controlled by halo angular momentum.

THE COSMIC HISTORY OF THE SPIN OF DARK MATTER HALOS WITHIN THE LARGE-SCALE STRUCTURE

We use N-body simulations to investigate the evolution of the orientation and magnitude of dark matter halo angular momentum within the large scale structure since z=3. We look at the evolution of the alignment of halo spins with filaments and with each other, as well as the spin parameter, which is a measure of the magnitude of angular momentum. It was found that the angular momentum vectors of dark matter haloes at high redshift have a weak tendency to be orthogonal to filaments and high mass haloes have a stronger orthogonal alignment than low mass haloes. Since z=1, the spins of low mass haloes have become weakly aligned parallel to filaments, whereas high mass haloes kept their orthogonal alignment. This recent parallel alignment of low mass haloes casts doubt on tidal torque theory as the sole mechanism for the build up of angular momentum. We see evidence for bulk flows and the broadening of filaments over time in the alignments of halo spin and velocities. We find a significant alignment of the spin of neighboring dark matter haloes only at very small separations, $r<0.3$Mpc/h, which is driven by substructure. A correlation of the spin parameter with halo mass is confirmed at high redshift.

The angular momentum of cold dark matter haloes with and without baryons

We investigate the magnitude and internal alignment of the angular momentum of cold dark matter haloes in simulations with and without baryons. We analyse the cumulative angular momentum profiles of hundreds of thousands of haloes in the Millennium simulation and in a smaller, but higher resolution, simulation, in total spanning 5 orders of magnitude in mass. For haloes of a given mass, the median specific angular momentum increases as j(<r) proportional to r. The direction of the vector varies considerably with radius: the median angle between the inner (< 0.25 Rvir) and total (< Rvir) angular momentum vectors is about 25degr. To investigate how baryons affect halo spin, we use another high resolution simulation, which includes gas cooling, star formation and feedback. This simulation produces a sample of galaxies with a realistic distribution of disc-to-total ratios. The formation of the galaxy spins up the dark matter within 0.1 Rvir such that the specific halo angular momentum increases by approx 50% in the median. The dark matter angular momentum becomes better aligned, but there remains a broad distribution of (mis-)alignments between the halo and the central galaxy, with a median angle between their angular momenta of ~ 30degr. Galaxies have a range of orientations relative to the shape of the halo: half of them have their minor axes misaligned by more than 45degr. Finally, we align a sample of haloes according to the orientation of their galaxies and stack the projected mass distributions. Although the individual haloes are aspherical, galaxy--halo misalignments produce a stacked mass distribution that is indistinguishable from circular. If the misalignment found in our simulations is realistic, it will be extremely difficult for weak lensing studies to measure the shape of dark haloes using this technique.

The Nonlinear Evolution of Galaxy Intrinsic Alignments

The non-Gaussian contribution to the intrinsic halo spin alignments is analytically modeled and numerically detected. Assuming that the growth of non-Gaussianity in the density fluctuations caused the tidal field to have nonlinear-order effect on the orientations of the halo angular momentum, we model the intrinsic halo spin alignments as a linear scaling of the density correlations on large scales, which is different from the previous quadratic-scaling model based on the linear tidal torque theory. Then, we analyze the halo catalogs from the recent high-resolution Millennium Run simulation at four different redshifts (z=0,0.5,1 and 2) and measure quantitatively the degree of the nonlinear effect on the halo spin alignments and its changes with redshifts. A clear signal of spin correlations is found on scales as large as 10 Mpc/h at z=0, which marks a detection of the nonlinear tidal effect on the intrinsic halo alignments. We also investigate how the nonlinear effect depends on the intrinsic properties of the halos. It is found that the degree of the nonlinear tidal effect increases as the halo mass scale decreases, the halo specific angular momentum increases, and the halo peculiar velocity decreases. We discuss implication of our result on the weak gravitational lensing.

The impact of box size on the properties of dark matter haloes in cosmological simulations

We investigate the impact finite simulation box size has on the structural and kinematic properties of cold dark matter (CDM) haloes forming in cosmological simulations. Our approach involves generating a single realization of the initial power spectrum of density perturbations and studying how truncation of this power spectrum on scales larger than Lcut= 2π/kcut affects the structure of dark matter haloes at z= 0. In particular, we have examined the cases of Lcut=fcutLbox with fcut= 1 (i.e. no truncation), 1/2, 1/3 and 1/4. In common with previous studies, we find that the suppression of long wavelength perturbations reduces the strength of clustering, as measured by a suppression of the two-point correlation function ξ(r), and reduces the numbers of the most massive haloes, as reflected in the depletion of the high-mass end of the mass function n(M). However, we note that the number density of ‘intermediate’-mass haloes (M∼ 1013h−1 M⊙) is enhanced by the suppression of long wavelength perturbations – by as much as ∼20 per cent for fcut= 1/4– and that this effect becomes more pronounced as fcut is decreased. Such a trend is in good agreement with the predictions of the Sheth–Tormen prescription. Interestingly, we find that truncation of the initial power spectrum has little impact on the internal properties of haloes. The masses of high-mass haloes decrease in a systematic manner as Lcut is reduced, but the distribution of concentrations is unaffected. On the other hand, the median spin parameter is ∼15 per cent lower in runs with fcut < 1. We argue that this is an imprint of the linear growth phase of the halo's angular momentum by tidal torquing, and that the absence of any measurable trend in concentration or halo shape reflects the importance of virialization and complex mass-accretion histories for these quantities. These results are of interest for studies that require high-mass resolution and statistical samples of simulated haloes, such as simulations of the population of first stars. Our analysis shows that large-scale tidal fields have relatively little effect on the internal properties of CDM haloes and hence may be ignored in these studies.

Rotation of Galaxy Dark Matter Halos

AbstractThe degree to which outer dark matter halos of spiral galaxies rotate with the disk is sensitive to their accretion history and may be probed with associated satellite galaxies. We use the Steward Observatory Bok telescope to measure the sense of rotation of nearby isolated spirals and combine these data with those of their associated satellites (drawn from SDSS) to directly test predictions from numerical simulations. We aim to constrain models of galaxy formation by measuring the projected component of the halo angular momentum that is aligned with that of spiral galaxy disks, Jz. We find the mean bulk rotation of the ensemble satellite system to be co-rotating with the disk with a velocity of 22 ± 13 km/s, in general agreement with previous observational studies and suggesting that galaxy disks could be formed by halo baryons collapsing by a factor of ≈10. We also find a prograde satellite fraction of 51% and Jz, of the satellite system to be positively correlated with the disk, albeit at low significance (2655 ± 2232 kpc km/s).

The Peculiar Velocities of Satellites of External Disk Galaxies

We analyze the angular distribution and the orbital rotation directions of a sample of carefully-selected satellite galaxies extracted from the Sloan Digital Sky Survey (SDSS). We also study these statistics in an N-body simulation of cosmological structure formation set within the LCDM paradigm under various assumptions for the orientations of disk angular momenta. Assuming that the angular momenta of the disks are aligned with the angular momenta of the inner regions of their host dark matter halos, we find that the fraction of simulated satellite halos that exhibit prograde motion is 0.55-0.60, with larger satellites more likely to be prograde. In our observational sample, approximately 60% of the satellites exhibit prograde motion, a result that is broadly consistent with the simulated sample. Contrary to several recent studies, our observational sample of satellite galaxies show no evidence for being anisotropically distributed about their primary disks. Again, this result is broadly consistent with our simulated sample of satellites under the assumption that disk and halo angular momenta are aligned. However, the small size of our observational sample does not yet allow us to distinguish between various assumptions regarding the orientations of disks in their halos. Finally, we assessed the importance of contamination by interlopers on the measured prograde and retrograde statistics.