Dark Matter Halos Of Galaxies Research Articles

Growth of Structure Seeded by Primordial Black Holes

We discuss the possibilities for primordial black holes (PBHs) to grow via the accretion of dark matter. In agreement with previous works, we find that accretion during the radiation-dominated era does not lead to a significant mass increase. However, during matter-domination, PBHs may grow by up to two orders of magnitude in mass through the acquisition of large dark matter halos. We discuss the possibility of PBHs being an important component in dark matter halos of galaxies as well as their potential to explain the ultra-luminous x-ray sources (ULXs) observed in nearby galactic disks. We point out that although PBHs are ruled out as the dominant component of dark matter, there is still a great deal of parameter space open to them playing a role in the modern-day universe. For example, a primordial halo population of PBHs each at $10^{2.5} M_\odot$ making up 0.1% of the dark matter grow to $10^{4.5} M_\odot$ via the accumulation of dark matter halos to account for $\sim 10%$ of the dark matter mass by a redshift of $z \approx 30$. These intermediate mass black holes may then ``light up'' when passing through molecular clouds, becoming visible as ULXs at the present day, or they may form the seeds for supermassive black holes at the centers of galaxies.

Weak lensing measurements of dark matter halos of galaxies from COMBO-17

Received / Accepted Abstract. We present a measurement of mass estimates for dark matter halos around galaxies from the COMBO- 17 survey using weak gravitational lensing. COMBO-17 is particularly useful for this kind of investigation because it covers observations in 17 optical filters from which accurate photometric redshifts and spectral classification for objects with R < 24 are derived. This allows us to select lens and source galaxies from their redshifts and to thus avoid any uncertainties from estimates of the source redshift distribution. We study galaxy lenses at redshifts zd = 0.2 − 0.7 by fitting the normalization of either singular isothermal spheres (SIS) or Navarro- Frenk-White (NFW) profiles to the whole lens sample; we then consider halos around blue and red subsamples separately. We also constrain the scaling of halo mass with light. For the NFW model, we find virial masses M � vir = 3.9

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).

A brighter past: galaxy luminosity function at high redshifts

Using the conditional luminosity function — the luminosity distribution of galaxies in a dark matter halo as a function of the halo mass — we present an empirical model to describe the redshift evolution of the rest B-band galaxy luminosity function (LF). The model is compared to various estimates of the LF, in rest UV and B bands, out to a redshift of 6, including estimates of LFs of galaxy types separated to red and blue galaxies. Using the observed LFs out to z~ 5, we present a general constraint on the redshift evolution of the central galaxy-halo mass relation. The increase in the number density of luminous galaxies, at the bright end of the LF, can be explained as due to a brightening of the luminosity of galaxies present in dark matter halo centres, relative to the luminosity of central galaxies in similar mass haloes today. The lack of strong evolution in the faint end of the LF, however, argues against a model involving pure-luminosity evolution at all halo mass scales. The increase in luminosity at the bright end compensates the rapid decline in the number density of massive haloes as the redshift is increased. The decline in group to cluster-mass dark matter haloes out to a redshift of ~2 is not important as the central galaxy luminosity flattens at halo masses around 1013 M⊙. At redshifts ~2–3, however, the density of bright galaxies begins to decrease due to the rapid decline in the number density of dark matter haloes at mass scales around and below 1013 M⊙. Based on a comparison of our predictions to the measured ultraviolet (UV) LF of galaxies at redshifts ~3, we estimate the probability distribution of halo masses to host Lyman break galaxies (LBGs). This probability for galaxies brighter than AB-absolute magnitudes of -21, with a number density of ~5 × 10−3h3 Mpc−3, peaks at a halo mass of ~7 × 1011h−1 M⊙ with a 68 per cent confidence level of (4–21) × 1011h−1 M⊙. These estimates are consistent with the mass estimates for LBGs using two-point clustering statistics and recent estimates of halo masses based on spectroscopic observations. For galaxies brighter than AB-absolute magnitudes of -21 at z~ 6, the halo mass scale is a factor of ~2 smaller; the LF predictions at z~ 6 are consistent with measured estimates in the literature. Based on the models, we also predict the LF of galaxies at redshifts greater than 6 and also the bias factor of galaxies at redshifts greater than 3; these predictions will soon be tested with observational data. In general, to explain high-redshift LFs, galaxies in dark matter haloes around 1012 M⊙ must increase in luminosity by a factor of ~4–6 between today and redshift of 6.

Interpreting the Relationship between Galaxy Luminosity, Color, and Environment

We study the relationship between galaxy luminosity, color, and environment in a cosmological simulation of galaxy formation. We compare the predicted relationship with that found for SDSS galaxies and find that the model successfully predicts most of the qualitative features seen in the data, but also shows some interesting differences. Specifically, the simulation predicts that the local density around bright red galaxies is a strong increasing function of luminosity, but does not depend much on color at fixed luminosity. Moreover, we show that this is due to central galaxies in dark matter halos whose baryonic masses correlate strongly with halo mass. The simulation also predicts that the local density around blue galaxies is a strong increasing function of color, but does not depend much on luminosity at fixed color. We show that this is due to satellite galaxies in halos whose stellar ages correlate with halo mass. Finally, the simulation fails to predict the luminosity dependence of environment observed around low luminosity red galaxies. However, we show that this is most likely due to the simulation's limited resolution. A study of a higher resolution, smaller volume simulation suggests that this dependence is caused by the fact that all low luminosity red galaxies are satellites in massive halos, whereas intermediate luminosity red galaxies are a mixture of satellites in massive halos and central galaxies in less massive halos.

The influence of redshift information on galaxy-galaxy lensing measurements

We investigate how galaxy-galaxy lensing measurements depend on the knowledge of redshifts for lens and source galaxies. Galaxy-galaxy lensing allows one to study dark matter halos of galaxies statistically us- ing weak gravitational lensing. Redshift information is required to reliably distinguish foreground lens galaxies from background source galaxies and to convert the measured shear into constraints on the lens model. Without spectroscopy or multi-colour information, redshifts can be drawn from independently estimated probability dis- tributions. The COMBO-17 survey provides redshifts for both lens and source galaxies. It thus offers the unique possibility to do this investigation with observational data. We find that it is of great importance to know the redshifts of individual lens galaxies in order to constrain the properties of their dark matter halos. Whether the redshifts are derived from UBV RI or the larger number of filters available in COMBO-17 is not very important. In contrast, knowledge of individual source redshifts improves the measurements only very little over the use of statistical source redshift distributions as long as the source redshift distribution is known accurately.

The Dark Halo of NGC 5963 as a Constraint on Dark Matter Self‐Interaction at the Low‐Velocity Regime

Self-interacting dark matter has been proposed as a hypothesis to explain the shallow central slopes of the density profiles of dark matter halos in galaxies. In order to be consistent with observational studies at scales of galaxy clusters, the cross section should scale inversely with the velocity of collision. In this paper we consider the mass density profile of the halo of the low surface brightness (LSB) galaxy NGC 5963 to place an upper limit on the dark matter cross section for collisions with velocities ~150 km s-1, i.e., at the low-velocity regime. After calibrating against cosmological simulations, we found that the large inferred dark matter concentration and central dark matter density in NGC 5963 are inconsistent with an effective collisional cross section per unit of mass >0.2 cm2 g-1. Corrections were applied in order to account for reduction of the core by the adiabatic contraction caused by cooling baryons. Our limits, which involve a number of simplifying, but always conservative, assumptions, exclude the last permitted interval for velocity-dependent cross sections to explain the flat density core in LSB galaxies. Implications for the nature of dark matter are also discussed.

Galaxy occupation statistics of dark matter haloes: observational results

Abstract We study the occupation statistics of galaxies in dark matter haloes using galaxy groups identified from the Two-degree Field Galaxy Redshift Survey with the halo-based group finder of Yang et al.. The occupation distribution is considered separately for early- and late-type galaxies, as well as in terms of central and satellite galaxies. The mean luminosity of the central galaxies scales with halo mass approximately as Lc∝M2/3 for haloes with masses M &amp;lt; 1013h−1M⊙, and as Lc∝M1/4 for more massive haloes. The characteristic mass of 1013h−1M⊙ is consistent with the mass scale where galaxy formation models suggest a transition from efficient to inefficient cooling. Another characteristic halo mass scale, M∼ 1011h−1M⊙, which cannot be probed directly by our groups, is inferred from the conditional luminosity function (CLF) that matches the observed galaxy luminosity function and clustering. For a halo of given mass, the distribution of Lc is rather narrow. A detailed comparison with mock galaxy redshift surveys indicates that this implies a fairly deterministic relation between Lc and the halo mass. The satellite galaxies, however, are found to follow a Poissonian number distribution, in excellent agreement with the occupation statistics of dark matter subhaloes. This provides strong support for the standard lore that satellite galaxies reside in subhaloes. The central galaxies in low-mass haloes are mostly late-type galaxies, while those in massive haloes are almost all early types. We also measure the CLF of galaxies in haloes of given mass. Over the mass range that can be reliably probed with the present data, 13.3 ≲ log[M/(h−1M⊙)]≲ 14.7, the CLF is reasonably well fitted by a Schechter function. Contrary to recent claims based on semi-analytical models of galaxy formation, the presence of central galaxies does not show up as a strong peak at the bright end of the CLF. The CLFs obtained from the observational data are in good agreement with the CLF model obtained by matching the observed luminosity function and large-scale clustering properties of galaxies in the standard Λ cold dark matter model.

Large scale inhomogeneity and local dynamical friction

We investigate the effect of a density gradient on Chandrasekhar's dynamical friction formula based on the method of 2-body encounters in the local approximation. We apply these generalizations to the orbit evolution of satellite galaxies in Dark Matter haloes. We find from the analysis that the main influence occurs through a position-dependent maximum impact parameter in the Coulomb logarithm, which is determined by the local scale-length of the density distribution. We also show that for eccentric orbits the explicit dependence of the Coulomb logarithm on position yields significant differences for the standard homogeneous force. Including the velocity dependence of the Coulomb logarithm yields ambigous results. The orbital fits in the first few periods are further improved, but the deviations at later times are much larger. The additional force induced by the density gradient, the inhomogeneous force, is not antiparallel to the satellite motion and can exceed 10% of the homogeneous friction force in magnitude. However, due to the symmetry properties of the inhomogeneous force, there is a deformation and no secular effect on the orbit at the first order. Therefore the inhomogeneous force can be safely neglected for the orbital evolution of satellite galaxies. For the homogeneous force we compare numerical N-body calculations with semi-analytical orbits to determine quantitatively the accuracy of the generalized formulae of the Coulomb logarithm in the Chandresekhar approach. With the local scale-length as the maximum impact parameter we find a significant improvement of the orbital fits and a better interpretation of the quantitative value of the Coulomb logarithm.

Galaxy-Galaxy Lensing Studies from COMBO-17

We study the dark matter halos of galaxies with galaxy-galaxy lensing using the COMBO-17 survey. This survey offers an unprecedented data set for studying lens galaxies at $z=0.2-0.7$ including redshift information and spectral classification from 17 optical filters for objects brighter than $R=24$ . So far, redshifts and classification for the lens galaxies have mainly been available for local surveys like the Sloan Digital Sky Survey (SDSS). Further, redshifts for the source galaxies have typically not been available at all but had to be estimated from redshift probability distribution which – for faint surveys – even had to be extrapolated. To study the dark matter halos we parametrize the lens galaxies as singular isothermal spheres (SIS) or by Navarro-Frenk-White (NFW) profiles. In both cases we find a dependence of the velocity dispersion or virial radius, respectively, on lens luminosity and colour. For the SIS model, we are able to reproduce the Tully-Fisher/Faber-Jackson relation on a scale of $150h^{-1}~\mathrm{kpc}$ . For the NFW profile we also calculate virial masses, mass-to-light ratios and rotation velocities. Finally, we investigate differences between the three survey fields used here. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Galaxy-galaxy weak lensing in the Sloan Digital Sky Survey: intrinsic alignments and shear calibration errors

Galaxy–galaxy lensing has emerged as a powerful probe of the dark matter haloes of galaxies, but is subject to contamination if intrinsically aligned satellites of the lens galaxy are used as part of the source sample. We present a measurement of this intrinsic shear using 200 747 lens galaxies from the Sloan Digital Sky Survey (SDSS) spectroscopic sample and a sample of satellites selected using photometric redshifts. The mean intrinsic shear at transverse separations of 30–446 h−1 kpc is constrained to be −0.0062 < Δγ < +0.0066 (99.9 per cent confidence, including identified systematics), which limits contamination of the galaxy–galaxy lensing signal to at most ∼15 per cent on these scales. We present these limits as a function of transverse separation and lens luminosity. We furthermore investigate shear calibration biases in the SDSS, which can also affect galaxy–galaxy lensing, and conclude that the shear amplitude is calibrated to better than 18 per cent. This includes noise-induced calibration biases in the ellipticity, which are small for the sample considered here, but which can be more important if low signal-to-noise ratio or poorly resolved source galaxies are used.

Major mergers of galaxy haloes: cuspy or cored inner density profile?

We present the results from a series of collisionless N-body simulations of major mergers of galaxy dark matter haloes with density profiles having either inner cusps or cores. Our simulations range from 2 × 10 5 to 10 7 particles, allowing us to probe the phase-space distribution of dark matter particles in the innermost regions (less than 0.005 virial radii) of cold dark matter haloes, a subject of much recent debate. We find that a major merger of two cored haloes yields a cored halo and does not result in a cuspy profile seen in many cosmological simulations. This result is unchanged if we consider mergers with parent mass ratios of 3 : 1 instead of 1 : 1. Mergers of a cuspy halo with either a cored halo or a second cuspy halo of equal mass, on the other hand, produce cuspy haloes with a slightly reduced inner logarithmic slope. Cuspy haloes, once formed, therefore appear resilient to major mergers. We find the velocity structure of the remnants to be mildly anisotropic, with a Maxwellian velocity distribution near the centre but not in the outer portions of the final haloes. Violent relaxation is effective only during the early phase of mergers, with phase mixing likely to be the dominant relaxation process at late times.

Orbit Evolution of Satellite Galaxies in Dark Matter Haloes

We investigate the effect of the inhomogeneity of the background distribution on dynamical friction. We find a generalised Coulomb logarithm with position dependent maximum impact parameter scaling with the local scale length, which is usually much smaller than the distance to the centre of the background system. We apply the new formula to N-body calculations of satellite galaxies in Dark Matter haloes and find a systematic improvement of the orbit fits. Additionally a first order force not parallel to the motion of the massive object appears, which can be neglected at least in spherical systems due to the lack of a secular effect.

Properties of galaxy dark matter halos from weak lensing

We present the results of a study of the average mass profile around galaxies using weak gravitational lensing. We use 45.5 deg2 of RC band imaging data from the Red-Sequence Cluster Survey (RCS) and define a sample of ~ 1.2 × 105 lenses with 19.5 &lt; RC &lt; 21, and a sample of ~ 1.5 × 106 background galaxies with 21.5 &lt; R &lt; 24.We constrain the power law scaling relations between the B-band luminosity and the mass and size of the halo, and find that the results are in excellent agreement with observed luminosity–line-width relations. Under the assumption that the luminosity does not evolve with redshift, the best fit NFW model yields a mass M200 = (8.8±0.7) × 1011h–1M⊙ and a scale radius rs = I6.7+3.7–3.0h–1 kpc for a galaxy with a fiducial luminosity of Lb = 1010h–2LB⊙. the latter result is in excellent agreement with predictions from numerical simulations for a halo of this mass. We also observe a signficant anisotropy of the lensing signal around the lenses, implying that the halos are flattened and aligned with the light distribution. We find an average (projected) halo ellipticity of 〈ehalo〉 = 0.20+0.04–0.05, in fair agreement with results from numerical simulations of CDM. Alternative theories of gravity (without dark matter) predict an isotropic lensing signal, which is excluded with 99.5% confidence. Hence, our results provide strong support for the existence of dark matter.

Galaxy disruption in a halo of dark matter.

The relics of disrupted satellite galaxies have been found around the Milky Way and Andromeda, but direct evidence of a satellite galaxy in the early stages of disruption has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxy's dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued buildup of galaxy halos.

Lensing by galaxies in CNOC2 fields

We have observed two blank fields of approximately 30 × 23 arcmin2 using the William Herschel Telescope. The fields have been studied as part of the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), and spectroscopic redshifts are available for 1125 galaxies in the two fields. We measured the lensing signal caused by large-scale structure, and found that the result is consistent with current, more accurate measurements. We study the galaxy—galaxy lensing signal of three overlapping samples of lenses (one with and two without redshift information), and detect a significant signal in all cases. The estimates for the velocity dispersion of an galaxy agree well for the various samples. The best-fitting singular isothermal sphere model to the ensemble-averaged tangential distortion around the galaxies with redshifts yields a velocity dispersion of σ*= 130+15−17 km s−1, or a circular velocity of V*c= 184+22−25 km s−1 for an L*B galaxy, in good agreement with other studies. We use a maximum-likelihood analysis, where a parametrized mass model is compared with the data, to study the extent of galaxy dark matter haloes. Making use of all available data, we find σ*= 111 ± 12 km s−1 (68.3 per cent confidence, marginalized over the truncation parameter s) for a truncated isothermal sphere model in which all galaxies have the same mass-to-light ratio. The value of the truncation parameter s is not constrained that well, and we find s*= 260+124−73h−1 kpc (68.3 per cent confidence, marginalized over σ*), with a 99.7 per cent confidence lower limit of 80 h−1 kpc. Interestingly, our results provide a 95 per cent confidence upper limit of 556 h−1 kpc. The galaxy—galaxy lensing analysis allows us to estimate the average mass-to-light ratio of the field, which can be used to estimate Ωm. The current result, however, depends strongly on the assumed scaling relation for s.

Analytic Cross Sections for Substructure Lensing

The magnifications of the images in a strong gravitational lens system are sensitive to small mass clumps in the lens potential; this effect has been used to infer the amount of substructure in galaxy dark matter halos. I study the theory of substructure lensing to identify important general features and to compute analytic cross sections that will facilitate further theoretical studies. I show that the problem of a clump anywhere along the line of sight to a lens can be mapped onto an equivalent problem of a clump in a simple convergence and shear field; clumps at arbitrary redshifts are therefore not difficult to handle in calculations. For clumps modeled as singular isothermal spheres (SISs), I derive simple analytic estimates of the cross section for magnification perturbations of a given strength. The results yield two interesting conceptual points: First, lensed images with positive parity are always made brighter by SIS clumps; images with negative parity can be brightened but are much more likely to be dimmed. Second, the clumps need not lie within the lens galaxy; they can be moved in redshift by several tenths and still have a significant lensing effect. Isolated small halos are expected to be common in hierarchical structure formation models, but it is not yet known whether they are abundant enough compared to clumps inside lens galaxies to affect the interpretation of substructure lensing.

Bar‐driven Dark Halo Evolution: A Resolution of the Cusp‐Core Controversy

Simulations predict that the dark matter halos of galaxies should have central cusps, while those inferred from observed galaxies do not have cusps. We demonstrate, using both linear perturbation theory and n-body simulations, that a disk bar, which should be ubiquitous in forming galaxies, can produce cores in cuspy CDM dark matter profiles within five bar orbital times. Simulations of forming galaxies suggest that one of Milky Way size could have a 10 kpc primordial bar; this bar will remove the cusp out to approximately 5 kpc in approximately 1.5 gigayears, while the disk only loses approximately 8% of its original angular momentum. An inner Lindblad-like resonance couples the rotating bar to orbits at all radii through the cusp, transferring the bar pattern angular momentum to the dark matter cusp, rapidly flattening it. This resonance disappears for profiles with cores and is responsible for a qualitative difference in bar driven halo evolution with and without a cusp. This bar induced evolution will have a profound effect on the structure and evolution of almost all galaxies. Hence, both to understand galaxy formation and evolution and to make predictions from theory it is necessary to resolve these dynamical processes. Unfortunately, correctly resolving these important dynamical processes in ab initio calculations of galaxy formation is a daunting task, requiring at least 4,000,000 halo particles using our SCF code, and probably requiring many times more particles when using noisier tree, direct summation, or grid based techniques, the usual methods employed in such calculations.

Two-Dimensional Galaxy-Galaxy Lensing: A Direct Measure of the Flattening and Alignment of Light and Mass in Galaxies

We propose a new technique to directly measure the shapes of dark matter halos of galaxies using weak gravitational lensing. Extending the standard galaxy-galaxy lensing method, we show that the shape parameters of the mass distribution of foreground galaxies can be measured from the two-dimensional shear field derived from background galaxies. This enables the comparison of the ellipticity of the mass distribution with that of the light in galaxies as well as an estimate of the degree of alignment between the stellar component and dark matter. We choose the specific case of an elliptical, isothermal profile and estimate the feasibility and significance of the detection of this signal. The prospects for applying this technique are excellent with large ongoing surveys like the Sloan Digital Sky Survey. The expected signal is smaller but comparable in significance to that of the mass in standard galaxy-galaxy lensing analyses. Since shapes of halos depend on the degree of dissipation and the transfer of angular momentum during galaxy assembly, constraints obtained from the analysis will provide important input to models of galaxy formation.

Fermion-Fermion Stars**The project supported partially by National Natural Science Foundation of China under Grant Nos 19745008 and 19835040

The fermion-fermion stars, i.e., the dark matter self-gravitating systems made from two kinds of fermions with different masses, are considered. We review the stability of the systems, present a comparison between the maxima of gravitational redshift for fermion stars, compact stars, Bondi stars, bonson stars and fermion-fermion stars, and then investigate rotation curves of fermion-fermion stars (two-component concentric spheres) which might be polytropic dark matter halos of galaxies. Results show that the fermion-fermion stars would give rotation curves with flat part at large radii. This presents a striking contrast with the rotation curve of a single component fermion star which has no flat parts.