We forecast constraints on primordial non-Gaussianity achievable from forthcoming surveys by exploiting the scale-dependent halo bias introduced on large scales by non-Gaussian initial conditions. We explore the performance of exploiting both the shape of the galaxy power-spectrum on large scales and the cross-correlation of galaxies with Cosmic Microwave Background maps through the Integrated Sachs Wolfe effect. We find that future surveys can detect primordial non-Gaussianity of the local form with a non-Gaussianity parameter $|f_{\rm NL}|$ of order unity. This is particularly exciting because, while the simplest single-field slow-roll models of inflation predict a primordial $|f_{\rm NL}|\ll 1$, this signal sources extra contributions to the effective $f_{\rm NL}$ of large-scale structures that are expected to be above our predicted detection threshold.
We present a new, galaxy-halo model of large-scale structure, in which the galaxies entering a given sample are the fundamental objects. Haloes attach to galaxies, in contrast to the standard halo model, in which galaxies attach to haloes. The galaxy-halo model pertains mainly to the relationships between the power spectra of galaxies and mass, and their cross-power spectrum. With surprisingly little input, an intuition-aiding approximation to the galaxy-matter cross-correlation coefficient R(k) emerges, in terms of the halo mass dispersion. This approximation seems valid to mildly non-linear …
Galaxy-galaxy lensing is a powerful probe of the relation between galaxies and dark matter halos, but its theoretical interpretation requires a careful modeling of various contributions, such as the contribution from central and satellite galaxies. For this purpose, a phenomenological approach based on the halo model has been developed, allowing for fast exploration of the parameter space of models. In this paper, we investigate the ability of the halo model to extract information from the g-g weak lensing signal by comparing it to high-resolution dissipationless simulations that resolve subha…
Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation mod…
We investigate the origin of halo assembly bias, the dependence of halo clustering on assembly history. We relate halo assembly to peak properties measured in the Lagrangian space of the initial linear Gaussian random density field, and show how these same Lagrangian properties determine large-scale bias. We focus on the two regimes where assembly bias has been observed to be significant: at masses very large and very small compared to the nonlinear mass scale. At high masses, we show that assembly bias is expected from the statistics of the peaks of Gaussian random fluctuations, and we show t…
In hierarchical models of gravitational clustering, virialized haloes are biased tracers of the matter distribution. As discussed by Mo & White (1996), this bias is nonlinear and stochastic. They developed a model which allows one to write down analytic expressions for the mean of the bias relation, in the initial Lagrangian, and the evolved, Eulerian spaces. We provide analytic expressions for the higher order moments as well. We also show how the Mo-White approach can be extended to compute the evolution, not just of the haloes, but of the dark matter distribution itself. The model predictio…
The relationship between galaxies and dark matter can be characterized by the halo mass of the central galaxy and the fraction of galaxies that are satellites. Here we present observational constraints from the SDSS on these quantities as a function of r-band luminosity and stellar mass using galaxy-galaxy weak lensing, with a total of 351,507 lenses. We use stellar masses derived from spectroscopy and virial halo masses derived from weak gravitational lensing to determine the efficiency with which baryons in the halo of the central galaxy have been converted into stars. We find that an L* gal…
The relics of disrupted satellite galaxies around the Milky Way and Andromeda have been found, but direct evidence of a satellite galaxy in the early stages of being disrupted 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 build-up of galaxy halos.
Using a large galaxy group catalogue constructed from the SDSS, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early types, late types, and intermediate types, based on their colour and specific star formation rate. At fixed luminosity, the early type fraction increases with increasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass scale. We argue that the previous claim of a characteristic feature on galaxy group scales i…
Here we present results from a maximum likelihood analysis of galaxy-galaxy weak lensing effects as measured in a 12.5' x 12.5' field obtained at the Nordic Optical Telescope, on La Palma, Spain. The analysis incorporates photometric redshifts and gives circular velocities consistent with previous weak lensing work.
Gravitational lensing of background compact objects like active galactic nuclei and quasars, by extended intermediate mass lenses such as globular clusters and and dark matter clumps with masses 10^5 - 10^8 M_sun, is considered. It is shown that observational study of the galaxy-quasar's associations is a powerful direct observational test of the nature of massive galaxy haloes. Optical interferometric observations with VLTI and Keck instruments are able to constrain masses and number of substructure halo objects. Evidence of gravitational lensing by globular clusters in haloes of spiral and e…
We have explored the galaxy disk/extended halo gas kinematic relationship using rotation curves (Keck/ESI) of ten intermediate redshift galaxies which were selected by MgII halo gas absorption observed in quasar spectra. Previous results of six edge-on galaxies, probed along their major axis, suggest that observed halo gas velocities are consistent with extended disk-like halo rotation at galactocentric distances of 25-72 kpc. Using our new sample, we demonstrate that the gas velocities are by and large not consistent with being directly coupled to the galaxy kinematics. Thus, mechanisms other…
We use a series of high-resolution N-body simulations of a `Milky-Way' halo, coupled to semi-analytic techniques, to study the formation of our own Galaxy and of its stellar halo. Our model Milky Way galaxy is a relatively young system whose physical properties are in quite good agreement with observational determinations. In our model, the stellar halo is mainly formed from a few massive satellites accreted early on during the galaxy's lifetime. The stars in the halo do not exhibit any metallicity gradient, but higher metallicity stars are more centrally concentrated than stars with lower abu…
Using a high-resolution cosmological $N$-body simulation, we identify the ejected population of subhalos, which are halos at redshift $z=0$ but were once contained in more massive `host' halos at high redshifts. The fraction of the ejected subhalos in the total halo population of the same mass ranges from 9% to 4% for halo masses from $\sim 10^{11}$ to $\sim 10^{12}\msun$. Most of the ejected subhalos are distributed within 4 times the virial radius of their hosts. These ejected subhalos have distinct velocity distribution around their hosts in comparison to normal halos. The number of subhalo…
A possible relation between the high dispersion in metallicity of metal-poor halo stars and the minor merger processes in the history of the Galaxy is presented. The foreign populations of stars in the satellites through minor merger processes, make considerable contributions to the Galactic halo, therefore alter the intrinsic distribution of metallicities of the halo stars. Our model reproduces the observed trends in the metallicity distribution of the extremely metal-poor halo field stars in the Galaxy. Taken all the parameters for our model, a density of $1.9\times 10^{-3}kpc^{-3}$ of extre…
The multiple images observed in galaxy cluster Abell 1689 provide strong constraints not only on the mass distribution of the cluster but also on the ensemble properties of the cluster galaxies. Using parametric strong lensing models for the cluster, and by assuming well motivated scaling laws between the truncation radius s and the velocity dispersion sigma of a cluster galaxy we are able to derive sizes of the dark matter halos of cluster galaxies. For the scaling law expected for galaxies in the cluster environment (s propto sigma), we obtain s = 64^{+15}_{-14} (sigma / 220 km/s) kpc. For t…
Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a g…
Using the empirical relations between the central galaxy luminosity and the halo mass, and between the total galaxy luminosity in a halo and the halo mass, we construct the galaxy luminosity function (LF). To the luminosity of the central galaxy in a halo of a given mass we assign log-normal scatter with a mean calibrated against the observations. In halos where the total galaxy luminosity exceeds that of the central galaxy, satellite galaxies are distributed as a power-law in luminosity. Combined with the halo mass function, this description reproduces the observed characteristics of the gala…
We model the luminosity-dependent projected two-point correlation function of DEEP2 (z~1) and SDSS (z~0) galaxies within the Halo Occupation Distribution (HOD) framework. At both epochs, there is a tight correlation between central galaxy luminosity and halo mass, with the slope and scatter decreasing for larger halo masses, and the fraction of satellite galaxies decreasing at higher luminosity. Central L* galaxies reside in halos a few times more massive at z~1 than at z~0. We find little evolution in the relation between mass scales of host halos for central galaxies and satellite galaxies a…
Conditional luminosity function (CLF) of dark matter halos supersedes simple models on the number of galaxies as a function of the halo mass, or the so-called halo occupation number, by assigning a luminosity distribution to those galaxies. Suggestions have now been made that both the clustering strength and properties of hosted galaxies of a given dark matter halo depends also on the age and the environment of that halo in addition to halo mass. Based on simple CLF models, we find that at least one suggestion that made use of central galaxy properties may be affected by uncertainties in the g…
We derive an analytic formula of genus for dark matter halos assuming that the primordial mass density field obeys the random-Gaussian statistics. In particular, we for the first time take account of the nonlinear nature of the halo biasing, in addition to the nonlinear gravitational evolution of the underlying mass distribution. We examine in detail the extent to which the predicted genus for halos depends on the redshift, smoothing scale of the density field, and the mass of those halos in representative cold dark matter models. In addition to the full model predictions, we derive an explici…
In this letter we present a morphological comparison between giant radio halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity, P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for mini--halos: mini--halo clusters share the same region of giant halo clusters in the (P_{1.4},L_X) plane, whereas they are clearly separated in the (P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be more than 50 times larger than that of giant halos, implying a very efficient…
We study the prospects for the current microlensing searches, which have recently discovered several candidates, to yield useful information about the flattening of the Galaxy dark matter halo. Models of HI warps and N-body simulations of galaxy formation suggest that disks commonly form in oblate halos with a tilt between the disk and halo symmetry axes. The microlensing optical depth for the Large Magellanic Cloud depends sensitively on the disk-halo tilt angle in the Milky Way. We conclude that a much larger spread in values for $τ(LMC)$ is consistent with rotation curve constraints than pr…
A new method for measuring gravitational lensing with high redshift type Ia supernovae is investigated. The method utilizes correlations between foreground galaxies and supernova brightnesses to substantially reduce possible systematic errors and increase the signal to noise ratio. It is shown that this lensing signal can be related to the mass, size and substructure of galaxy halos and galaxy clusters if dark matter consists of microscopic particles. This technique may be particularly useful for measuring the size of dark matter halos, a measurement to which the lensing of galaxies is not wel…
We use a simplified version of the halo model with a power law power spectrum to study scale dependence in galaxy bias at the very large scales relevant to baryon oscillations. In addition to providing a useful pedagogical explanation of the scale dependence of galaxy bias, the model provides an analytic tool for studying how changes in the Halo Occupation Distribution (HOD) impact the scale dependence of galaxy bias on scales between 10 and 1000 Mpc/h, which is useful for interpreting the results of complex N-body simulations. We find that changing the mean number of galaxies per halo of a gi…
Gravitational lensing has now become a popular tool to measure the mass distribution of structures in the Universe on various scales. Here we focus on the study of galaxy's scale dark matter halos with galaxy-galaxy lensing techniques: observing the shapes of distant background galaxies which have been lensed by foreground galaxies allows us to map the mass distribution of the foreground galaxies. The lensing effect is small compared to the intrinsic ellipticity distribution of galaxies, thus a statistical approach is needed to derive some constraints on an average lens population. An advantag…
We compute two-point correlation functions and measure the shear signal due to galaxy-galaxy lensing for 80,000 optically identified and 5,700 radio-loud AGN from Data Release 4 (DR4) of the Sloan Digital Sky Survey. Halo occupation models are used to estimate halo masses and satellite fractions for these two types of AGN. The large sample size allows us to separate AGN according to the stellar mass of their host galaxies. We study how the halo masses of optical and radio AGN differ from those of the parent population at fixed M*. Halo masses deduced from clustering and from lensing agree sati…
The rate at which galaxy halos form is thought to play a key role in explaining many observable cosmological phenomena such as the initial epoch at which luminous matter forms and the distribution of active galaxies. Here we show how Press-Schechter theory can be used to provide a simple, completely analytic model of the halo formation rate. This model shows good agreement with both Monte-Carlo and N-body simulation results.
In the CDM cosmological framework structures grow from merging with smaller structures. Merging should have observable effects on galaxies including destroying disks and creating spheroids. This proceeding aims to give a brief overview of how mergers occur in cosmological simulations. In this regard it is important to understand that dark matter halo mergers are not galaxy mergers; a theory of galaxy formation is necessary to connect the two. Mergers of galaxies in hydrodynamical simulations show a stronger dependence on mass than halo mergers in N-body simulations. If one knows how to connect…
We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have determined the halo occupation distribution, which describes how galaxies reside within dark matter halos, using the observed luminosity function and clustering of 40,696 0.2<z<1.0 red galaxies in Bootes. Half of 10^{11.9} Msun/h halos host a red central galaxy, and this fraction increases with increasing halo mass. We do not observe any evolution of the relationship between red galaxy stellar mass and host halo mass, although we expect both galaxy stellar masses and halo masses to evolve over cos…
We use a SDSS galaxy group catalogue to study the dependence of galaxy activity on stellar mass, halo mass, and group hierarchy (centrals vs. satellites). We split our galaxy sample in star-forming galaxies, galaxies with optical AGN activity and radio sources. We find a smooth transition in halo mass as the activity of central galaxies changes from star formation to optical AGN activity to radio emission. Star-forming centrals preferentially reside in haloes with M<10^{12} Msun, central galaxies with optical-AGN activity typically inhabit haloes with M \sim 10^{13} Msun, and centrals emitting…
We present the first galaxy-galaxy weak lensing results using early data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). These results are based on ~22 sq. deg. of i' data. From this data, we estimate the average velocity dispersion for an L* galaxy at a redshift of 0.3 to be 137 +- 11 km/s, with a virial mass, M_{200}, of 1.1 +- 0.2 \times 10^{12} h^{-1} Msun and a rest frame R-band mass-to-light ratio of 173 +- 34 h Msun/Lsun. We also investigate various possible sources of systematic error in detail. Additionally, we separate our lens sample into two sub-samples, divided by …
We report optical photometry and HI synthesis observations of the southern edge-on S_c,d galaxy IC5249. The observations were carried out primarily to determine if IC5249 has a red halo surrounding it similar to the halo that has been recently found surrounding the northern edge-on galaxy NGC5907. The galaxy IC5249 is almost perfectly edge-on, and it shows no direct sign of warping or flaring out to its optical radius. It has a low luminosity and a slow rotation velocity. We find that it has a remarkably similar halo to NGC5907. The halo is visible at a distance of 3kpc from the disk of the ga…
The nonlinear perturbation theory of gravitational instability is extended to include effects of both biasing and redshift-space distortions, which are inevitable in predicting observable quantities in galaxy surveys. The precise determination of scales of baryon acoustic oscillations is crucial to investigate the nature of dark energy by galaxy surveys. We find that a local Lagrangian bias and redshift-space distortions are naturally incorporated in our formalism of perturbation theory with a resummation technique via the Lagrangian picture. Our formalism is applicable to any biasing scheme w…
It has been shown by Paczyński that gravitational microlensing is a useful method to detect brown dwarfs in the dark halo of our galaxy. At present several experiments are carried out to monitor several million stars in the Large Magellanic Cloud with the aim to find such microlensing events. Here I discuss the possibility to use as targets stars in the Andromeda galaxy. Such an experiment would be sensitive to both the brown dwarfs in our halo and in the one of M31. The optical depth $τ$ to gravitational microlensing due to brown dwarfs in the halo of M31 turns out to be $\sim 10^{-6}$, which…
The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative distributions of galaxies and dark matter within halos. We calculate predicted HODs for a Lambda-CDM cosmological model using an SPH hydrodynamic simulation and a semi-analytic (SA) galaxy formation model. Although the two methods predict different galaxy mass functions, their HOD predictions agree remarkably well. For mass-selected samples, the mean occupation <N(M)> exhib…
We derive an analytic model for the redshift evolution of linear-bias, allowing for interactions and merging of the mass-tracers, by solving a second order differential equation based on linear perturbation theory and the Friedmann-Lemaitre solutions of the cosmological field equations. We then study the halo-mass dependence of the bias evolution, using the dark matter halo distribution in a $Λ$CDM simulation in order to calibrate the free parameters of the model. Finally, we compare our theoretical predictions with available observational data and find a good agreement. In particular, we find…
The observed flaring of HI disk in the outer region of galaxies has been used in the past to determine the shape of the dark matter halo. Previous studies based on this concept suggest a slightly oblate halo (axis ratio ~ 0.8) for our Galaxy. We reinvestigate this problem by calculating the HI scaleheight in the outer Galaxy to a larger radial distance, and by studying its dependence on the shape and the density profile of the halo. We find that a simple isothermal infinite halo of any shape- oblate or prolate, is not able to account for the observed flaring. Instead we show that a spherical h…
We combine the measurements of luminosity dependence of bias with the luminosity dependent weak lensing analysis of dark matter around galaxies to derive the galaxy bias and constrain nonlinear mass and cosmological parameters. We take advantage of theoretical and simulation predictions that predict that while halo bias is rapidly increasing with mass for high masses, it is nearly constant in low mass halos. We use a new weak lensing analysis around the same SDSS galaxies to determine their halo mass probability distribution. These halo mass probability distributions are used to predict the bi…
Ultra-light scalar fields provide an interesting alternative to WIMPS as halo dark matter. In this paper we consider the effect of embedding a supermassive black hole within such a halo, and estimate the absorption probability and the accretion rate of dark matter onto the black hole. We show that the accretion rate would be small over the lifetime of a typical halo, and hence that supermassive central black holes can coexist with scalar field halos.
