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We propose a novel method to quantify the assembly histories of dark matter haloes with the redshift evolution of the mass-weighted spatial variance of their progenitor haloes, that is, the protohalo size history. We find that the protohalo size history for each individual halo at z ∼ 0 can be described by a double power-law function. The amplitude of the fitting function strongly correlates to the central-to-total stellar mass ratios of descendant haloes. The variation of the amplitude of the protohalo size history can induce a strong halo assembly bias effect for massive haloes. This effect is detectable in observation using the central-to-total stellar mass ratio as a proxy of the protohalo size. The correlation to the descendant central-to-total stellar mass ratio and the halo assembly bias effect seen in the protohalo size are much stronger than that seen in the commonly adopted half-mass formation time derived from the mass accretion history. This indicates that the information loss caused by the compression of halo merger trees to mass accretion histories can be captured by the protohalo size history. Protohalo size thus provides a useful quantity to connect protoclusters across cosmic time and to link protoclusters with their descendant clusters in observations. [ABSTRACT FROM AUTHOR]

We propose an efficient and robust method to estimate the halo concentration based on the first moment of the density distribution, which is |$R_1\equiv \int _0^{r_{\rm vir}}4\pi r^3\rho (r)\mathrm{ d}r/M_{\rm vir}/r_{\rm vir}$|⁠. We find that R 1 has a monotonic relation with the concentration parameter of the Navarro–Frenk–White (NFW) profile, and that a cubic polynomial function can fit the relation with an error |$\lesssim 3~{{\ \rm per\ cent}}$|⁠. Tests on ideal NFW haloes show that the conventional NFW profile fitting method and the V max/ V vir method produce biased halo concentration estimation by |$\approx 10~{{\ \rm per\ cent}}$| and |$\approx 30~{{\ \rm per\ cent}}$|⁠ , respectively, for haloes with 100 particles. In contrast, the systematic error for our R 1 method is smaller than 0.5  per cent even for haloes containing only 100 particles. Convergence tests on realistic haloes in N -body simulations show that the NFW profile fitting method underestimates the concentration parameter for haloes with ≲300 particles by |$\gtrsim 20~{{\ \rm per\ cent}}$|⁠ , while the error for the R 1 method is |$\lesssim 8~{{\ \rm per\ cent}}$|⁠. We also show other applications of R 1, including estimating V max and the Einasto concentration c e ≡ r vir/ r −2. The calculation of R 1 is efficient and robust, and we recommend including it as one of the halo properties in halo catalogues of cosmological simulations. [ABSTRACT FROM AUTHOR]

We develop a method to identify protoclusters based on dark matter haloes represented by galaxy groups selected from surveys of galaxies at high redshift. We test the performance of this method on haloes in N -body simulations, and find that it can correctly identify more than 85 per cent of the true protoclusters with |$\gtrsim 95$|  per cent purity and with mass estimates typically within 0.25 dex from their true values. We show how the information provided by the protoclusters can be used to link galaxies in present-day clusters of galaxies with their high-redshift progenitors. Our tests show that the protoclusters identified by our method can recover reliably the progenitor stellar mass distribution of galaxies, thereby providing an avenue to investigate the formation and evolution of present-day galaxy clusters and their member galaxies. [ABSTRACT FROM AUTHOR]

We present a comparison of the physical properties of the ionized gas in the circumgalactic medium and intergalactic medium (IGM) at z ∼ 0 between observations and four cosmological hydrodynamical simulations: Illustris, TNG300 of the IllustrisTNG project, EAGLE, and one of the Magneticum simulations. For the observational data, we use the gas properties that are inferred from cross-correlating the Sunyaev–Zel'dovich effect (SZE) from the Planck CMB maps with haloes and large-scale structure. Both the observational and simulation results indicate that the integrated gas pressure in haloes deviates from the self-similar case, showing that feedback impacts haloes with |$M_{500}\sim 10^{12\!-\!13}\, {\rm M_\odot }$|⁠. The simulations predict that more than half the baryons are displaced from haloes, while the gas fraction inferred from our observational data roughly equals the cosmic baryon fraction throughout the |$M_{500}\sim 10^{12\!-\!14.5}\, {\rm M_\odot }$| halo mass range. All simulations tested here predict that the mean gas temperature in haloes is about the virial temperature, while that inferred from the SZE is up to one order of magnitude lower than that from the simulations (and also from X-ray observations). While a remarkable agreement is found for the average properties of the IGM between the observation and some simulations, we show that their dependence on the large-scale tidal field can break the degeneracy between models that show similar predictions otherwise. Finally, we show that the gas pressure and the electron density profiles from simulations are not well described by a generalized NFW profile. Instead, we present a new model with a mass-dependent shape that fits the profiles accurately. [ABSTRACT FROM AUTHOR]

Identifying galaxy groups from redshift surveys of galaxies plays an important role in connecting galaxies with the underlying dark matter distribution. Current and future high- z spectroscopic surveys, usually incomplete in redshift sampling, present both opportunities and challenges to identifying groups in the high- z Universe. We develop a group finder that is based on incomplete redshift samples combined with photometric data, using a machine learning method to assign halo masses to identified groups. Test using realistic mock catalogues shows that |$\gtrsim \! 90{{\ \rm per\ cent}}$| of true groups with halo masses |$\rm {\it M}_h \gtrsim 10^{12} M_{\odot }\,h^{-1}$| are successfully identified, and that the fraction of contaminants is smaller than |$10{{\ \rm per\ cent}}$|⁠. The standard deviation in the halo mass estimation is smaller than 0.25 dex at all masses. We apply our group finder to zCOSMOS-bright and describe basic properties of the group catalogue obtained. [ABSTRACT FROM AUTHOR]

We examine the thermal energy contents of the intergalactic medium (IGM) over three orders of magnitude in both mass density and gas temperature using thermal Sunyaev–Zeldovich effect (tSZE). The analysis is based on Planck tSZE map and the cosmic density field, reconstructed in the Sloan Digital Sky Survey Data Release 7 volume and sampled on a grid of cubic cells of $$(1\, h^{-1}{\rm Mpc})^3$$, together with a matched filter technique employed to maximize the signal to noise. Our results show that the pressure–density relation of the IGM is roughly a power law given by an adiabatic equation of state, with an indication of steepening at densities higher than about 10 times the mean density of the Universe. The implied average gas temperature is $$\sim$$ 104 K in regions of mean density, $$\rho _{\rm m} \sim {\overline{\rho }}_{\rm m}$$, increasing to about $$10^5\, {\rm K}$$ for $$\rho _{\rm m} \sim 10\, {\overline{\rho }}_{\rm m}$$, and to $$\gt$$ 106 K for $$\rho _{\rm m} \sim 100\, {\overline{\rho }}_{\rm m}$$. At a given density, the thermal energy content of the IGM is also found to be higher in regions of stronger tidal fields, likely due to shock heating by the formation of large-scale structure and/or feedback from galaxies and active galactic nuclei. A comparison of the results with hydrodynamic simulations suggests that the current data can already provide interesting constraints on galaxy formation. [ABSTRACT FROM AUTHOR]

We apply a halo-based group finder to four large redshift surveys, the 2MRS (Two Micron All-Sky Redshift Survey), 6dFGS (Six-degree Field Galaxy Survey), SDSS (Sloan Digital Sky Survey) and 2dFGRS (Two-degree Field Galaxy Redshift Survey), to construct group catalogues in the low-redshift Universe. The group finder is based on that of Yang et al. but with an improved halo mass assignment so that it can be applied uniformly to various redshift surveys of galaxies. Halo masses are assigned to groups according to proxies based on the stellar mass/luminosity of member galaxies. The performances of the group finder in grouping galaxies according to common haloes and in halo mass assignments are tested using realistic mock samples constructed from hydrodynamical simulations and empirical models of galaxy occupation in dark matter haloes. Our group finder finds ∼94 per cent of the correct true member galaxies for 90-95 per cent of the groups in the mock samples; the halo masses assigned by the group finder are un-biased with respect to the true halo masses, and have a typical uncertainty of ∼0.2 dex. The properties of group catalogues constructed from the observational samples are described and compared with other similar catalogues in the literature. [ABSTRACT FROM AUTHOR]

We show that the ratio between the stellar mass of central galaxy and the mass of its host halo, fc ≡ M*,c/Mh, can be used as an observable proxy of halo assembly time, in that galaxy groups with higher fc assembled their masses earlier. Using SDSS groups of Yang et al., we study how fc correlates with galaxy properties such as colour, star formation rate, metallicity, bulge-to-disc ratio, and size. Central galaxies of a given stellar mass in groups with fc > 0.02 tend to be redder in colour, more quenched in star formation, smaller in size, and more bulge dominated, as fc increases. The trends in colour and star formation appear to reverse at fc < 0.02, reflecting a downsizing effect that galaxies in massive haloes formed their stars earlier although the host haloes themselves assembled later (lower fc). No such reversal is seen in the size of elliptical galaxies, suggesting that their assembly follows halo growth more closely than their star formation. Satellite galaxies of a given stellar mass in groups of a given halo mass tend to be redder in colour, more quenched in star formation and smaller in size as fc increases. For a given stellar mass, satellites also tend to be smaller than centrals. The trends are stronger for lower mass groups. For groups more massive than ~1013M☉, a weak reversed trend is seen in colour and star formation. The observed trends in star formation are qualitatively reproduced by an empirical model based on halo age abundance matching, but not by a semi-analytical model tested here. [ABSTRACT FROM AUTHOR]

We investigate the ability of current implementations of galaxy group finders to recover colour-dependent halo occupation statistics. To test the fidelity of group catalogue inferred statistics, we run three different group finders used in the literature over a mock that includes galaxy colours in a realistic manner. Overall, the resulting mock group catalogues are remarkably similar, and most colour-dependent statistics are recovered with reasonable accuracy. However, it is also clear that certain systematic errors arise as a consequence of correlated errors in group membership determination, central/satellite designation, and halo mass assignment. We introduce a new statistic, the halo transition probability (HTP), which captures the combined impact of all these errors. As a rule of thumb, errors tend to equalize the properties of distinct galaxy populations (i.e. red versus blue galaxies or centrals versus satellites), and to result in inferred occupation statistics that are more accurate for red galaxies than for blue galaxies. A statistic that is particularly poorly recovered from the group catalogues is the red fraction of central galaxies as a function of halo mass. Group finders do a good job in recovering galactic conformity, but also have a tendency to introduce weak conformity when none is present. We conclude that proper inference of colour-dependent statistics from group catalogues is best achieved using forward modelling (i.e. running group finders over mock data) or by implementing a correction scheme based on the HTP, as long as the latter is not too strongly model dependent. [ABSTRACT FROM AUTHOR]

We use a set of observational data for galaxy cold gas mass fraction and gas phase metallicity to constrain the content, inflow and outflow of gas in central galaxies hosted by haloes with masses between 1011 and 1012M⊙. The gas contents in high-redshift galaxies are obtained by combining the empirical star formation histories and star formation models that relate star formation rate with the cold gas mass in galaxies. We find that the total baryon mass in low-mass galaxies is always much less than the universal baryon mass fraction since z = 2, regardless of star formation model adopted. The data for the evolution of the gas phase metallicity require net metal outflow at z ≲ 2, and the metal loading factor is constrained to be about 0.01, or about 60 per cent of the metal yield. Based on the assumption that galactic outflow is more enriched in metal than both the interstellar medium and the material ejected at earlier epochs, we are able to put stringent constraints on the upper limits for both the net accretion rate and the net mass outflow rate. The upper limits strongly suggest that the evolution of the gas phase metallicity and gas mass fraction for low-mass galaxies at z < 2 is not compatible with strong outflow. We speculate that the low star formation efficiency of low-mass galaxies is owing to some preventative processes that prevent gas from accreting into galaxies in the first place. [ABSTRACT FROM AUTHOR]

We infer mechanisms of galaxy formation for a broad family of semi-analytic models (SAMs) constrained by the K-band luminosity function and H i mass function of local galaxies using tools of Bayesian analysis. Even with a broad search in parameter space the whole model family fails to match to constraining data. In the best-fitting models, the star formation and feedback parameters in low-mass haloes are tightly constrained by the two data sets, and the analysis reveals several generic failures of models that similarly apply to other existing SAMs. First, based on the assumption that baryon accretion follows the dark matter accretion, large mass-loading factors are required for haloes with circular velocities lower than 200 km s−1, and most of the wind mass must be expelled from the haloes. Second, assuming that the feedback is powered by Type II supernovae with a Chabrier initial mass function, the outflow requires more than 25 per cent of the available supernova kinetic energy. Finally, the posterior predictive distributions for the star formation history are dramatically inconsistent with observations for masses similar to or smaller than the Milky Way mass. The inferences suggest that the current model family is still missing some key physical processes that regulate the gas accretion and star formation in galaxies with masses below that of the Milky Way. [ABSTRACT FROM PUBLISHER]

Using a high-resolution cosmological N-body simulation, we identify the ejected population of subhaloes, which are haloes at redshift but were once contained in more massive ‘host’ haloes at high redshifts. The fraction of the ejected subhaloes in the total halo population of the same mass ranges from 9 to 4 per cent for halo masses from to . Most of the ejected subhaloes are distributed within four times the virial radius of their hosts. These ejected subhaloes have distinct velocity distribution around their hosts in comparison to normal haloes. The number of subhaloes ejected from a host of given mass increases with the assembly redshift of the host. Ejected subhaloes in general reside in high-density regions, and have a much higher bias parameter than normal haloes of the same mass. They also have earlier assembly times, so that they contribute to the assembly bias of dark matter haloes seen in cosmological simulations. However, the assembly bias is not dominated by the ejected population, indicating that large-scale environmental effects on normal haloes are the main source for the assembly bias. [ABSTRACT FROM AUTHOR]

We use galaxy groups selected from the Sloan Digital Sky Survey (SDSS) together with mass models for individual groups to study the galaxy–galaxy lensing signals expected from galaxies of different luminosities and morphological types. We compare our model predictions with the observational results obtained from the SDSS by Mandelbaum et al. for the same samples of galaxies. The observational results are well reproduced in a Λ cold dark matter (ΛCDM) model based on the Wilkinson Microwave Anisotropy Probe ( WMAP) 3-yr data, but a ΛCDM model with higher σ8, such as the one based on the WMAP 1-yr data, significantly overpredicts the galaxy–galaxy lensing signal. We model, separately, the contributions to the galaxy–galaxy lensing signals from different galaxies: central versus satellite, early type versus late type and galaxies in haloes of different masses. We also examine how the predicted galaxy–galaxy lensing signal depends on the shape, density profile and the location of the central galaxy with respect to its host halo. [ABSTRACT FROM AUTHOR]

Galaxy clustering and galaxy–galaxy lensing probe the connection between galaxies and their dark matter haloes in complementary ways. Since the clustering of dark matter haloes depends on cosmology, the halo occupation statistics inferred from the observed clustering properties of galaxies are degenerate with the adopted cosmology. Consequently, different cosmologies imply different mass-to-light ratios for dark matter haloes. Galaxy–galaxy lensing, which yields direct constraints on the actual mass-to-light ratios, can therefore be used to break this degeneracy, and thus to constrain cosmological parameters. In this paper, we establish the link between galaxy luminosity and dark matter halo mass using the conditional luminosity function (CLF), , which gives the number of galaxies with luminosities in the range that reside in a halo of mass M. We constrain the CLF parameters using the galaxy luminosity function and the luminosity dependence of the correlation lengths of galaxies. The resulting CLF models are used to predict the galaxy–galaxy lensing signal. For a cosmology that agrees with constraints from the cosmic microwave background, i.e. , the model accurately fits the galaxy–galaxy lensing data obtained from the Sloan Digital Sky Survey. For a comparison cosmology with , however, we can accurately fit the luminosity function and clustering properties of the galaxy population, but the model predicts mass-to-light ratios that are too high, resulting in a strong overprediction of the galaxy–galaxy lensing signal. We conclude that the combination of galaxy clustering and galaxy–galaxy lensing is a powerful probe of the galaxy–dark matter connection, with the potential to yield tight constraints on cosmological parameters. Since this method mainly probes the mass distribution on relatively small (non-linear) scales, it is complementary to constraints obtained from the galaxy power spectrum, which mainly probes the large-scale (linear) matter distribution. [ABSTRACT FROM AUTHOR]

We develop a new method to reconstruct the cosmic density field from the distribution of dark matter haloes above a certain mass threshold. Our motivation is that well-defined samples of galaxy groups/clusters, which can be used to represent the dark halo population, can now be selected from large redshift surveys of galaxies, and our ultimate goal is to use such data to reconstruct the cosmic density field in the local Universe. Our reconstruction method starts with a sample of dark matter haloes above a given mass threshold. Each volume element in space is assigned to the domain of the nearest halo according to a distance measure that is scaled by the virial radius of the halo. The distribution of the mass in and around dark matter haloes of a given mass is modelled using the cross-correlation function between dark matter haloes and the mass distribution within their domains. We use N-body cosmological simulations to show that the density profiles required in our reconstruction scheme can be determined reliably from large cosmological simulations, and that our method can reconstruct the density field accurately using haloes with masses down to (above which samples of galaxy groups can be constructed from current large redshift surveys of galaxies). Working in redshift space, we demonstrate that the redshift distortions due to the peculiar velocities of haloes can be corrected in an iterative way. We also describe some applications of our method. [ABSTRACT FROM AUTHOR]

The kinematics of satellite galaxies reflect the masses of the extended dark matter haloes in which they orbit, and thus shed light on the mass–luminosity relation (MLR) of their corresponding central galaxies. In this paper, we select a large sample of centrals and satellites from the Sloan Digital Sky Survey and measure the kinematics (velocity dispersions) of the satellite galaxies as a function of the r-band luminosity of the central galaxies. Using the analytical framework presented in More, van den Bosch & Cacciato, we use these data to infer both the mean and the scatter of the MLR of central galaxies, carefully taking account of selection effects and biases introduced by the stacking procedure. As expected, brighter centrals on average reside in more massive haloes. In addition, we find that the scatter in halo masses for centrals of a given luminosity, , also increases with increasing luminosity. As we demonstrate, this is consistent with , which reflects the scatter in the conditional probability function , being independent of halo mass. Our analysis of the satellite kinematics yields , in excellent agreement with constraints from clustering and group catalogues, and with predictions from a semi-analytical model of galaxy formation. We thus conclude that the amount of stochasticity in galaxy formation, which is characterized by , is well constrained, independent of halo mass and in a good agreement with current models of galaxy formation. [ABSTRACT FROM AUTHOR]

According to the current paradigm, galaxies initially form as disc galaxies at the centres of their own dark matter haloes. During their subsequent evolution, they may undergo a transformation to a red, early-type galaxy, thus giving rise to the build-up of the red sequence. Two important, outstanding questions are (i) which transformation mechanisms are most important and (ii) in what environment do they occur. In this paper, we study the impact of transformation mechanisms that operate only on satellite galaxies, such as strangulation, ram-pressure stripping and galaxy harassment. Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey, we compare the colours and concentrations of satellites galaxies to those of central galaxies of the same stellar mass, adopting the hypothesis that the latter are the progenitors of the former. On average, satellite galaxies are redder and more concentrated than central galaxies of the same stellar mass, indicating that satellite-specific transformation processes do indeed operate. Central-satellite pairs that are matched in both stellar mass and colour, however, show no average concentration difference, indicating that the transformation mechanisms operating on satellites affect colour more than morphology. We also find that the colour and concentration differences of matched central-satellite pairs are completely independent of the mass of the host halo (not to be confused with the subhalo) of the satellite galaxy, indicating that satellite-specific transformation mechanisms are equally efficient in host haloes of all masses. This strongly rules against mechanisms that are thought to operate only in very massive haloes, such as ram-pressure stripping or harassment. Instead, we argue that strangulation is the main transformation mechanism for satellite galaxies. Finally, we determine the relative importance of satellite quenching for the build-up of the red sequence. We find that roughly 70 per cent of red-sequence satellite galaxies with had their star formation quenched as satellites. This drops rapidly with increasing stellar mass, reaching virtually zero at . Therefore, a very significant fraction of red satellite galaxies were already quenched before they became a satellite. [ABSTRACT FROM AUTHOR]

We study the three-dimensional and projected shapes of galaxy groups in the Sloan Digital Sky Survey Data Release 4, and examine the alignment between the orientation of the central galaxy and the spatial distribution of satellite galaxies. The projected ellipticity of a group is measured using the moments of the discrete distribution of its member galaxies. We infer the three-dimensional and projected axis ratios of their dark matter haloes by comparing the measured ellipticity distributions with those obtained from Monte Carlo simulations of projected, triaxial dark matter haloes with different axis ratios. We find that the halo shape has a strong dependence on the halo mass. While the haloes of low-mass groups are nearly spherical, those of massive groups tend to be prolate. For groups containing at least four members, the statistical distribution of their measured ellipticities does not have a strong dependence on the colours of their central galaxies. Our analysis further shows that the average three-dimensional axis ratio for haloes with is about 1 : 0.46 : 0.46, resulting in a projected axis ratio of ∼0.77. Our results for the alignment between the orientation of the central galaxy of a group and the distribution of their satellite galaxies are in broad agreement with those obtained by Yang et al. The distribution of satellite galaxies preferentially aligns with the major axis of the central galaxy, with a clear dependence on both halo mass and galaxy colours. In particular, the alignment is stronger in more massive groups, and the strongest alignment is seen between red centrals and the distribution of red satellites. For groups with blue centrals, no significant alignment is detected. Finally, we examine how the observed alignment can be reproduced with the information about the halo axis ratios. The observed alignment signal can be reproduced if the angle between the major axis of the central galaxy and the projected major axis of the host halo has a Gaussian distribution with a mean of 0° and a dispersion of ∼23°. This dispersion is larger for groups with blue centrals than those with red centrals. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the distribution of satellite galaxies is preferentially aligned with the major axis of their central galaxy. The strength of this alignment has been found to depend strongly on the colours of the satellite and central galaxies, and only weakly on the mass of the halo in which the galaxies reside. In this paper we study whether these alignment signals, and their dependence on galaxy and halo properties, can be reproduced in a hierarchical structure formation model of a ΛCDM concordance cosmology. To that extent we use a large N-body simulation which we populate with galaxies following a semi-analytical model for galaxy formation. We find that if the orientation of the central galaxy is perfectly aligned with that of its dark matter halo, then the predicted central–satellite alignment signal is much stronger than observed. If, however, the minor axis of a central galaxy is perfectly aligned with the angular momentum vector of its dark matter halo, we can accurately reproduce the observed alignment strength as a function of halo mass and galaxy colour. Although this suggests that the orientation of central galaxies is governed by the angular momentum of their dark matter haloes, we emphasize that any other scenario in which the minor axes of central galaxy and halo are misaligned by ∼40° (on average) will match the data equally well. Finally, we show that dependence of the alignment strength on the colour of the central galaxy is most likely an artefact due to interlopers in the group catalogue. The dependence on the colour of the satellite galaxies, on the other hand, is real and owes to the fact that red satellites are associated with subhaloes that were more massive at their time of accretion. [ABSTRACT FROM AUTHOR]

We use the conditional luminosity function (CLF) and data from the 2-degree Field Galaxy Redshift Survey (2dFGRS) to constrain the average relation between light and mass in a Lambda cold dark matter (ΛCDM) cosmology with and (hereafter WMAP3 cosmology). Reproducing the observed luminosity dependence of the galaxy two-point correlation function results in average mass-to-light ratios that are ∼35 per cent lower than those in a ΛCDM cosmology with and (hereafter WMAP1 cosmology). This removes an important problem with previous halo occupation models which had a tendency to predict cluster mass-to-light ratios that were too high. For the WMAP3 cosmology, our model yields average mass-to-light ratios, central galaxy luminosities, halo occupation numbers, satellite fractions and luminosity-gap statistics, that are all in excellent agreement with those obtained from a 2dFGRS group catalogue and from other independent studies. We also use our CLF model to compute the probability distribution , that a central galaxy of luminosity Lcen resides in a halo of mass M. We find this distribution to be much broader than what is typically assumed in halo occupation distribution models, which has important implications for the interpretation of satellite kinematics and galaxy–galaxy lensing data. Finally, reproducing the luminosity dependence of the pairwise peculiar velocity dispersions in the 2dFGRS requires relatively low mass-to-light ratios for clusters and a satellite fraction that decreases strongly with increasing luminosity. This is only marginally consistent with the constraints obtained from the luminosity dependence of the galaxy two-point correlation function. We argue that a cosmology with parameters between those of the WMAP1 and WMAP3 cosmologies is likely to yield results with a higher level of consistency. [ABSTRACT FROM AUTHOR]

We use a high-resolution N-body simulation to study how the formation of cold dark matter haloes is affected by their environments, and how such environmental effects produce the age dependence of halo clustering observed in recent N-body simulations. We estimate, for each halo selected at redshift , an ‘initial’ mass Mi defined to be the mass enclosed by the largest sphere which contains the initial barycentre of the halo particles and within which the mean linear density is equal to the critical value for spherical collapse at . For haloes of a given final mass, Mh, the ratio Mi/ Mh has large scatter, and the scatter is larger for haloes of lower final masses. Haloes that form earlier on average have larger Mi/ Mh, and so correspond to higher peaks in the initial density field than their final masses imply. Old haloes are more strongly clustered than younger ones of the same mass because their initial masses are larger. The age dependence of clustering for low-mass haloes is entirely due to the difference in the initial/final mass ratio. Low-mass old haloes are almost always located in the vicinity of big structures, and their old ages are largely due to the fact that their mass accretions are suppressed by the hot environments produced by the tidal fields of the larger structure. The age dependence of clustering is weaker for more massive haloes because the heating by large-scale tidal fields is less important. [ABSTRACT FROM AUTHOR]

Galaxy–galaxy lensing is rapidly becoming one of the most promising means to accurately measure the average relation between galaxy properties and halo mass. In order to obtain a signal of sufficient signal-to-noise ratio, one needs to stack many lens galaxies according to their property of interest, such as luminosity or stellar mass. Since such a stack consists of both central and satellite galaxies, which contribute very different lensing signals, the resulting shear measurements can be difficult to interpret. In the past, galaxy–galaxy lensing studies either have completely ignored this problem, have applied rough isolation criteria in an attempt to preferentially select ‘central’ galaxies, or have tried to model the contribution of satellites explicitly. However, if one is able to a priori split the galaxy population in central and satellite galaxies, one can measure their lensing signals separately. This not only allows a much cleaner measurement of the relation between halo mass and their galaxy populations, but also allows a direct measurement of the subhalo masses around satellite galaxies. In this paper, we use a realistic mock galaxy redshift survey to show that galaxy groups, properly selected from large galaxy surveys, can be used to accurately split the galaxy population in centrals and satellites. Stacking the resulting centrals according to their group mass, estimated from the total group luminosity, allows a remarkably accurate recovery of the masses and density profiles of their host haloes. In addition, stacking the corresponding satellite galaxies according to their projected distance from the group centre yields a lensing signal that can be used to accurate measure the masses of both subhaloes and host haloes. We conclude that an application of galaxy–galaxy lensing measurements to group catalogues extracted from large galaxy redshift surveys offers a unique opportunity to accurately constrain the galaxy–dark matter connection. [ABSTRACT FROM AUTHOR]

Successfully reproducing the galaxy luminosity function (LF) and the bimodality in the galaxy distribution requires a mechanism that can truncate star formation in massive haloes. Current models of galaxy formation consider two such truncation mechanisms: strangulation, which acts on satellite galaxies, and active galactic nucleus (AGN) feedback, which predominantly affects central galaxies. The efficiencies of these processes set the blue fraction of galaxies, fblue( L, M), as a function of galaxy luminosity, L, and halo mass, M. In this paper, we use a galaxy group catalogue extracted from the Sloan Digital Sky Survey (SDSS) to determine fblue( L, M). To demonstrate the potential power of these data as a benchmark for galaxy formation models, we compare the results to the semi-analytical model for galaxy formation of Croton et al. Although this model accurately fits the global statistics of the galaxy population, as well as the shape of the conditional LF, there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model. In particular, the model predicts (i) too many faint satellites in massive haloes, (ii) a blue fraction of satellites that is much too low, and (iii) a blue fraction of centrals that is too high and with an inverted luminosity dependence. In the same order, we argue that these discrepancies owe to (i) the neglect of tidal stripping in the semi-analytical model, (ii) the oversimplified treatment of strangulation, and (iii) improper modelling of dust extinction and/or AGN feedback. The data presented here will prove useful to test and calibrate future models of galaxy formation and, in particular, to discriminate between various models for AGN feedback and other star formation truncation mechanisms. [ABSTRACT FROM AUTHOR]

We use galaxy groups selected from the Sloan Digital Sky Survey to examine the alignment between the orientation of the central galaxy (defined as the brightest group member) and the distribution of satellite galaxies. By construction, we therefore only address the alignment on scales smaller than the halo virial radius. We find a highly significant alignment of satellites with the major axis of their central galaxy. This is in qualitative agreement with the recent study of Brainerd, but inconsistent with several previous studies who detected a preferential minor-axis alignment. The alignment strength in our sample is strongest between red central galaxies and red satellites. On the contrary, the satellite distribution in systems with a blue central galaxy is consistent with isotropic. We also find that the alignment strength is stronger in more massive haloes and at smaller projected radii from the central galaxy. In addition, there is a weak indication that fainter (relative to the central galaxy) satellites are more strongly aligned. We present a detailed comparison with previous studies, and discuss the implications of our findings for galaxy formation. [ABSTRACT FROM AUTHOR]

Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey Data Release 2, 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 late- (early-)type fraction of galaxies increases (decreases) with decreasing 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 is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak, especially over the range : galaxy type depends more strongly on halo mass than on luminosity. In agreement with previous studies, the late- (early-)type fraction increases (decreases) with increasing halocentric radius. However, we find that this radial dependence is present in haloes of all masses probed (down to ), while previous studies did not find any radial dependence in haloes with . We argue that this discrepancy owes to the fact that we have excluded central galaxies from our analysis. We also find that the properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early-type fraction of satellites is significantly higher in a halo with an early-type central galaxy than in a halo of the same mass but with a late-type central galaxy. This phenomenon, which we call ‘galactic conformity’, is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate-type galaxies is always ∼20 per cent, independent of luminosity, independent of halo mass, independent of halocentric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolution. [ABSTRACT FROM AUTHOR]

We use recent observations of the H i mass function to constrain galaxy formation. The data conflict with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disc of cold gas that is depleted by star formation and supernova-driven outflows until the disc becomes gravitationally stable. Assuming a star formation threshold density supported by both theory and observations, this model predicts H i masses that are much too large. The reason is simple: supernova feedback requires star formation, which in turn requires a high surface density for the gas. Heating by the ultraviolet background can reduce the amount of cold gas in haloes with masses , but is insufficient to explain the observed H i mass function. A consistent model can be found if low-mass haloes are embedded in a pre-heated medium, with a specific gas entropy ∼10 keV cm2. In addition, such a model simultaneously matches the faint-end slope of the galaxy luminosity function without the need for any supernova-driven outflows. We propose a pre-heating model where the medium around low-mass haloes is pre-heated by gravitational pancaking. Because gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with were embedded in pancakes of masses at . The formation of such pancakes heats the gas to a temperature of and compresses it to an overdensity of ∼10. Such gas has a cooling time that exceeds the age of the Universe at , and has a specific entropy of ∼15 keV cm2, almost exactly the amount required to explain the stellar and H i mass functions. [ABSTRACT FROM AUTHOR]

We determine cross-correlation function between galaxies and galaxy groups, using both the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). Groups are identified using the halo-based group finder developed by Yang et al., which is optimized to associate to a group those galaxies which belong to the same dark matter halo. Our galaxy–group cross-correlation function is therefore a surrogate for the galaxy–halo cross-correlation function. We study the cross-correlation as a function of group mass, and as a function of the luminosity, stellar mass, colour, spectral type and specific star formation rate of the galaxies. All these cross-correlation functions show a clear transition from the ‘one-halo’ to the ‘two-halo’ regimes on a scale comparable to the virial radius of the groups under consideration. On scales larger than the virial radius, all cross-correlation functions are roughly parallel, consistent with the linear bias model. In particular, the large-scale correlation amplitudes are higher for more massive groups, and for brighter and redder galaxies. In the ‘one-halo’ regime, the cross-correlation function depends strongly on the definition of the group centre. We consider both a luminosity-weighted centre (LW centre) and a centre defined by the location of the brightest group galaxy (BG centre). With the first definition, the bright early-type galaxies in massive groups are found to be more centrally concentrated than the fainter, late-type galaxies. Using the BG centre, and excluding the brightest galaxy from the cross-correlation analysis, we only find significant segregation in massive groups for galaxies of different spectral types (or colours or specific star formation rates). In haloes with masses , there is a significant deficit of bright satellite galaxies. Comparing the results from the 2dFGRS with those obtained from realistic mock samples, we find that the distribution of galaxies in groups is much less concentrated than dark matter haloes predicted by the current ΛCDM model. [ABSTRACT FROM AUTHOR]

The brightest galaxy in a dark matter halo is expected to reside at rest at the centre of the halo. In this paper, we test this ‘Central Galaxy Paradigm’ (CGP) using group catalogues extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). For each group, we compute a parameter , which is defined as the difference between the velocity of the brightest group galaxy and the average velocity of the other group members (hereafter satellites), normalized by the unbiased estimator of the velocity dispersion of the satellite galaxies. Because the redshift surveys suffer from incompleteness effects and the group selection criterion unavoidably selects interlopers, a proper comparison between data and model needs to take this into account. To this extent, we use detailed mock galaxy redshift surveys (MGRSs), which are analysed in exactly the same way as the data, thus allowing for a fair comparison. We show that the CGP is only consistent with the data in haloes with , while in more massive haloes the data indicate a non-zero offset between the brightest galaxy and the satellites. This indicates that either central galaxies reside at the minimum of the dark matter potential, but that the halo itself is not yet fully relaxed, or, that the halo is relaxed, but that the central galaxy oscillates in its potential well. The former is consistent with the fact that the velocity bias of the brightest halo galaxies is larger in more massive haloes, while the latter may be indicative of cored, rather than cusped, dark matter haloes. We discuss several implications of these findings, including mass estimates based on satellite kinematics, strong gravitational lensing, halo occupation models, and the frequency and longevity of lopsidedness in disc galaxies. [ABSTRACT FROM AUTHOR]

We analyse the two-point correlation function (2PCF) of galaxy groups identified from the 2-degree Field Galaxy Redshift Survey with the halo-based group finder recently developed by Yang et al. With this group catalogue we are able to estimate the 2PCFs for systems ranging from isolated galaxies to rich clusters of galaxies. The real-space correlation length obtained for these systems ranges from∼4 to∼15 h−1 Mpc, respectively. The observed correlation amplitude (and the corresponding bias factor) as a function of group abundance is well reproduced by associating galaxy groups with dark matter haloes in the standardΛ-cold dark matter model. Redshift distortions are clearly detected in the redshift-space correlation function, the degree of which is consistent with the assumption of gravitational clustering and halo bias in the cosmic density field. In agreement with previous studies we find a strong increase of the correlation length with the mean intergroup separation. Although well-determined observationally, we show that current theoretical predictions are not yet accurate enough to allow for stringent constraints on cosmological parameters. Finally, we use our results to explore the power-law nature of the 2PCF of galaxies. We split the 2PCF into one- and two-group terms, equivalent to the one- and two-halo terms in halo occupation models, and show that the power-law form of the 2PCF is broken, when only including galaxies in the more massive systems. [ABSTRACT FROM AUTHOR]

Using detailed mock galaxy redshift surveys (MGRSs) we investigate the abundance and radial distribution of satellite galaxies. The mock surveys are constructed using large numerical simulations and the conditional luminosity function (CLF), and are compared against data from the Two Degree Field Galaxy Redshift Survey (2dFGRS). We use Monte Carlo Markov chains to explore the full posterior distribution of the CLF parameter space, and show that the average relation between light and mass is tightly constrained and in excellent agreement with our previous models and with that of Vale&Ostriker. The radial number density distribution of satellite galaxies in the 2dFGRS reveals a pronounced absence of satellites at small projected separations from their host galaxies. This is (at least partly) owing to the overlap and merging of galaxy images in the 2dFGRS parent catalogue. Owing to the resulting close-pair incompleteness we are unfortunately unable to put meaningful constraints on the radial distribution of satellite galaxies; the data are consistent with a radial number density distribution that follows that of the dark matter particles, but we cannot rule out alternatives with a constant number density core. Marginalizing over the full CLF parameter space, we show that in aΛCDM concordance cosmology the observed abundances of host and satellite galaxies in the 2dFGRS indicate a power spectrum normalization of. The same cosmology but withis unable to match simultaneously the abundances of host and satellite galaxies. This confirms our previous conclusions based on the pairwise peculiar velocity dispersions and the group multiplicity function. [ABSTRACT FROM AUTHOR]

We present models for the three-point correlation function (3PCF) of both dark matter and galaxies. We show that models based on the halo model can reasonably match the dark-matter 3PCF obtained from high-resolution N-body simulations. On small scales the 3PCF is sensitive to details regarding the density distributions of dark-matter haloes. On larger scales the results are very sensitive to the abundance of the few most prominent haloes. Using the conditional luminosity function, we also construct models for the 3PCF of galaxies, which we test against large mock galaxy samples. The bias of the galaxy distribution with respect to the dark matter, and the finite number of galaxies that can be hosted by individual haloes, significantly reduce the normalized three-point correlation function with respect to that of dark matter. Contrary to the 3PCF of the dark matter, the galaxy 3PCF is much less sensitive to details regarding the spatial number density distribution of galaxies in individual haloes or to the abundance of the few most massive systems. Finally, we show that our model based on the conditional luminosity function is in good agreement with results obtained from the 2-degree Field Galaxy Redshift Survey. In particular, the model nicely reproduces the observational finding that the 3PCF for early-type galaxies is slightly higher than that of late-type galaxies, and that there is no significant dependence of the 3PCF on galaxy luminosity. [ABSTRACT FROM AUTHOR]

Using detailed mock galaxy redshift surveys (MGRSs) we investigate to what extent the kinematics of large samples of satellite galaxies extracted from flux-limited surveys can be used to constrain halo masses. Unlike previous studies, which focused only on satellites around relatively isolated host galaxies, we try to recover the average velocity dispersion of satellite galaxies in all haloes, as a function of the luminosity of the host galaxy. We show that previous host-satellite selection criteria (SC) yield relatively large fractions of interlopers and with a velocity distribution that, contrary to what has been assumed in the past, differs strongly from uniform. We show that with an iterative, adaptive selection criterion one can obtain large samples of hosts and satellites, with strongly reduced interloper fractions, that allow an accurate measurement of σsat(Lhost) over 2.5 orders of magnitude in host luminosity. We use the conditional luminosity function (CLF) to make predictions and show that satellite weighting, which occurs naturally when stacking many host-satellite pairs to increase the signal-to-noise ratio, introduces a bias towards higher σsat(Lhost) compared to the true, host-averaged mean. A further bias, in the same direction, is introduced when using flux-limited, rather than volume-limited, surveys. We apply our adaptive selection criterion to the Two Degree Field Galaxy Redshift Survey (2dFGRS) and obtain a sample of 12 569 satellite galaxies and 8132 host galaxies. We show that the kinematics of these satellite galaxies are in excellent agreement with the predictions based on the CLF, after taking account of the various biases. We thus conclude that there is independent dynamical evidence to support the mass-to-light ratios predicted by the CLF formalism. [ABSTRACT FROM AUTHOR]

In two recent papers, we developed a powerful technique to link the distribution of galaxies to that of dark matter haloes by considering halo occupation numbers as a function of galaxy luminosity and type. In this paper we use these distribution functions to populate dark matter haloes in high-resolution N-body simulations of the standard ΛCDM cosmology with and . Stacking simulation boxes of and 300 h−1 Mpc with 5123 particles each we construct mock galaxy redshift surveys out to a redshift of with a numerical resolution that guarantees completeness down to 0.01 L*. We use these mock surveys to investigate various clustering statistics. The predicted two-dimensional correlation function reveals clear signatures of redshift space distortions. The projected correlation functions for galaxies with different luminosities and types, derived from , match the observations well on scales larger than ∼3 h−1 Mpc. On smaller scales, however, the model overpredicts the clustering power by about a factor two. Modelling the ‘finger-of-God’ effect on small scales reveals that the standard ΛCDM model predicts pairwise velocity dispersions (PVD) that are ∼400 km s−1 too high at projected pair separations of ∼1 h−1 Mpc. A strong velocity bias in massive haloes, with (where σgal and σdm are the velocity dispersions of galaxies and dark matter particles, respectively) can reduce the predicted PVD to the observed level, but does not help to resolve the overprediction of clustering power on small scales. Consistent results can be obtained within the standard ΛCDM model only when the average mass-to-light ratio of clusters is of the order of in the B-band. Alternatively, as we show by a simple approximation, a ΛCDM model with may also reproduce the observational results. We discuss our results in light of the recent WMAP results and the constraints on σ8 obtained independently from other observations. [ABSTRACT FROM AUTHOR]

ABSTRACT McKay et al. have recently used measurements of weak galaxy–galaxy lensing in the Sloan Digital Sky Survey to estimate the cross-correlation between galaxies and the projected dark matter density field. They have derived a relation between aperture mass within a radius of 260 h[sup -1] kpc, M[sub 260] , and lens galaxy luminosity that does not depend on galaxy luminosity, type or environment. In this paper, we study the cross-correlation between galaxies and dark matter using galaxy catalogues constructed from a high-resolution N-body simulation of a ΛCDM universe. We show that the simulation reproduces the observational results reasonably well, except that the predicted mass-to-light ratio is about a factor of 2 too high. In the simulation, M[sub 260] is approximately equal to the halo virial mass for L* galaxies. M[sub 260] overestimates the virial mass for fainter galaxies and underestimates it for brighter galaxies. We use the simulations to show that under certain circumstances the halo virial mass may be recovered by fitting a Navarro–Frenk–White (NFW) profile to the projected galaxy–mass correlation function. If we apply our method to the observations, we find that L* galaxies typically reside in haloes of ∼2 × 10[sup 12]h[sup -1]M⊙ , consistent with halo masses estimated from the observed Tully–Fisher relation. In the simulations, the halo virial mass scales with galaxy luminosity as L[sup 1.5] for central galaxies in haloes and for galaxies in low-density regions. For satellite galaxies, and for galaxies in high-density regions, there is no simple relation between galaxy luminosity and halo mass and care must be exercised when interpreting the lensing results. [ABSTRACT FROM AUTHOR]