The two-point correlation of galaxy groups: probing the clustering of dark matter haloes

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 standard Λ-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.     

Tidal effects and the environment dependence of halo assembly

We explore a possible origin for the puzzling anti-correlation between the formation epoch of galactic dark-matter haloes and their environment density. This correlation has been revealed from cosmological N -body simulations and is in conflict with the extended Press–Schechter model of halo clustering. Using similar simulations, we first quantify the straightforward association of an early formation epoch with a reduced mass-growth rate at late times. We then find that a primary driver of suppressed growth, by accretion and mergers, is tidal effects dominated by a neighbouring massive halo. The tidal effects range from a slowdown of the assembly of haloes due to the shear along the large-scale filaments that feed the massive halo to actual mass loss in haloes that pass through the massive halo. Using the restricted three-body problem, we show that haloes are prone to tidal mass loss within 1.5 virial radii of a larger halo. Our results suggest that the dependence of the formation epoch on environment density is a secondary effect induced by the enhanced density of haloes in filaments near massive haloes where the tides are strong. Our measures of assembly rate are particularly correlated with the tidal field at high redshifts   z ∼ 1  .     

Halo masses for optically selected and for radio-loud AGN from clustering and galaxy–galaxy lensing

We compute two-point correlation functions and measure the shear signal due to galaxy–galaxy lensing for 80 000 optically identified and 5700 radio-loud active galactic nuclei (AGN) from Data Release 4 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 satisfactorily. Radio AGN are found in more massive haloes than optical AGN: in our samples, their mean halo masses are  1.6 × 10¹³  and  8 × 10¹¹  h ⁻¹ M_⊙  , respectively. Optical AGN follow the same relation between stellar mass and halo mass as galaxies selected without regard to nuclear properties, but radio-loud AGN deviate significantly from this relation. The dark matter haloes of radio-loud AGN are about twice as massive as those of control galaxies of the same stellar mass. This boost is independent of radio luminosity, and persists even when our analysis is restricted to field galaxies. The large-scale gaseous environment of the galaxy clearly plays a crucial role in producing observable radio emission. The dark matter halo masses that we derive for the AGN in our two samples are in good agreement with recent models in which feedback from radio AGN becomes dominant in haloes where gas cools quasi-statically.     

The clustering evolution of the galaxy distribution

We follow the evolution of the galaxy population in a ΛCDM cosmology by means of high-resolution N -body simulations in which the formation of galaxies and their observable properties are calculated using a semi-analytic model. We display images of the spatial distribution of galaxies in the simulations that illustrate its evolution and provide a qualitative understanding of the processes responsible for the various biases that develop. We consider three specific statistical measures of clustering at     and     : the correlation length (in both real and redshift space) of galaxies of different luminosity, the morphology–density relation and the genus curve of the topology of galaxy isodensity surfaces. For galaxies with luminosity below L ∗, the     correlation length depends very little on the luminosity of the sample, but for brighter galaxies it increases very rapidly, reaching values in excess of 10  h ⁻¹ Mpc. The 'accelerated' dynamical evolution experienced by galaxies in rich clusters, which is partly responsible for this effect, also results in a strong morphology–density relation. Remarkably, this relation is already well-established at     . The genus curves of the galaxies are significantly different from the genus curves of the dark matter, however this is not a result of genuine topological differences but rather of the sparse sampling of the density field provided by galaxies. The predictions of our model at     will be tested by forthcoming data from the 2dF and Sloan galaxy surveys, and those at     by the DEEP and VIRMOS surveys.     

Measuring the distribution of galaxies between haloes

We develop a method to measure the probability, P ( N;   M ), of finding N galaxies in a dark matter halo of mass M from the theoretically determined clustering properties of dark matter haloes and the observationally measured clustering properties of galaxies. Knowledge of this function and the distribution of the dark matter completely specifies all clustering properties of galaxies on scales larger than the size of dark matter haloes. Furthermore, P ( N;   M ) provides strong constraints on models of galaxy formation, since it depends upon the merger history of dark matter haloes and the galaxy–galaxy merger rate within haloes. We show that measurements from a combination of the Two Micron All Sky Survey and Sloan Digital Sky Survey or Two-degree Field Galaxy Redshift Survey data sets will allow P ( N;   M ) averaged over haloes occupied by bright galaxies to be accurately measured for N =0–2 .