More precisely, it has been found that the fluctuations in their number density, δ g is related to the underlying mass overdensity field δ m through a simple, linear biasing relation δ g∼ 1.2 δ m ( Tegmark, Zaldarriaga & Hamilton 2001 Taylor et al. Mid-infrared-selected galaxies also constitute a very interesting population of objects since they are also known to trace the underlying mass distribution in the local Universe in an almost unbiased fashion. 2002), the smaller correlation measured in the DEEP2 survey implies that this is not a strongly biased sample of objects either, since the clustering of the dark mass is expected to decrease with redshift in a similar way ( Coil et al. Since 2dF b j-selected galaxies are known to trace the underlying mass density field in an unbiased way ( Lahav et al. (2004) has shown that the two-point correlation function of DEEP2 galaxies with a median redshift of z= 1.14 is consistent with that measured by Adelberger (2003) in a very deep ( z∼ 3) sample of Lyman break galaxies but is significantly smaller than the correlation measured in the local ( z∼ 0) 2dF galaxy sample. First, results based on early data look very promising indeed. 2004) fields and the DEEP2 redshift survey ( Davis et al. Several deep redshift surveys of optically selected galaxies, such as the Keck surveys of the GOODS-north ( Wirth et al. In particular, strong constraints on galaxy evolution models can be obtained by measuring the relative clustering of different extragalactic objects as a function of redshifts, for which very deep galaxy samples are required. Indeed, the very fact that the spatial clustering of galaxies is related to their physical properties represents an important observational test for all theories of galaxy formation. The evidence that different galaxy populations might give different biased pictures of the mass distribution has complicated but also enriched the interpretation of galaxy clustering.

2001) rather than on luminosity ( Szapudi et al. On the other hand, the clustering of infrared- (IR-) selected galaxies seems to depend on their infrared colour ( Hawkins et al. 2002 and references therein), morphological and spectral type ( Hermit et al. The clustering of optically selected galaxies has been found to depend on galaxy luminosity ( Norberg et al. However, it is now well established that the clustering of galaxies at low redshift depends on a variety of factors, implying that not all types of galaxies can be regarded as unbiased mass tracers. This is true as long as galaxies trace the underlying mass density field. The primary statistical tool for characterizing galaxy clustering is the spatial two-point correlation function ξ( s), since, in the current paradigm of structure formation, the galaxy two-point correlation function is directly related to the initial power spectrum of mass density fluctuations. Investigating the redshift-space distribution of galaxies has long been regarded as a fundamental aspect of observational cosmology. Galaxies: clusters: general, galaxies: evolution, cosmology: observations, large-scale structure of Universe, infrared: galaxies 1 Introduction We also confirm that optically selected galaxies in the local redshift surveys, especially those of the SDSS sample, are significantly more clustered than infrared objects.

This suggests a lack of significant clustering evolution of infrared-selected objects out to z= 0.5 that is further confirmed by the consistency found between the correlation functions measured in a local ( z < 0.2) and a distant (0.2 < z < 0.5) subsample of ELAIS-S1 galaxies.

This result is in good agreement with the redshift-space correlation function measured in more local samples of mid-infrared-selected galaxies such as the IRAS Point Source Catalog (PSC z) redshift survey. We detected a positive signal in the correlation function that in the range of separations 1–10 h −1 Mpc is well approximated by a power law with a slope γ= 1.4 ± 0.25 and a correlation length s 0= 5.4 ± 1.2 h −1 Mpc, at the 90 per cent significance level. We used a subsample of the ELAIS-S1 catalogue covering approximately 4 deg 2 and consisting of 148 objects selected at 15 μm with a flux >0.5 mJy and a redshift z < 0.5. We investigate the clustering properties of galaxies in the recently completed ELAIS-S1 redshift survey through their spatial two-point autocorrelation function.

Redshift space drawings