Galaxy clustering

Spatial Distribution of Galaxies:

  • Two point correlation function (2PCF): We have investigated whether Labini's group claim, that the 2PCF at large scales behavior in galaxy surveys (BAO, Universe homogenization) cannot be trusted due to the limited volume effect, is correct. We have demonstrated that all 2PCF estimators verifies a relation called integral constraint, which is not necessary by the real 2PCF, which biases correlation function estimators. But we showed using simulations of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) that the effect of the constraint is very small for current galaxy surveys (Labatie, Starck, Lachieze-Rey, Statistical Methodology, 2011).
  • Baryonic Acoustic Oscillation (BAO): We have designed a specific wavelet adapted to search for shells, and exploit the physics of the process by making use of two different mass tracers, introducing a specific statistic to detect the BAO features. We have applied our method to the detection of BAO in a galaxy sample drawn from the Sloan Digital Sky Survey (SDSS). We have used the "main" catalogue to trace the shells, and the luminous red galaxies (LRG) as tracers of the high density central regions. Using this new method, we detect, with a high significance, that the LRG in our sample are preferentially located close to the centers of shell-like structures in the density field, with characteristics similar to those expected from BAO (Arnalte-Mur, Labatie, Clerc, Martínez,  Starck et al, A&A, 2012). Then we have studied the classical method for detecting BAOs and the assumptions that the method requires. We have also found that the approximation of a constant covariance matrix in the classical BAO analysis method can affect non negligibly both the BAO detection and cosmological parameter constraints (Labatie, Starck, Lachieze-Rey, ApJ,2012a) (Labatie, Starck, Lachieze-Rey, ApJ,2012b).
  • Multiscale morphology of the galaxy distribution: We have shown how to calculate the Minkowski Functionals (MFs) taking into account border effects of complex observational sample volumes. We have proposed a multi-scale extension of the MF, which gives us more information about how the galaxies are spatially distributed. This method has been applied to the 2dF Galaxy Redshift Survey data. The MMF clearly indicates an evolution of morphology with scale. We also compare the 2dF real catalogue with mock catalogues and found that Λ cold dark matter simulations roughly fit the data, except at the finest scale (Saar, Martinez, Starck and Donoho, MNRAS, 2007).
  • Galaxy clustering and the changing relationship between galaxies and haloes since z=1.2: We measured the galaxy spatial correlation function in multi-band optical data over 133 square degree in the CFHTLS-Wide survey, from z=0.2 to 1.2 (Coupon, Kilbinger et al., A&A, 2012). Comparing these observations to a semi-analytical model of the matter distribution in the Universe, including a prescription how galaxies populate halos, a so-called halo occupation distribution (HOD) model, we determine the evolution of the luminosity-to-mass (L/M) ratios for stellar-mass selected galaxy samples. A maximum L/M is reached at halo masses of 6.3 × 1011 at low redshift. This mass increases with redshift, indicating “anti-hierarchical” evolution or “down-sizing”, where galaxies formed more efficiently in larger halos in the past.