The ALHAMBRA survey: An empirical estimation of the cosmic variance for merger fraction studies based on close pairs

Abstract

Aims. Our goal is to estimate empirically the cosmic variance that affects merger fraction studies based on close pairs for the first time. Methods. We compute the merger fraction from photometric redshift close pairs with 10 h −1 kpc ≤ r p ≤ 50 h −1 kpc and Δv ≤ 500 km s −1 and measure it in the 48 sub-fields of the ALHAMBRA survey. We study the distribution of the measured merger fractions that follow a log-normal function and estimate the cosmic variance σ v as the intrinsic dispersion of the observed distribution. We develop a maximum likelihood estimator to measure a reliable σ v and avoid the dispersion due to the observational errors (including the Poisson shot noise term). Results. The cosmic variance σ v of the merger fraction depends mainly on (i) the number density of the populations under study for both the principal (n 1 ) and the companion (n 2 ) galaxy in the close pair and (ii) the probed cosmic volume V c . We do not find a significant dependence on either the search radius used to define close companions, the redshift, or the physical selection (luminosity or stellar mass) of the samples. Conclusions. We have estimated the cosmic variance that affects the measurement of the merger fraction by close pairs from observations. We provide a parametrisation of the cosmic variance with n 1 , n 2 , and V c , σ v ∝ n −0.54(n 2 /n 1 ) −0.37 . Thanks to this prescription, future merger fraction studies based on close pairs could properly account for the cosmic variance on their results.

Galaxy clusters and groups in the ALHAMBRA survey

“…Finally, as other works have already claimed (López-Sanjuan et al 2014;Arnalte-Mur et al 2014;Molino et al 2014), the study of the distribution of the cluster properties in each of the different fields of ALHAM-BRA sets evidence on the cosmic variance at the level of the clustering of clusters. We notice striking differences between different fields in terms of mass and redshift distribution.…”

“…The simulation counts are only ∼ 8% smaller than the real data, a discrepancy that could be attributed to cosmic variance (e.g. López-Sanjuan et al 2014).…”

“…Molino et al 2014;López-Sanjuan et al 2014). In addition, we can take advantage of early searches on these fields to cross-correlate them with our detections and quantify the degree of agreement between different datasets and methodologies.…”

“…In this case, the results are very similar (<5% difference). Although a simple Gaussian is not the best approach in photometric surveys (López-Sanjuan et al 2014;Molino et al 2014), the bright galaxies centered in clusters present high S/N ratios, making reasonable the approximation. Therefore, we decided to use the second approach for computational and disk space purposes.…”

An accurate cluster selection function for the J-PAS narrow-band wide-field survey

“…This kind of data, in the frontier between spectroscopic and photometric surveys, has never been explored before and it will allow us to reliably map the large-scale structure in 3D down to fainter magnitudes and larger areas than previous spectroscopic samples. To realise the full scientific potential of these data, new algorithms and techniques are being developed and tested and cos-1 http://www.ipac.caltech.edu/wfirst/ mological constraints will be forecasted (Xavier et al 2014;López-Sanjuan et al 2014).…”