Cosmic Variance and Its Effect on the Luminosity Function Determination in Deep High z Surveys

TL;DR

This paper analyzes how cosmic variance impacts the measurement of galaxy luminosity functions in high-redshift surveys, highlighting potential biases and providing tools for correction.

Contribution

It introduces a method to quantify cosmic variance effects using correlation functions and synthetic catalogs, emphasizing the importance of environment effects on luminosity function fitting.

Findings

Cosmic variance causes both normalization and slope variations in luminosity functions.

Underdense fields show a steepening of the luminosity function slope.

A public cosmic variance calculator tool is provided for researchers.

Abstract

We study cosmic variance in deep high redshift surveys and its influence on the determination of the luminosity function for high redshift galaxies. For several survey geometries relevant for HST and JWST instruments, we characterize the distribution of the galaxy number counts. This is obtained by means of analytic estimates via the two point correlation function in extended Press-Schechter theory as well as by using synthetic catalogs extracted from N-body cosmological simulations of structure formation. We adopt a simple luminosity - dark halo mass relation to investigate the environment effects on the fitting of the luminosity function. We show that in addition to variations of the normalization of the luminosity function, a steepening of its slope is also expected in underdense fields, similarly to what is observed within voids in the local universe. Therefore, to avoid introducing artificial biases, caution must be taken when attempting to correct for field underdensity, such as in the case of HST UDF i-dropout sample, which exhibits a deficit of bright counts with respect to the average counts in GOODS. A public version of the cosmic variance calculator based on the two point correlation function integration is made available on the web.