The definition of environment and its relation to the quenching of galaxies at z = 1–2 in a hierarchical Universe

Abstract

A well calibrated method to describe the environment of galaxies at all redshifts is essential for the study of structure formation. Such a calibration should include well understood correlations with halo mass, and the possibility to identify galaxies which dominate their potential well (centrals), and their satellites. Focusing on z∼ 1 and 2 we propose a method of environmental calibration which can be applied to the next generation of low to medium resolution spectroscopic surveys. Using an up-to-date semi-analytic model of galaxy formation, we measure the local density of galaxies in fixed apertures on different scales. There is a clear correlation of density with halo mass for satellite galaxies, while a significant population of low mass centrals is found at high densities in the neighbourhood of massive haloes. In this case the density simply traces the mass of the most massive halo within the aperture. To identify central and satellite galaxies, we apply an observationally motivated stellar mass rank method which is both highly pure and complete, especially in the more massive haloes where such a division is most meaningful. Finally we examine a test case for the recovery of environmental trends: the passive fraction of galaxies and its dependence on stellar and halo mass for centrals and satellites. With careful calibration, observationally defined quantities do a good job of recovering known trends in the model. This result stands even with reduced redshift accuracy, provided the sample is deep enough to preserve a wide dynamic range of density.

Galaxy Environment in the 3D-HST Fields: Witnessing the Onset of Satellite Quenching at z ∼ 1–2

“…QUANTIFICATION OF THE ENVIRONMENT There are many ways to describe the environment in which a galaxy lives (e.g., Haas et al 2012;Muldrew et al 2012;Etherington & Thomas 2015). In this work, we apply to observational data the method we explored and calibrated in Fossati et al (2015) and based on the work of Wilman et al (2010). We use the number density of neighbouring galaxies within fixed cylindrical apertures because it is more sensitive to high overdensities, less biased by the viewing angle, more robust across cosmic times, and easier to physically interpret and calibrate than the N th nearest neighbour methods (Shattow et al 2013).…”

“…The main advantage of using multiple parameters is to break degeneracies which are otherwise dominant if only one parameter is used (e.g. to account for the role of stellar mass at low density, where halo mass depends more significantly on stellar mass than density; Fossati et al 2015). Fossati et al (2015) explored how the rank in stellar mass of a galaxy in an appropriate aperture can be a good discriminator of the central/satellite status for a galaxy.…”

“…These values are calibrated using the models (see Fossati et al 2015). We also limit the aperture between 0.35 and 1.00 Mpc.…”

“…For instance in galaxy pairs or small groups (where the mass of the real central and satellites are very close), it is difficult to use the stellar mass to robustly define which galaxy is the central. On the other hand, in the infalling regions beyond the virial radius of massive clusters, many central galaxies would be classified as satellites as analysed in detail in Fossati et al (2015). In this work, we go beyond the simple dichotomic definition that each galaxy is either central or satellite using the mass-rank only.…”

“…In Fossati et al (2015), we studied how to link a purely observational parameter space to physical quantities (e.g., halo mass, central/satellite status) by analysing a stellar mass limited sample extracted from semi-analytic models of galaxy formation. To do so, we computed a projected density field in the simulation box and we tested different definitions of density at different redshift accuracy.…”