Abstract
Galaxy populations at different cosmic epochs are often linked together by comoving cumulative number density in observational studies. Many theoretical works, however, have shown that the number densities of tracked galaxy populations evolve in bulk and spread out over time. We present a number density method for linking progenitor and descendant galaxy populations which takes both of these effects into account. We define probability distribution functions that capture the evolution and dispersion of galaxy populations in comoving number density space, and use these functions to assign galaxies at one redshift $z_f$ probabilities of being progenitors or descendants of a galaxy population at another redshift $z_0$. These probabilities are then used as weights for calculating distributions of physical properties such as stellar mass, star formation rate, or velocity dispersion within the progenitor/descendant population. We demonstrate that this probabilistic method provides more accurate predictions for the evolution of physical properties then either the assumption of a constant number density or the assumption of an evolving number density in a bin of fixed width by comparing the predictions against galaxy populations directly tracked through a cosmological simulation. We find that the constant number density method performs most poorly at recovering galaxy properties, the evolving number method density slightly better, and the probabilistic number density method best of all. The improvement is present for predictions of both stellar mass as well as inferred quantities such as star formation rate and velocity dispersion which were not included in the number density fits. We demonstrate that this method can also be applied robustly and easily to observational data, and provide a code package for doing so.
Highlights
Extragalactic observations have provided us with a wealth of data about how the global population of galaxies grows and evolves throughout cosmic time
Galaxies are ranked according to their stellar mass and assigned the comoving number density of all galaxies which are at least as massive
In T15, we examined how galaxy populations in the cosmological hydrodynamical simulation Illustris evolve in number density from 0 < z < 3, and found fits to the number density evolution which are strikingly similar to those from Behroozi et al (2013) and Leja, van Dokkum & Franx (2013)
Summary
Extragalactic observations have provided us with a wealth of data about how the global population of galaxies grows and evolves throughout cosmic time. Armed solely with a set of CMFs, one can predict the progenitors or descendants of a population of galaxies by identifying its number density at one redshift and finding the stellar mass corresponding to that same number density at other redshifts This method has been used to predict the evolution of galaxies’ stellar masses (van Dokkum et al 2013; Brammer et al 2011; Muzzin et al 2013) as well as numerous other properties including star formation rate (Papovich et al 2011; Fumagalli et al 2012), velocity dispersion (Bezanson et al 2011), gas content (Conselice et al 2013), and size (Patel et al 2013)
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