Star Formation and Merging in Massive Galaxies at z < 2

TL;DR

This study investigates the formation and evolution of massive galaxies at redshifts below 2, highlighting ongoing star formation and merging activities that contribute significantly to their growth.

Contribution

It provides new insights into the star formation history and merger rates of massive galaxies at z < 2, challenging previous assumptions of their early formation.

Findings

40% of massive galaxies at z~1 are actively forming stars.

Massive galaxies undergo nearly one major merger between z=0.4 and 1.4.

Number and mass density evolution at z<1.5 is limited to a factor of 2-3.

Abstract

Observing massive galaxies at various redshifts is one of the most straightforward and direct approaches towards understanding galaxy formation. There is now largely a consensus that the massive galaxy (M_* > 10^11 M_0) population is fully formed by z~1, based on mass and luminosity functions. However, we argue that the latest data can only rule out number and mass density evolution of a factor of > 2-3 at z < 1.5. We furthermore show that the star formation history of M_* > 10^11 M_0 galaxies reveals that 40+/-5% of galaxies with M_* > 10^11 M_0 at z~1 are undergoing star formation that effectively doubles their stellar mass between z = 0.4 - 1.4. These massive galaxies also undergo 0.9^+0.7_-0.5 major mergers during this same time period.