K+a Galaxies as the Aftermath of Gas-rich Mergers: Simulating the Evolution of Galaxies as Seen by Spectroscopic Surveys

Abstract

Models of poststarburst (or "K+A") galaxies are constructed by combining fully three-dimensional hydrodynamic simulations of galaxy mergers with radiative transfer calculations of dust attenuation. Spectral line catalogs are generated automatically from moderate-resolution optical spectra calculated as a function of merger progress in each of a large suite of simulations. The mass, gas fraction, orbital parameters, and mass ratio of the merging galaxies are varied systematically, showing that the lifetime and properties of the K+A phase are strong functions of the merger scenario. K+A durations are generally less than or similar to 0.1-0.3 Gyr, significantly shorter than the commonly assumed 1 Gyr, which is obtained only in rare cases, owing to a wide variation in star formation histories resulting from different orbital and progenitor configurations. Combined with empirical merger rates, the model lifetimes predict rapidly rising K+A fractions as a function of redshift that are consistent with results of large spectroscopic surveys, resolving tension between the observed K+A abundance and that predicted when one assumes the K+A duration is the lifetime of A stars (similar to 1 Gyr). These simulated spectra are spatially resolved on scales of about 1 kpc, and indicate that a centrally concentrated starburst causes the Balmer absorption strengths to increase toward the central few kiloparsecs of the remnant. The effects of dust attenuation, viewing angle, and aperture bias on our models are analyzed. In some cases, the K+A features are longer-lived and more pronounced when active galactic nucleus (AGN) feedback removes dust from the center, uncovering the young stars formed during the burst. In this picture, the K+A phase begins during or shortly after the bright starburst/AGN phase in violent mergers, and thus offers a unique opportunity to study the effects of quasar and star formation feedback on the gas reservoir and evolution of the remnant. Analytic fitting formulae are provided for the estimates of K+A incidence as a function of merger scenario.