Inferring the star formation histories of massive quiescent galaxies with bagpipes: evidence for multiple quenching mechanisms

Abstract

We present Bayesian Analysis of Galaxies for Physical Inference and Parameter\nEStimation, or BAGPIPES, a new Python tool which can be used to rapidly\ngenerate complex model galaxy spectra and to fit these to arbitrary\ncombinations of spectroscopic and photometric data using the MultiNest nested\nsampling algorithm. We extensively test our ability to recover realistic\nstar-formation histories (SFHs) by fitting mock observations of quiescent\ngalaxies from the MUFASA simulation. We then perform a detailed analysis of the\nSFHs of a sample of 9289 quiescent galaxies from UltraVISTA with stellar\nmasses, $M_* > 10^{10}\\ \\mathrm{M_\\odot}$ and redshifts $0.25 < z < 3.75$. The\nmajority of our sample exhibit SFHs which rise gradually then quench relatively\nrapidly, over $1{-}2$ Gyr. This behaviour is consistent with recent\ncosmological hydrodynamic simulations, where AGN-driven feedback in the\nlow-accretion (jet) mode is the dominant quenching mechanism. At $z > 1$ we\nalso find a class of objects with SFHs which rise and fall very rapidly, with\nquenching timescales of $< 1$ Gyr, consistent with quasar-mode AGN feedback.\nFinally, at $z < 1$ we find a population with SFHs which quench more slowly\nthan they rise, over $>3$ Gyr, which we speculate to be the result of\ndiminishing overall cosmic gas supply. We confirm the mass-accelerated\nevolution (downsizing) trend, and a trend towards more rapid quenching at\nhigher stellar masses. However, our results suggest that the latter is a\nnatural consequence of mass-accelerated evolution, rather than a change in\nquenching physics with stellar mass. We find $61\\pm8$ per cent of $z > 1.5$\nmassive quenched galaxies undergo significant further evolution by $z = 0.5$.\nBAGPIPES is available at https://bagpipes.readthedocs.io\n