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 EStimation, or BAGPIPES, a new Python tool which can be used to rapidly generate complex model galaxy spectra and to fit these to arbitrary combinations of spectroscopic and photometric data using the MultiNest nested sampling algorithm. We extensively test our ability to recover realistic star-formation histories (SFHs) by fitting mock observations of quiescent galaxies from the MUFASA simulation. We then perform a detailed analysis of the SFHs of a sample of 9289 quiescent galaxies from UltraVISTA with stellar masses, $M_* > 10^{10}\ \mathrm{M_\odot}$ and redshifts $0.25 < z < 3.75$. The majority of our sample exhibit SFHs which rise gradually then quench relatively rapidly, over $1{-}2$ Gyr. This behaviour is consistent with recent cosmological hydrodynamic simulations, where AGN-driven feedback in the low-accretion (jet) mode is the dominant quenching mechanism. At $z > 1$ we also find a class of objects with SFHs which rise and fall very rapidly, with quenching timescales of $< 1$ Gyr, consistent with quasar-mode AGN feedback. Finally, at $z < 1$ we find a population with SFHs which quench more slowly than they rise, over $>3$ Gyr, which we speculate to be the result of diminishing overall cosmic gas supply. We confirm the mass-accelerated evolution (downsizing) trend, and a trend towards more rapid quenching at higher stellar masses. However, our results suggest that the latter is a natural consequence of mass-accelerated evolution, rather than a change in quenching physics with stellar mass. We find $61\pm8$ per cent of $z > 1.5$ massive quenched galaxies undergo significant further evolution by $z = 0.5$. BAGPIPES is available at https://bagpipes.readthedocs.io

• The paper introduces BAGPIPES, a flexible Bayesian spectral modeling tool, to accurately recover star-formation histories in massive quiescent galaxies.
• It reveals a downsizing trend where more massive galaxies assemble earlier and a decline in formation redshift as observed redshift decreases.
• The study identifies three quenching regimes—rapid, moderate, and slow—each linked to distinct processes such as AGN feedback and gas depletion.

Inferring the Star-Formation Histories of Massive Quiescent Galaxies with BAGPIPES

The study presented by A. C. Carnall et al. explores the intricate task of reconstructing the star-formation histories (SFHs) of massive quiescent galaxies using a novel tool called BAGPIPES. This Python-based software utilizes Bayesian techniques to fit detailed model galaxy spectra to observational data. A significant piece of this work is its application in understanding the mechanisms that lead to the quenching of star formation in galaxies with stellar masses exceeding $10^{10}\, M_\odot$.

Methodology Overview

The paper introduces BAGPIPES, a spectral modeling framework designed for flexibility and speed in generating complex galaxy models. It employs the MultiNest nested sampling algorithm to explore high-dimensional parameter spaces efficiently, crucial for examining degenerate SFH models. The study conducts tests using simulated data from the Mufasa suite to ensure the reliability of various SFH parameterizations. The comparison revealed that the double-power-law parameterization, with specified priors, accurately recovers the SFHs without introducing sizable biases.

Subsequent to its validation, BAGPIPES was applied to a large sample from the UltraVISTA survey, targeting massive quiescent galaxies across a substantial redshift range (0.25 < $z$ < 3.75). The data comprised both broad-band photometric observations and median photometric redshifts from a well-calibrated catalogue, providing a robust testbed for the methodology.

Key Results and Analysis

The analysis yields several notable trends concerning the SFHs of quiescent galaxies:

1. Downsizing Phenomenon: Consistent with previous findings, a clear downsizing trend is observed, where more massive galaxies typically exhibit higher redshift formation (Fig. 5). This suggests an earlier assembly of stellar mass in these systems, aligning with the mass-accelerated evolution hypothesis.
2. Decline in Formation Redshift with Observed Redshift: The study documents a decrease in average formation redshift as observed redshift decreases. This is indicative of an ongoing assembly of the red sequence, pointing to the continual emergence of new galaxies quenching across the cosmos.
3. Quenching Mechanisms and Timescales: The detected SFH shapes are aggregated into three distinct types, each linked to different quenching mechanisms:
   • Rapid Quenching: Predominantly found at $z > 1$, possibly related to quasar-mode AGN feedback.
   • Moderate Quenching: The dominant mode across all samples, potentially resultant from jet-mode feedback.
   • Slow Quenching: Emergent at $z < 1$, possibly an effect of the diminishing cosmic gas reservoir.

Implications

This investigation has implications for the theoretical understanding of galaxy evolution, particularly in the context of quenching processes. It provides evidence supporting multiple quenching mechanisms and confirms the nuanced effects of environment and redshift on galaxy SFHs. The insights into the duration and timing of star formation interruptions are vital for calibrating models of galaxy formation and evolution.

Future Prospects

While the study significantly advances the field's understanding, it underscores the potential for further exploration using spectroscopic data to refine SFH reconstruction. The introduction of BAGPIPES offers a substantial tool for future research, potentially aiding in the disentanglement of quenching mechanisms in various environmental contexts and contributing to fine-tuning theoretical models.

In conclusion, the work of A. C. Carnall et al. embodies a robust move toward leveraging advanced statistical tools to decipher the complexities of galaxy evolution, with a marked emphasis on handling the quenching of star formation in massive galaxies.