# An Older, More Quiescent Universe from Panchromatic SED Fitting of the 3D-HST Survey

## TL;DR

This study uses a comprehensive Bayesian model to re-estimate galaxy properties from the 3D-HST survey, revealing an older, more quiescent universe consistent with dynamical data and improving understanding of cosmic star formation history.

## Contribution

It introduces the Prospector-$eta$ model with 14 parameters for more accurate galaxy parameter inference, demonstrating its effectiveness on a large galaxy sample.

## Key findings

- Stellar masses are 0.1-0.3 dex larger with older SFHs.
- SFRs are 0.1-1+ dex lower, especially from old star emission.
- Results reconcile cosmic star formation rate density with stellar mass growth.

## Abstract

Galaxy observations are influenced by many physical parameters: stellar masses, star formation rates (SFRs), star formation histories (SFHs), metallicities, dust, black hole activity, and more. As a result, inferring accurate physical parameters requires high-dimensional models which capture or marginalize over this complexity. Here we re-assess inferences of galaxy stellar masses and SFRs using the 14-parameter physical model Prospector-$\alpha$ built in the Prospector Bayesian inference framework. We fit the photometry of 58,461 galaxies from the 3D-HST catalogs at $0.5 < z < 2.5$. The resulting stellar masses are $\sim0.1-0.3$ dex larger than the fiducial masses while remaining consistent with dynamical constraints. This change is primarily due to the systematically older SFHs inferred with Prospector. The SFRs are $\sim0.1-1+$ dex lower than UV+IR SFRs, with the largest offsets caused by emission from "old" ($t>100$ Myr) stars. These new inferences lower the observed cosmic star formation rate density by $\sim0.2$ dex and increase the observed stellar mass growth by $\sim 0.1$ dex, finally bringing these two quantities into agreement and implying an older, more quiescent Universe than found by previous studies at these redshifts. We corroborate these results by showing that the Prospector-$\alpha$ SFHs are both more physically realistic and are much better predictors of the evolution of the stellar mass function. Finally, we highlight examples of observational data which can break degeneracies in the current model; these observations can be incorporated into priors in future models to produce new & more accurate physical parameters.