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

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-α 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 ∼ 0.1 − 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 ∼ 0.1 − 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 ∼ 0.2 dex and increase the observed stellar mass growth by ∼ 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-α 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.

High-resolution, 3D radiative transfer modelling

“…We find that the old stellar population is the dominant heating source in the bulge region, while both old and young stellar populations are equally responsible for the dust heating in the bar region. -We confirm a strong link between f young and the sSFR which was previously reported in the radiative transfer model analysis of M 51 (De Looze et al 2014), M 31 (Viaene et al 2017), and M 81 , as well as in studies of (Nersesian et al 2019) for the DustPedia galaxy sample and Leja et al (2019) for the 3D-HST galaxy sample, and provide a relation to calibrate the contribution of the old stellar population to dust heating in global SED modelling. -We confirm that the central regions and the two diametrically opposed ends of the bar are places of enhanced star formation and show that the bar in those galaxies affects the radial profiles of the f young and dust temperature.…”

“…The concluding remarks in Leja et al (2019) agree quite well with the picture we draw here by studying the properties of local galaxies on resolved scales, as we also infer that the older stellar population has a more prominent role on the heating of the diffuse dust. The relation between the sSFR and the relative fraction of dust heated by the star-forming regions or by the old stellar populations has now been observed in a wide range of galaxy types and using various modelling approaches.…”

“…This correlation was further confirmed in the radiation transfer models of M 31 (Viaene et al 2017) and M 81 . Radiation transfer models aside, others have found the same relationship at both local galaxy samples (Viaene et al 2016;Nersesian et al 2019) and intermediate redshift (z) galaxies (Leja et al 2019). Figure 8 shows the relative contribution from the young stellar populations to the dust heating responsible for the TIR emission as was calculated for each dust cell and for all four galaxies of this study, as a function of log sSFR.…”

“…To some degree the shift of the sSFRf young relation towards higher sSFR values with increasing redshift can be attributed to the increased SFR, at least in the regime 0.5 < z < 1.5. In addition, Leja et al (2019) showed that the old stellar populations in high-redshifts (1.5 < z < 2.5) are relatively younger and on average more luminous, contributing more to the dust heating, which explains the decrease in f young with redshift.…”

“…However, the contribution of the old, more evolved, stars to the dust heating can be non-negligible (e.g. Hinz et al 2004;Calzetti et al 2010;Bendo et al 2010Bendo et al , 2015Boquien et al 2011;Bendo et al 2012;Smith et al 2012;De Looze et al 2014;Viaene et al 2017;Leja et al 2019;Nersesian et al 2019) and therefore needs to be taken into account while estimating the star-formation rate (SFR) of a galaxy.…”