Surrogate modelling the Baryonic Universe I: The colour of star formation

TL;DR

This paper demonstrates that variations in star formation history primarily determine the optical and near-infrared colours of galaxies, with a simple, dominant influence along a single direction in colour space, supported by spectral modeling and analysis of SDSS data.

Contribution

It introduces a principal component analysis revealing that star formation history variations influence galaxy colours along a single dominant direction, simplifying understanding of galaxy colour diversity.

Findings

Galaxy colours are mainly affected by star formation history variations.

The projection of galaxy colours onto the SFH-direction correlates with recent stellar mass formation.

SDSS galaxy colours form an ellipsoid aligned with the SFH-direction.

Abstract

The spectral energy distribution of a galaxy emerges from the complex interplay of many physical ingredients, including its star formation history (SFH), metallicity evolution, and dust properties. Using GALAXPY, a new galaxy spectral prediction tool, and SFHs predicted by the empirical model UNIVERSEMACHINE and the cosmological hydrodynamical simulation IllustrisTNG, we isolate the influence of SFH on optical and near-infrared colours from 3200 to 10800 \r{A} at z=0. By carrying out a principal component analysis, we show that physically-motivated SFH variations modify galaxy colours along a single direction in colour space: the SFH-direction. We find that the projection of a galaxy's present-day colours onto the SFH-direction is almost completely regulated by the fraction of stellar mass that the galaxy formed over the last billion years. Together with cosmic downsizing, this results in galaxies becoming redder as their host halo mass increases. We additionally study the change in galaxy colours due to variations in metallicity, dust attenuation, and nebular emission lines, finding that these properties vary broad-band colours along distinct directions in colour space relative to the SFH-direction. Finally, we show that the colours of low-redshift SDSS galaxies span an ellipsoid with significant extent along two independent dimensions, and that the SFH-direction is well-aligned with the major axis of this ellipsoid. Our analysis supports the conclusion that variations in star formation history are the dominant influence on present-day galaxy colours, and that the nature of this influence is strikingly simple.