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