How complex are galaxies? A non-parametric estimation of the intrinsic dimensionality of wide-band photometric data

Galaxies are complex objects, yet the number of independent parameters to describe them remains unknown. We present here a non-parametric method to estimate the intrinsic dimensionality of large data sets. We apply it to wide-band photometric data drawn from the COSMOS2020 catalogue and a comparable mock catalogue from the HORIZON-AGN simulation. Our galaxy catalogues are limited in signal-to-noise ratio (SNR) in all optical and near-infrared bands. Our results reveal that most of the variance in the wide-band photometry of this galaxy sample can be described with at most 4.3 ± 0.5 independent parameters for star-forming galaxies and 2.9 ± 0.2 for passive ones, both in the observed and simulated catalogues. We identify one of these parameters to be noise-driven, and recover that stellar mass and redshift are two key independent parameters driving the magnitudes. Our findings support the idea that wide-band photometry does not provide more than one additional independent parameter for star-forming galaxies. Although our sample is not mass-limited and may miss some passive galaxies due to our cut in SNR, our work suggests that dimensionality reduction techniques may be effectively used to explore and analyse wide-band photometric data, provided the used latent space is at least four-dimensional.