Characterizing and understanding galaxies with two parameters

Abstract

ABSTRACT We report the discovery of a 2D Galaxy Manifold within the multidimensional luminosity space of local galaxies. The multidimensional luminosity space is constructed using 11 bands that span from far ultraviolet to near-infrared for redshift < 0.1 galaxies observed with GALEX, SDSS, and UKIDSS. The manifold captures the diversity of observed galaxies in terms of stellar-dominated emissions and ties the correlations of various physical properties to the manifold. We find that two latent parameters are sufficient to express 93.2 per cent of the variance in the galaxy sample, suggesting that this Galaxy Manifold is one of the most efficient representations of galaxies. The transformation between the observed luminosities and the manifold parameters as an analytic mapping is provided. The manifold representation provides accurate (precision = 0.85) morphological classifications with a simple linear boundary, and galaxy properties can be estimated with minimal scatter (0.12 and 0.04 dex for star formation rate and stellar mass, respectively) by calibrating with the 2D manifold location. Under the assumption that the manifold expresses the possible parameter space of galaxies, the evolution on the manifold is considered. We find that constant and exponentially decreasing star formation histories form almost orthogonal modes of evolution on the manifold. Through these simple models, we understand that the two modes are closely related to gas content, which suggests the close relationship of the manifold to gas accretion. Lastly, the found manifold suggests a paradigm where galaxies are characterized by their mass/scale and specific SFR, which agrees with previous studies of dimensionality reduction.