Simulations first to reveal fundamental plane of star formation
26 November 2015
Galaxy formation involves many inter-linked physical processes at any given time: the rate at which the galaxy's halo accretes mass from the intergalactic medium, the rate of shocking and cooling of this gas onto the galaxy, and the conversion of gas in the interstellar medium into stars. The complexity and non-linearity of these processes make it difficult to understand which processes dominate, and if and how this changes over time. The identification of scaling relations, that is the tight correlation between certain physical galaxy properties, can therefore be very valuable to reduce the number of properties in galaxy formation models and to formulate simple relations that capture the dominant paths along which galaxies evolve. While helpful for this reason, these relations cannot ultimately distinguish between cause and effect though. Cosmological simulations of galaxy formation are excellent testbeds as they allow examining causality directly. If reproducing the observed scaling relations adequately, these simulations can be used to better understand how galaxies evolve and to predict how scaling relations are established, how they evolve, and which processes determine the scatter around the main trends.
In their current publication, the research team around CAASTRO member Dr Claudia Lagos (ICRAR-UWA) investigated the correlations between different physical properties of star-forming galaxies in the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) cosmological hydrodynamical simulation suite over the redshift range 0<z<4.5. Their careful statistical analysis revealed that the neutral gas fraction, stellar mass and star formation rate account for most of the variance seen in the population of galaxies at all times. Galaxies trace a two-dimensional, nearly flat surface in the three-dimensional space of the properties above which the team names "Fundamental plane of star formation". The location of this plane varies little in time, whereas galaxies themselves move along the plane as their gas fraction and star formation rate decrease over time. The existence of this "fundamental plane of star formation" is a consequence of the self-regulation of galaxies in which the accretion of newly cooled gas and feedback outflows from stars and Active Galactic Nuclei balance each other out: the rate at which gas flows into and out from the galaxy are similar. Excitingly, using the insights from their simulations, the researchers found that real galaxies follow the same plane, based on a large compilation of observations spanning the redshift range 0<z<2.5. This is the first time that the existence of such a plane was initially established in simulations and then confirmed in observations.