Simulating the Fundamental Plane of Early Type Galaxies

Recent low-redshift observations have attempted to determine the star formation histories of elliptical galaxies by tracking correlations between the stellar population parameters (age and metallicity) and the structural parameters that enter the funda- mental plane (size Re and velocity dispersion Ï). These studies have found that velocity dispersion, rather than effective radius or dynamical mass, is the main predictor of a galaxyâs stellar age and metallicity. I apply an analytic model that predicts the structural properties of bulge-dominated galaxies from a semi-analytic model (SAM). Predicting the effective radius, velocity dispersion, luminosity, age, and metallicity of bulge-dominated galaxies enables a direct comparison to observations of early-type galaxies. While I find a tight correlation between age and metallicity and velocity dispersion, there is a stronger dependence of metallicity on effective radius than observations report. I find that the correlations with velocity dispersion arise as a result of the strong link between the assembly time of a galaxy and its velocity dispersion. Furthermore, minor mergers introduce a large amount of scatter in size, weakening any dependence on effective radius. I examine the formation and assembly histories of the simulated galaxies and find that galaxies with relatively low surface brightnesses have early formation and assembly times, short star formation timescales, and high dynamical-to-stellar mass ratios. In contrast their stellar mass-to-light ratios are similar to those of galaxies with higher surface brightnesses. These correlations all suggest that the scatter in the fundamental plane results from structural differences in the galaxies themselves and not passive fading from a young, bright stellar population.

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