Influence of Cooled Interstellar Gas on the Fundamental Plane of Elliptical Galaxies

Abstract

We explore the possible important influence of cooled interstellar gas on the fundamental plane of elliptical galaxies. Interstellar cooling is described by a parameterized sink term in the equation of continuity. Parameters that give the best fits to the X-ray observations of elliptical galaxy NGC 4472 are used as a template for other galaxies. These spatially dependent cooling parameters are then extended homologously to elliptical galaxies of lower mass for which X-ray observations of the hot gas are currently unavailable or impossible because of stronger relative X-ray emission from binary stars. Most of the interstellar gas cools within an effective radius where it can contribute an additional 10%-30% to the mass of the old stellar population. The absence of observed cooled gas and simple theoretical arguments support the hypothesis that the cooled gas is forming into low-mass stars, with implications that depend on the initial mass function (IMF) of this young stellar population. If the IMF includes only stars of very low mass, M☉ as is commonly believed, the cooled mass is optically dark. For this IMF, the mass-to-light ratios determined from stellar velocities systematically overestimate that of the old stellar population. Furthermore, the total mass and spatial distribution of the optically dark stellar population does not scale homologously with galactic luminosity or radius, so the total stellar mass-to-light ratio is expected to vary with galactic radius. These variations in stellar population and nonhomology can introduce pronounced deviations from the fundamental plane. We investigate cooled gas perturbations to the mass-to-light ratio for several idealized homologous elliptical galaxies and show that they can be incompatible with the observed thinness of the fundamental plane. However, if the IMF of the stellar population produced from cooled interstellar gas is optically luminous, its influence on observed stellar mass-to-light ratios and the fundamental plane would be lessened. If the IMF of a young, continuously forming, and optically luminous stellar population were sufficiently well behaved, it could account for about 10% of the optical light from large elliptical galaxies within re, possibly having important implications for understanding the fundamental plane.