The Fundamental Scaling Relations of Elliptical Galaxies

Abstract

We examine the fundamental scaling relations of elliptical galaxies formed through mergers. Using hundreds of simulations to judge the impact of progenitor galaxy properties on the properties of merger remnants, we find that gas dissipation provides an important contribution to tilt in the Fundamental Plane relation. Dissipationless mergers of disks produce remnants that occupy a plane similar to that delineated by the virial relation. As the gas content of progenitor disk galaxies is increased, the ti lt of the resulting Fundamental Plane relation increases and the slope of the Re - M⋆ relation steepens. For gas fractions fgas > 30%, the simulated Fundamental Plane scalings (Re ∝ σ 1.55 I -0.82 e ) approach those observed in the K-band (Re ∝ σ 1.53 I -0.79 e ). The dissipationless merging of spheroidal galaxies and the re-merging of disk galaxy remnants roughly maintain the tilt of the Fundamental Plane occupied by the progenitor ellipticals, approximately independent of the orbital energy or angular momentum. Dry merging of spheroidal systems at redshifts z < 1 is then expected to maintain the stellar-mass Fundamental Plane relations imprinted by gas-rich merging during the epoch of rapid spheroid and supermassive black hole growth at redshifts z ≈ 1 - 3. In our simulations, feedback from supermassive black hole growth has only a minor influence on the stellar-mass sca ling relations of spheroidal galaxies, but may play a role in maintaining the observed Fundamental Plane tilt at optical wavelengths by suppressing residual star formation in merger remnants. We estimate that ≈ 40 - 100% of the Fundamental Plane tilt induced by structural properties, as opposed to stellar population effects, owes to trends in the central total-to-stellar mass ratio Mtotal/M⋆ produced by dissipation. Gas cooling allows for an increase in central s tellar phase-space density relative to dissipationless mergers, thereby decreasing the central Mtotal/M⋆. Lower mass systems obtain greater phase-space densities than higher mass systems, producing a galaxy mass-dependent central Mtotal/M⋆ and a corresponding tilt in the Fundamental Plane. We account for these trends in the importance of dissipation with galaxy mass in terms of the inefficient cooling of collisionally heated gas in massi ve halos and dynamically varying gas consumption timescales in smaller systems.