Elliptical galaxies comprise the majority of luminous galaxies in our universe. Ellipticals have long been thought to form through gas-rich "major" mergers of two roughly equal-mass spiral galaxies. We propose instead that ellipticals form through gas-poor, mainly minor mergers of spirals in groups. This hypothesis is tested using a novel sample of hundreds of numerical simulations of mergers in groups of three to twenty-five spiral galaxies. These simulations are accompanied by mock observations of the central remnants in each group, comparing to data on ellipticals from galaxy surveys.
The simulated merger remnants have similar surface brightness profiles to observed ellipticals - if the spirals begin with concentrated bulges. The remnants follow tight size-luminosity and velocity dispersion-luminosity relations (<0.12 dex scatter), with similar slopes as observed. Stochastic merging can produce tight scaling relations if the merging galaxies follow tight scaling relations themselves. However, the remnants are too large and have too low dispersions at fixed luminosity. Some remnants show substantial (v/sigma > 0.1) rotational support, but most are slow rotators with v/sigma << 0.5.Ellipticals also follow a tight "fundamental plane" scaling relation between size, mean surface brightness and velocity dispersion: R ~ (sigma^a)(mu^b). This relation has tiny (<0.05 dex) scatter and significantly different coefficients from the expected scaling (a "tilt"). The remnants lie on a similar fundamental plane, with even smaller scatter than observed and a tilt in the correct sense (albeit weaker than observed). This tilt is mainly driven by variable dark matter fractions, such that massive merger remnants have larger central dark matter fractions than their lower-mass counterparts. The origin of this mass-dependent dark matter fraction and fundamental plane tilt is examined in detail, linking with dynamical masses and the virial theorem.
Contrary to previous studies, massive ellipticals can originate from multiple, mainly minor and dry mergers. However, significant gas dissipation may be needed to produce lower-mass, rapidly-rotating ellipticals.