SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation

TL;DR

This study compares simulated galaxy merger remnants with observed elliptical galaxies, finding that gas-rich mergers with star formation reproduce observed properties better than dry mergers, and hierarchical mergers explain slow rotators.

Contribution

It demonstrates that gas-rich mergers with star formation match the SAURON elliptical galaxy kinematic trends, challenging the binary merger formation scenario.

Findings

Gas-rich mergers reproduce the SAURON relationship for flattened ellipticals.

Dry mergers do not produce the low ellipticity, low anisotropy ellipticals.

Hierarchical mergers of starbursting subunits align with observed slow rotators.

Abstract

The intrinsic anisotropy delta and flattening epsilon of simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analysed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation cannot reproduce the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is however found for merger simulations of disks with gas fractions >= 20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of merger remnants however does not generate the slowly-rotating SAURON ellipticals which are characterized by low ellipticities epsilon < 0.25 and low anisotropies. This indicates that at least some ellipticals on the red galaxy sequence did not form by binary mergers of disks or early-type galaxies. We show that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend of delta with epsilon. The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits with the angular momentum of the system determining its location on the relation.