Formation of the Small Magellanic Cloud: ancient major merger as a solution to the kinematical differences between old stars and HI gas

TL;DR

This paper proposes that an ancient major merger between gas-rich dwarf irregulars explains the Small Magellanic Cloud's current kinematic differences between old stars and HI gas, supported by simulations showing distinct stellar and gas dynamics.

Contribution

It introduces a novel merger scenario that accounts for the SMC's kinematic properties, supported by simulations of dwarf-dwarf mergers resulting in a spheroidal stellar component and rotating HI disk.

Findings

Simulations produce a spheroidal stellar component with little rotation.

Simulations generate a rotating extended HI gas disk.

The model explains the observed kinematic differences in the SMC.

Abstract

Recent observations of the Small Magellanic Cloud (SMC) have revealed that the HI gas shows a significant amount of rotation (V_c 60 km/s), while no or little rotation is evident for the old stellar populations. We suggest that this unique kinematical difference between these components in the SMC can be caused by a major merger event which occurred in the early stage of the SMC formation. Our simulations show that dissipative dwarf-dwarf merging can transform two gas-rich dwarf irregulars into a new dwarf, which consists of a spheroidal stellar component and a rotating extended HI disk. The remnant of this dwarf-dwarf merging shows significantly different kinematics between stars and gas, in the sense that a gas disk rotates rapidly while a stellar component shows little rotation. We thus suggest that the simulated dwarf having a dynamically hot spheroid and an extended gas disk finally evolves into the present SMC after efficient stripping of the outer gas via tidal fields of the Galaxy and the Large Magellanic Cloud. We also suggest that spatial distributions and kinematics of RGB and AGB stars with different ages in the possible spheroidal component of the SMC can provide valuable information on whether and when a past major merger event really occurred in the SMC.