The 3D structure of the Small Magellanic Cloud

TL;DR

This study maps the 3D structure of the Small Magellanic Cloud using red clump and RR Lyrae stars, revealing a spheroidal or slightly ellipsoidal shape with evidence of past merger activity.

Contribution

It provides the first detailed 3D structural analysis of the SMC using multiple stellar populations and estimates its shape, orientation, and merger history.

Findings

SMC has a large line of sight depth.

The SMC is approximated as a triaxial ellipsoid.

Evidence suggests a past merger event 4-5 Gyr ago.

Abstract

The 3D structure of the inner Small Magellanic Cloud (SMC) is investigated using the red clump (RC) stars and the RR Lyrae stars (RRLS). The V and I pass bands photometric data from the Optical Gravitational Lensing Experiment (OGLE III) catalogue are used. The mean dereddened I${_0}$ magnitude and the dispersion in the I${_0}$ magnitudes are used to estimate the relative positions of different regions in the SMC and the line of sight (LOS) depth across the SMC respectively. From the analysis of both the populations we found that the north eastern part of the SMC is closer to us and the line of sight depth across the SMC is large. The radial density profile of the RC stars suggest that they are more likely to be distributed in a nearly spheroidal system. An elongation along the NE-SW direction is seen in the distribution of the RC stars. Based on these results the observed SMC is approximated as a triaxial ellipsoidal galaxy. The parameters of the ellipsoid obtained using both the populations are found to be similar. The above estimated parameters depend on the data coverage of the SMC. Using the RRLS with equal coverage in all the three axes (data within 3 degrees in X, Y and Z axes), we estimated an axes ratio of 1:1.33:1.61 with an inclination, i = 2.6 deg and position angle of the projection of the ellipsoid, \phi = 70.2 deg. Our tidal radius estimates and the recent observational studies suggest that the full extent of the SMC in the XY plane is of the order of the front to back distance estimated along the line of sight. These results suggest that the structure of the SMC is spheroidal or slightly ellipsoidal. We propose that the SMC experienced a merger with another dwarf galaxy at ~ 4-5 Gyr ago, and the merger process was completed in another 2-3 Gyr. This resulted in a spheroidal distribution comprising of stars older than 2 Gyr.