Morphology and metallicity of the Small Magellanic Cloud using RRab stars

TL;DR

This study analyzes the 3D structure and metallicity distribution of the Small Magellanic Cloud using RRab stars from OGLE-III data, revealing a possible bulge and no radial metallicity gradient.

Contribution

It provides a detailed 3D structural model of the SMC and investigates its metallicity distribution, including the first evidence of a bulge component.

Findings

Identified NE arm and main body in 3D structure.

No radial metallicity gradient detected.

Estimated the SMC's axes ratios and orientation parameters.

Abstract

We present a study of three-dimensional structure of the Small Magellanic Cloud (SMC). The $V$- and $I$-band light curves of the fundamental mode RR Lyrae stars (RRab) obtained by the Optical Gravitational Lensing Experiment (OGLE)-III project were utilized in order to comprehend the SMC structure. The $[Fe/H]-P-\phi_{31}$ relation of \citet{jurc96} is exploited to obtain the metallicities. From the three-dimensional RRab distance distributions, northeast (NE) arm and main body of the galaxy is identified. Combining metallicities with spatial distribution of these tracers, no radial metallicity gradient in the SMC has been detected. Dividing the entire sample into three parts: northeastern (NE), central and southwestern (SW), we find that the central part has a significantly larger line of sight depth as compared to rest of the parts, indicating that the SMC may have a bulge. Results obtained from the $I$-band data seem to be reliable and were further substantiated using the \citet{smol05} relation. Distribution of SMC RRab stars were modeled as a tri-axial ellipsoid. Errors in structural parameters of the SMC ellipsoid were obtained from Monte Carlo simulations. We estimated the axes ratios of the galaxy as $1.00\pm 0.000:1.310\pm 0.029:8.269\pm0.934$, the inclination of the longest axis with line of sight $i = 2^{\circ}.265\pm 0^{\circ}.784$, and the position angle of the line of nodes $\theta_{\text{lon}}=74^{\circ}.307\pm 0^{\circ}.509$ from the variance weighted $I$-band determinations.