Outside-In Evolution with a Twist: Metallicity Gradients and Asymmetries in the SMC

TL;DR

Using Ca II triplet spectroscopy of 3,697 SMC red giants, the study derives homogeneous metallicities and constructs a two-dimensional metallicity map. It reveals a global negative radial gradient of $[Fe/H]$ consistent with outside-in evolution, with a tentative positive gradient in the central region suggesting radial migration or central enrichment. Azimuthal variations show flatter gradients in the east/south and steeper ones in the north/west, aligning with tidal effects from the LMC, while eastern distance/velocity bifurcations do not correspond to metallicity differences, implying a common chemical origin. A notable metal-poor population is present in the outskirts, especially to the east, yet depth variations do not drive the main trends, underscoring a complex, interaction-influenced chemical structure for the SMC.

Abstract

Taking advantage of the near-infrared calcium triplet lines, we determine metallicities for a sample of more than 3,500 red giant stars in the field of the Small Magellanic Cloud (SMC). We find a median metallicity of [Fe/H]=-1.05$\pm$0.01 dex with a negative metallicity gradient of -0.064$\pm$0.007 dex deg$^{-1}$ between 1.2°~to 6.0°consistent with an outside-in evolution scenario. For the first time, we detect hints of a positive metallicity gradient within 1.2°, likely reflecting radial migration or centralised chemical enrichment. Azimuthal metallicity asymmetries are detected, with flatter gradients in the eastern and southern quadrants and steeper ones in the north and west. They are consistent with tidal interaction effects from the Large Magellanic Cloud (LMC). Finally, in spite of a clear distance and velocity bifurcations in the east, they seem to share a common chemical origin, in agreement with other studies.

Figures (12)

• Figure 1: Spatial distribution of our sample (blue points) overimposed to the Hi map 2015Kalberla. The location of the objects studied by C08: Carrera2008 and P10 and P16: Parisi2010Parisi2016. The four quadrants and the two example ellipses used in our analysis are also shown as references (see text for details).
• Figure 2: Residual metallicities of each cluster across the four bands. The cross symbols illustrate the discrepancies between the metallicities in this work and those of D14, both using the $K_s$ band. The blue dashed lines are the standard deviation of the differences of the CaT calibration (Fig. 3 of Navabi et al. ( submitted.).
• Figure 3: The metallicity distribution of our sample with both K$_s$ (red solid line) and GaiaG (blue dashed line) magnitudes. The details of these distributors are summarized in Table \ref{['tab:metallicity_distributions']}.
• Figure 4: The metallicity gradient across semi-major axis of the ellipses as described in Sec.\ref{['sec:radial']}. For comparison, we include the median metallicities of field stars from C08, P10, P16: Carrera2008, Parisi2010, Parisi2016, D14 and clusters from P09, P15, P22, B22: Parisi2009, Parisi2015, Parisi2022, and Bortoli2022, respectively (see inset). The top of the x-axis shows the corresponding distance in kpc.
• Figure 5: Normalised [Fe/H] distributions for RGB stars in the four quadrants of the SMC: East (blue), South (red), West (black), and North (green). A bootstrap sampling method was applied here to determine the statistics for each quadrant listed in Table \ref{['tab:metallicity_distributions']}.