[Paper Review] SMC west halo: a slice of the galaxy that is being tidally stripped? Star clusters trace age and metallicity gradients

Bruno Dias, L. O. Kerber+3 more|arXiv (Cornell University)|Apr 11, 2016

Stellar, planetary, and galactic studies65 references3 citations

TL;DR

This study analyzes nine star clusters in the SMC's west halo (WH) using V(B-V) color-magnitude diagrams from SOAR telescope photometry to derive homogeneous ages, metallicities, distances, and reddenings. It confirms strong age and metallicity gradients in the WH, with a low-dispersion age-metallicity relation compatible with tidal stripping by the LMC, suggesting the WH is a tidally stripped remnant of the SMC’s interaction with the LMC.

ABSTRACT

(ABRIDGED) The evolution and structure of the Magellanic Clouds is presently under debate. The classical scenario where both the Large and Small Magellanic Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an alternative where the LMC and SMC are in their first close passage to our Galaxy. Detailed studies of stellar populations in the galaxies should constrain the proposed scenarios. In particular, the west halo of the SMC was recently characterized with radial trends in age and metallicity which indicates tidal disruption. We increase the sample of star clusters in the west halo of the SMC with homogeneous age, metallicity, and distance derivations, to determine better age and metallicity gradients. Comparisons of observed and synthetic V,(B-V) colour-magnitude diagrams are used to derive parameters for west halo star clusters. We derived age and metallicity for the reference cluster NGC 152 compatible with literature parameters. Age and metallicity gradients are confirmed in the west halo: 2.6 +/- 0.6 Gyr/deg and -0.19 +/- 0.09 dex/deg, respectively. Age-metallicity relation for the west halo has low dispersion in metallicity and it is compatible with a burst model of chemical enrichment. All WH clusters seem to follow the same predicted stellar distribution, with exception of AM-3 that should belong to the counter-bridge. Bruck 6 is only 130 +/- 40 Myr old and it could have been formed during a recent tidal interaction of the SMC-LMC. We suggest that it is crucial to split the SMC cluster population in groups: main body, wing/bridge, counter-bridge and west halo. This is the way to analyse the complex star formation and dynamical history of our neighbour. In particular we show that west halo has clear age and metallicity gradients and age-metallicity relation, also compatible with the dynamical model of tidal influence of the LMC over the SMC.

Motivation & Objective

• To increase the sample of star clusters in the SMC west halo with homogeneous age, metallicity, and distance measurements.
• To test whether age and metallicity gradients in the WH support tidal stripping by the LMC during a close encounter.
• To compare cluster positions and kinematics with dynamical models of SMC-LMC interaction to assess orbital history.
• To propose a region-based classification of SMC clusters (main body, wing, bridge, counter-bridge, WH) for improved analysis of star formation and dynamical history.
• To identify key clusters like Brück 6 and AM-3 as tracers of recent tidal interactions and structural components of the SMC.

Proposed method

• Photometric analysis of V(B-V) color-magnitude diagrams (CMDs) from SOAR 4.1m telescope observations.
• Comparison of observed CMDs with synthetic CMDs to derive age, metallicity, distance, and reddening for each cluster.
• Artificial star tests to assess photometric completeness and reliability of cluster member selection.
• Statistical comparison with a control field to identify cluster members and reduce field star contamination.
• Radial gradient analysis of age and metallicity across the WH using cluster positions relative to the SMC center.
• Model comparison with the dynamical simulation of Besla et al. (2011) to test consistency of cluster distribution with tidal stripping predictions.

Experimental results

Research questions

• RQ1 Do star clusters in the SMC west halo exhibit radial gradients in age and metallicity consistent with tidal stripping?
• RQ2 Is the age-metallicity relation in the WH consistent with a burst-like chemical enrichment model following tidal interaction?
• RQ3 How do the spatial distributions of WH clusters compare with predictions from dynamical models of SMC-LMC interaction?
• RQ4 Can clusters like AM-3 and Brück 6 be assigned to specific tidal structures (e.g., counter-bridge, Magellanic bridge) based on their age and metallicity?
• RQ5 Is the observed discontinuity in age and metallicity between the main body and WH indicative of a distinct dynamical origin?

Key findings

• The west halo (WH) of the SMC exhibits a significant age gradient of 2.6 ± 0.6 Gyr/° with a negative linear coefficient of -3.7 ± 1.8 Gyr, indicating younger clusters at larger radii.
• A metallicity gradient of -0.19 ± 0.09 dex/° is measured, with WH clusters at [Fe/H] ≈ -0.8, significantly higher than the main body’s [Fe/H] ≈ -1.3 at the boundary.
• The age-metallicity relation for WH clusters shows low dispersion and is consistent with a burst chemical enrichment model (Pagel & Tautvaisiene, 1998), supporting a short-lived enrichment event.
• Cluster Brück 6 is the youngest in the sample at 130 ± 40 Myr, consistent with formation during the tidal interaction that created the WH and the Magellanic bridge.
• AM-3, though located in the WH region, follows the age and metallicity trends of the counter-bridge, suggesting it is a stripped counter-bridge cluster rather than part of the WH.
• The spatial distribution of WH clusters aligns qualitatively with the dynamical model of Besla et al. (2011), which predicts stellar debris from SMC-LMC tidal interaction, particularly in the outer regions between the main body and tidal arms.