The formation history of the Small Magellanic cloud ( SMC ) is unraveled based on the results of our new chemical evolution models constructed for the SMC , highlighting the observed anomaly in the age-metallicity relation for star clusters in the SMC .
We first propose that evidence of a major merger is imprinted in the age-metallicity relation as a dip in [ Fe/H ] .
Our models predict that the major merger with a mass ratio of 1:1 to 1:4 occurred at \sim 7.5 Gyr ago , with a good reproduction of the abundance distribution function of field stars in the SMC .
Furthermore , our models predict a relatively large scatter in [ Mg/Fe ] for -1.4 \leq { [ Fe / H ] } -1.1 as a reflection of a looping feature resulting from the temporally inverse progress of chemical enrichment , which can be tested against future observational results .
Given that the observed velocity dispersion ( \sim 30 km s ^ { -1 } ) of the SMC is much smaller than that ( \sim 160 km s ^ { -1 } ) of the Galactic halo , our finding strongly implies that the predicted merger event happened in a small group environment that was far from the Galaxy and contained a number of small gas-rich dwarfs comparable to the SMC .
This theoretical view is extensively discussed in the framework that considers a connection with the formation history of the Large Magellanic cloud .