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We present the first comparison between the lifetime star formation histories(SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera forSurveys aboard the Hubble Space Telescope, we obtained deep optical imaging ofAndromeda II (M$_{V} = -$12.0; log(M$_{\star}$/M$_{\odot}$) $\sim$ 6.7) andAndromeda XVI (M$_{V} = -$7.5; log(M$_{\star}$/M$_{\odot}$) $\sim$ 4.9)yielding color-magnitude diagrams that extend at least 1 magnitude below theoldest main sequence turnoff, and are similar in quality to those available forthe MW companions. And II and And XVI show strikingly similar SFHs: both formed50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$\sim$5-0.5)and both were abruptly quenched $\sim$ 5 Gyr ago (z$\sim$0.5). The predominanceof intermediate age populations in And XVI makes it qualitatively differentfrom faint companions of the MW and clearly not a pre-reionization fossil.Neither And II nor And XVI appears to have a clear analog among MW companions,and the degree of similarity in the SFHs of And II and And XVI is not seenamong comparably faint-luminous pairs of MW satellites. These findings providehints that satellite galaxy evolution may vary substantially among hosts ofsimilar stellar mass. Although comparably deep observations of more M31satellites are needed to further explore this hypothesis, our results underlinethe need for caution when interpreting satellite galaxies of an individualsystem in a broader cosmological context.

Comparing M31 and Milky Way Satellites: The Extended Star Formation Histories of Andromeda II and Andromeda XVI