We present the analysis of Lick absorption-line indices from three separate long-slit spectroscopic observations of the nearby isolated elliptical galaxy NGCÂ 821 .
The three data sets present a consistent picture of the stellar population within one effective radius , in which strong gradients are evident in both luminosity-weighted age and metallicity .
The central population exhibits a young age of \sim 4 Â Gyr and a metallicity \sim 3 times solar .
At one effective radius the age has risen to \sim 12 Â Gyr and the metallicity fallen to less than \sim \frac { 1 } { 3 } solar .
The low metallicity population around one effective radius appears to have an exclusively red horizontal branch , with no significant contribution from the blue horizontal branch evident in some globular clusters of the same age and metallicity .
Despite the strong central age gradient , we demonstrate that only a small fraction ( \leq 10 % ) of the galaxy ’ s stellar mass can have been created in recent star formation events .
We consider possible star formation histories for NGCÂ 821 and find that the most likely cause of the young central population was a minor merger or tidal interaction that caused NGCÂ 821 to consume its own gas in a centrally concentrated burst of star formation 1 to 4 Gyr ago .