We present deep photometry of the Carina dwarf Spheroidal galaxy in the B , V filters from CTIO/MOSAIC , out to and beyond the tidal radius of r _ { ell } \approx 0.48 degrees .
The accurately calibrated photometry is combined with spectroscopic metallicity distributions of Red Giant Branch stars to determine the detailed star formation and chemical evolution history of Carina . The star formation history confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail , as a function radius from the centre .
Two main episodes of star formation occurred at old ( > 8 Gyr ) and intermediate ( 2 - 8 Gyr ) ages , both enriching stars starting from low metallicities ( [ Fe/H ] < - 2 dex ) .
By dividing the SFH into two components , we determine that 60 \pm 9 percent of the total number of stars formed within the intermediate age episode .
Furthermore , within the tidal radius ( 0.48 degrees or 888 pc ) a total mass in stars of 1.07 \pm 0.08 \times 10 ^ { 6 } M _ { \odot } was formed , giving Carina a stellar mass-to-light ratio of 1.8 \pm 0.8 . Combining the detailed star formation history with spectroscopic observations of RGB stars , we are able to determine the detailed age-metallicity relation of each episode and the timescale of \alpha -element evolution of Carina from individual stars .
The oldest episode displays a tight age-metallicity relation during \approx 6 Gyr with steadily declining \alpha -element abundances and a possible \alpha -element “ knee ” visible at [ Fe/H ] \approx - 2.5 dex .
The intermediate age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in \alpha -element abundances with a slope much steeper than observed in the old episode , starting from [ Fe/H ] = - 1.8 dex and [ Mg/Fe ] \approx 0.4 dex and declining to Mg-poor values ( [ Mg/Fe ] \leq - 0.5 dex ) .
This indicates clearly that both episodes of star formation formed from gas with different abundance patterns , inconsistent with simple evolution in an isolated system .