A MegaCam Survey of Outer Halo Satellites. VI: The Spatially Resolved
  Star Formation History of the Carina Dwarf Spheroidal Galaxy
  We present the spatially resolved star formation history (SFH) of the Carina
dwarf spheroidal galaxy, obtained from deep, wide-field g,r imaging and a
metallicity distribution from the literature. Our photometry covers $\sim2$
deg$^2$, reaching up to $\sim10$ times the half-light radius of Carina with a
completeness higher than $50\%$ at $g\sim24.5$, more than one magnitude fainter
than the oldest turnoff. This is the first time a combination of depth and
coverage of this quality has been used to derive the SFH of Carina, enabling us
to trace its different populations with unprecedented accuracy. We find that
Carina's SFH consists of two episodes well separated by a star formation
temporal gap. These episodes occurred at old ($>10$ Gyr) and intermediate
($2$-$8$ Gyr) ages. Our measurements show that the old episode comprises the
majority of the population, accounting for $54\pm5\%$ of the stellar mass
within $1.3$ times the King tidal radius, while the total stellar mass derived
for Carina is $1.60\pm0.09\times 10^{6} M_{\rm{\odot}}$, and the stellar
mass-to-light ratio $1.8\pm0.2$. The SFH derived is consistent with no recent
star formation which hints that the observed blue plume is due to blue
stragglers. We conclude that the SFH of Carina evolved independently of the
tidal field of the Milky Way, since the frequency and duration of its star
formation events do not correlate with its orbital parameters. This result is
supported by the age/metallicity relation observed in Carina, and the gradients
calculated indicating that outer regions are older and more metal poor.
