We have examined the star formation history (SFH) of Andromeda VII (And VII), the brightest and most massive dwarf spheroidal (dSph) satellite of the Andromeda galaxy (M 31). Although M 31 is surrounded by several dSph companions with old stellar populations and low metallicity, it has a metal-rich stellar halo with an age of 6$-$8 Gyr. This indicates that any evolutionary association between the stellar halo of M 31 and its dSph system is frail. Therefore, the question is whether And VII (a high-metallicity dSph located $sim$220 kpc from M 31), can be associated with M 31s young, metal-rich halo. Here, we perform the first reconstruction of the SFH of And VII employing long-period variable (LPV) stars. As the most-evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars, the birth mass of LPVs can be determined by connecting their near-infrared photometry to theoretical evolutionary tracks. We found 55 LPV candidates within two half-light radii, using multi-epoch imaging with the Isaac Newton Telescope in the $i$ and $V$ bands. Based on their birth mass function, the star-formation rate (SFR) of And VII was obtained as a function of cosmic time. The main epoch of star formation occurred $simeq 6.2$ Gyr ago with a SFR of $0.006pm0.002$ M$_odot$ yr$^{-1}$. Over the past 6 Gyr, we find slow star formation, which continued until 500 Myr ago with a SFR $sim0.0005pm0.0002$ M$_odot$ yr$^{-1}$. We determined And VIIs stellar mass $M=(13.3pm5.3)times10^6$ M$_odot$ within a half-light radius $r_{frac{1}{2}}=3.8pm0.3$ arcmin and metallicity $Z=0.0007$, and also derived its distance modulus of $mu=24.38$ mag.
تحميل البحث