Merger traces in the spatial distribution of stellar populations in the Fornax dSph galaxy

Abstract

We present a comprehensive and detailed study of the stellar populations of the Fornax dwarf spheroidal galaxy. We analyse their spatial distributions along the main body of the galaxy, obtaining their surface density maps, together with their radial density profiles. Results are based on the largest and most complete catalogue of stars in Fornax, with more than $3.5\times10^{5}$ stars covering the main body of the galaxy up to $V \sim 24$. We find a differentiated structure in Fornax depending on the stellar ages. Old stars ($\gtrsim 10$ Gyr) follow an elliptical distribution well fitted by King profiles with relatively large core radius ($r_{\rm c} = 760\pm60$ pc). On another hand, young populations ($\lesssim 3$ Gyr) concentrate in the central region of the galaxy ($r_{\rm c} = 210\pm10$ pc), and are better fitted by Sersic profiles with $0.8 < n < 1.2$, indicating some discy shape. These stars show strong asymmetries and substructures not aligned with the main optical axes of Fornax. This together with the observed differences between metallicity and age distribution maps strongly suggests accretion of material with different angular momentum. These results lead us to propose a scenario in which Fornax has suffered a major merger at $z\sim1$.