The bursty star formation history of the Fornax dwarf spheroidal galaxy revealed with the HST

We present a new star formation history (SFH) of the dwarf spheroidal galaxy Fornax characterized by unprecedented high precision and age resolution. It reveals that star formation has proceeded in sharp bursts separated by periods of low-level or quiescent activity. The SFH was derived by applying the well-tested technique of colour-magnitude diagram fitting to two extremely deep colour magnitude diagrams obtained with data from the Hubble Space Telescope, and sampling stellar populations at the centre and at one core radius. The attained age resolution at different epochs not only allowed us to observe the major bursts of star formation at $10.6\pm2.4$ (centre) and $10.9\pm1.3$ Gyr ago (core radius), but also a strong burst at $4.6\pm0.4$ Gyr ago and recent intermittent episodes of activity during the period $\sim2-0.2$ Gyr ago. Detailed testing using mock stellar populations allowed us to estimate the duration of the main bursts and to study the occurrence of low-level star formation between bursts. The two regions in Fornax were observed to have many common features in their SFH, with activity at the same epochs and similar age-metallicity relationship. However, clear indications of a spatial population gradient were also found, with mean stellar ages increasing with radius and star formation episodes being more prolonged in the centre. We calculated an orbit of Fornax relative to the closest dwarfs and Milky Way and observed a correspondence between the main intermediate-age and young SFH events and pericentric passages of Fornax around the Milky Way, possibly indicating tidally induced star formation.