The Complex Star Formation History of NGC 1569

We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained with the Hubble Space Telescope NICMOS/NIC2 in the F110W ( J ) and F160W ( H ) near-infrared (NIR) filters and interpreted with the synthetic color-magnitude diagram method. The galaxy has experienced complex star formation (SF) activity. The best fit to the data is found by assuming three episodes of activity in the last 1–2 Gyr. The most recent and strong episode constrained by these NIR data started ∼3.7 × 10 7 yr ago and ended ∼1.3 × 10 7 yr ago, although we cannot exclude the possibility that up to three SF episodes occurred in this time interval. The average star formation rate (SFR) of the episode is ∼3.2 M ⊙ yr -1 kpc -2 , in agreement with literature data. A previous episode produced stars between ∼1.5 × 10 8 and ∼4 × 10 7 yr ago, with a mean SFR about two-thirds lower than the mean SFR of the youngest episode. An older SF episode occurred about 1 × 10 9 yr ago. All these SFRs are 2–3 orders of magnitude higher than those derived for late-type dwarfs of the Local Group. In all cases an initial mass function similar to Salpeter's allows for a good reproduction of the data, but we cannot exclude flatter mass functions. These results have been obtained adopting a distance of 2.2 Mpc and a reddening E ( B - V ) = 0.56. A larger distance would require younger episodes and higher SFRs. We have explored some possible scenarios using the astrated mass in the best-fit model, in order to constrain the past star formation history. We cannot rule out a low past SFR, but we can safely conclude that the last 1–2 Gyr have been peculiar.