2005AJ....129.2203A

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 ∼3.7x10⁷ yr ago and ended ∼1.3x10⁷ 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/kpc2, in agreement with literature data. A previous episode produced stars between ∼1.5x10⁸ and ∼4x10⁷ yr ago, with a mean SFR about two-thirds lower than the mean SFR of the youngest episode. An older SF episode occurred about 1x10⁹ 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.