The Complex Star Formation History of NGC 1569 L. Angeretti 1, M. Tosi 2, L. Greggio 3, E. Sabbi 1, A. Aloisi 4, C. Leitherer 4 The object The observations The method The results Conclusions NGC 1569 Type: dwarf irregular galaxy Distance = 2.2 Mpc (Israel et al. ‘88) Galactic E(B-V) = 0.56 (Israel et al. ‘88) Metallicity: Z = (Calzetti et al. ’94) Mass: M dyn = 3.3 × 10 8 M ☉, 1/3 of which is HI (Israel et al. ’88) Contains 2 Super Star Clusters, many small clusters and HII regions NGC 1569 shows the signs of a recent and strong Star Formation:  Arms and bridges observed in HI  Observations of grains in IR wavelengths are consistent with presence of shocks  Filaments and arcs observed in H  and X-ray Best Model*: 3 episodes of star formation in the last  1 Gyr (Fig.4). Young episode: 37  13 Myr SFR  3.2 M ☉ yr -1 kpc -2 Intermediate episode: 150  40 Myr SFR  1 M ☉ yr -1 kpc -2 Old episode: 600  300 Myr SFR  1.1 M ☉ yr -1 kpc -2 (*) SFRs derived assuming Salpeter’s IMF (mass range 0.1  120 M ☉ ). Adopting Kroupa’s ‘03 IMF the SFRs are 40% lower than with Salpeter’s The SFH of the best model is plotted as solid lines in Fig.5 Other solutions (dotted lines in Fig.5)  Up to 3 episodes in the age interval of the young episode  We obtain acceptable models for any starting epoch in the range 2  0.6 Gyr ago for the old episode  We cannot rule out an IMF flatter than Salpeter’s in the mass range 0.6  120 M ☉ The observations : Obtained with NICMOS/NIC2 (HST)  Field of view: 19.2” x 19.2” equivalent to 205 x 205 pc 2 (d = 2.2 Mpc)  Pixel size: 0.4 pc Filters: F110W (J) and F160W (H) Data reduction (Aloisi et al 2001) : Perfomed with DAOPHOT Selection criteria:   2 < 1.5 and sharpness < 0.4 Limiting magnitude:  24.5 in m F110W Artificial star experiments were perfomed to obtain completeness and photometric errors distributions (1) Bologna University, Italy (2) INAF-Bologna, Italy (3) INAF-Padova, Italy (4) STScI, USA The last Gyr could be a peculiar epoch for NGC 1569! What could be the cause of this recent and strong SF? There is an HI cloud located at a projected distance of 5 kpc from NGC1569 and connected to the galaxy by a bridge. This cloud may have triggered the SF, as suggested by Stil & Israel ‘98. A possible scenario Astrated stellar mass in the simulations (assuming Kroupa ’03 IMF): 1.6 ×10 7 M ☉ Lum. mass (M/L=10): 3.3 ×10 7 M ☉ Remaining stellar mass: 1.7×10 7 M ☉ Mean SFR (age range 13  1 Gyr ago):  7 × M ☉ yr -1 kpc -2  This rate is 3-4 orders of magnitude lower than in the last Gyr  We have simulated an old episode adopting these parameters. We cannot rule out a low SF activity in the past The Synthetic CMD method: The synthetic CMD is constructed via Monte Carlo extractions of mass-ages pairs, assuming IMF, SF law, initial and final epochs of the SF activity Each synthetic star is placed in the CMD considering photometric properties of data, distance and reddening Incompleteness test is then performed and photometric error is assigned The SFH is obtained when the synthetic CMD reproduces the major features of the observed CMD and contains the same number of objects For more details: Angeretti et al. submitted to AJ Fig. 1 NIC2 image Fig. 2 Observed CMD Fig. 3 Scheme of the synthetic CMD method Fig. 4 Synthetic CMD (Best Model) Fig. 5 SFH