Transcript The Complex Star Formation History of NGC 1569

The Complex Star Formation History of NGC 1569
L. Angeretti1, M. Tosi2, L. Greggio3, E. Sabbi1, A. Aloisi4, C. Leitherer4
(1) Bologna University, Italy
(2) INAF-Bologna, Italy
(3) INAF-Padova, Italy
(4) STScI, USA
NGC 1569
 Type: dwarf irregular galaxy
 Distance = 2.2 Mpc (Israel et al. ‘88)
The
object
 Galactic E(B-V) = 0.56 (Israel et al. ‘88)
 Metallicity: Z = 0.004 (Calzetti et al. ’94)
Mass: Mdyn = 3.3 × 108 M☉, 1/3 of which
is HI (Israel et al. ’88)

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
 Contains 2 Super Star Clusters, many
Fig. 1 NIC2 image
small clusters and HII regions
The observations :
Data reduction (Aloisi et al 2001):
 Obtained with NICMOS/NIC2 (HST)
 Perfomed with DAOPHOT
 Field of view: 19.2” x 19.2” equivalent
The
observations
to 205 x 205 pc2 (d = 2.2 Mpc)
 Pixel size: 0.4 pc
 Selection criteria:
 2 < 1.5 and sharpness < 0.4
 Limiting magnitude: 24.5 in mF110W
 Filters: F110W (J) and F160W (H)
Artificial star experiments were perfomed to
obtain completeness and photometric errors
distributions
Fig. 2 Observed CMD
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
The
method
 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
Fig. 3 Scheme of the synthetic CMD method
Best Model*: 3 episodes of star formation
in the last 1 Gyr (Fig.4).
The SFH of the best model is plotted as
solid lines in Fig.5
 Young episode:
37  13 Myr
The
results
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
Fig. 4 Synthetic CMD (Best Model)
SFRs are 40% lower than with Salpeter’s
A possible scenario
 Astrated stellar mass in the simulations
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 last Gyr could be a peculiar epoch for
NGC 1569!
(assuming Kroupa ’03 IMF): 1.6×107 M☉
 Lum. mass (M/L=10): 3.3×107 M☉
 Remaining stellar mass: 1.7×107 M☉
Conclusions
 Mean SFR (age range 13  1 Gyr ago):
7×10-4 M☉ yr-1 kpc-2
 This rate is 3-4 orders of magnitude
lower than in the last Gyr
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.
 We have simulated an old episode
Fig. 5 SFH
adopting these parameters. We cannot
rule out a low SF activity in the past
For more details:
Angeretti et al. submitted to AJ