Received 06th March 1998
"The Magellanic Clouds and Other Dwarf Galaxies
of the Bonn/Bochum-Graduiertenkolleg
Preliminary Results on the Resolved Stellar Population of I Zw 18
A. Aloisi1, L. Greggio1, M. Tosi2,
M. Clampin3, A. Nota3, and
, Università di Bologna, Bologna, Italy
2Osservatorio Astronomico di
, Bologna, Italy
3Space Telescope Science
, Baltimore, USA
To infer the star formation history and initial mass function (IMF)
of the resolved stellar population in the Blue Compact Dwarf Galaxy
I Zw 18, we have applied the method of comparing the observed
colour-magnitude diagram (CMD) with synthetic ones based on homogeneous
sets of stellar evolutionary tracks.
To this purpose we have retrieved and re-reduced archive
Here we present preliminary results on our data reduction and CMD simulations.
Models seem to indicate a quite flat IMF, with an exponent in the range
α ∼ 1.5 - 2.0, and a relatively old star formation activity
stopped or drastically reduced 3 - 5 Myr ago.
It is not yet possible to definitively distinguish between the single-burst
or two-burst scenarios proposed for this galaxy, but the latter seems to
provide better results.
1. Data Reduction
Our data on I Zw 18 are taken from
I Zw 18 has been observed in the broadband filters F336W, F555W and
F814W of HST/WFPC2 in November 1994
(the galaxy was centered in the PC1 CCD), and CMDs obtained from photometric
reduction have been published by Hunter & Thronson (1995).
We have performed a new photometric reduction of archive data
to obtain a direct estimate of all parameters involved in the CMD simulations
(distribution of photometric errors, incompleteness, etc.).
Figure 1a shows the CMD in the
HST F555W and F814W bands containing
all the stars with photometric error σ < 0.2 mag in both bands
2. Synthetic CMDs
In order to deduce the star formation history and IMF of I Zw 18 from the
CMD obtained by data reduction, we are carrying on several simulations
(see Tosi et al. 1991 and Greggio et al. 1998 for more details on the
simulation code) considering two sets of stellar evolutionary tracks:
Geneva tracks with Z = 0.001 (Schaller et al. 1992) and Padova tracks
with Z = 0.0004 (Fagotto et al. 1994).
These are the only ones available with a metallicity compatible with that
of I Zw 18, the most metal poor galaxy ever known with
In order to obtain the transformation from the theoretical to the observational
plane, we have assumed a distance modulus
(m-M)0 = 30 mag, corresponding to a distance of
10 Mpc, and we have adopted a reddening E(B-V) = 0.04
(Hunter & Thronson 1995).
3. Preliminary Results
In the CMD of I Zw 18 we have sampled stars with a magnitude V < 27.5.
At the assumed distance modulus of the galaxy this means
MV < -2.5,
corresponding to stars more massive than 4 Msun in
their post-main sequence phase, and therefore with lifetimes <0.2 Gyr.
A comparison of the observed stellar distribution with synthetic diagrams has
not allowed us so far to understand if in this galaxy the star formation
activity has occurred in one single burst or in two bursts separated by a
both these star formation scenarios give diagrams which are not in perfect
agreement with the observations.
Anyway we have recognized the following general trends in the star formation
- both in a single-burst and in a two-burst scenarios, simulations
indicate an IMF flatter than Salpeter (α = 2.35)
in order to avoid too many faint red stars.
Values of the IMF exponent more consistent with data are in the range
α = 1.5 - 2.0;
- in a single-burst scenario star formation cannot have started later
than 30 Myr ago to reproduce the observed faint stars (both blue
and red), but it should have stopped or been drastically reduced around
3 - 5 Myr ago to avoid stars brighter than V < 22 not
observed in I Zw 18.
In Fig. 1b we show an example
of a single young burst started 10 Myr ago and still ongoing
(Padova tracks with α = 1.5):
the simulation is not able to predict stars fainter than
V ∼ 26,
to obtain both faint and bright red stars and to avoid stars brighter
than V ∼ 22.
In Fig. 1c we present instead
one of the single-burst models which better seems to reproduce the
observed CMD (Padova tracks with α = 1.5)
with an exponentially decreasing and still ongoing star formation
activity started 0.5 Gyr ago and with an e-folding time of
There are still too many red stars of intermediate brightness in
the synthetic CMD;
- in a two-burst scenario we generally obtain the best consistency with
observations, especially if we consider an old burst started
0.5 - 0.3 Gyr ago and finished 50 Myr ago and a more recent
burst started 30 Myr ago and stopped 3 - 5 Myr ago.
In Fig. 1d we show our best
two-burst model obtained considering a first episode from 0.5 Gyr
to 50 Myr ago, and a second one from 30 Myr to 5 Myr ago
(Padova tracks with α = 1.5).
[Click here to see Fig. 1!]
- Fagotto F., Bressan A., Bertelli G., Chiosi C., 1994, A&AS 104, 365
- Greggio L., Tosi M., Clampin M., De Marchi G., Leitherer C., Nota A.,
Sirianni M., 1998, ApJ, submitted
- Hunter D.A., Thronson H.A. Jr., 1995, ApJ 452, 238
- Schaller G., Schaerer D., Meynet G., Maeder A., 1992, A&AS 96, 269
- Tosi M., Greggio L., Marconi G., Focardi P., 1991, AJ 102, 951
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|First version: ||24th||July,||1998
|Last update: ||28th||September,||1998
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