Issue |
A&A
Volume 420, Number 1, June II 2004
|
|
---|---|---|
Page(s) | 89 - 96 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20040102 | |
Published online | 14 May 2004 |
SED, age distribution and evolutionary history of M 33
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
Corresponding author: J. Li, ljl@vega.bac.pku.edu.cn
Received:
10
June
2003
Accepted:
15
January
2004
We present CCD spectrophotometry of the nearby spiral
galaxy M 33 using images obtained with National Astronomical
Observatories of China (NAOC) cm Schmidt telescope in a
broad U-band and 13 intermediate-band filters from 4000 to 10 000 Å. The observations cover the whole area of M 33, with a total
integration of 39.08 h from September 23, 1995 to August 28,
2000. The spectral energy distributions (SEDs) for each area of
M 33 are obtained. With the aid of an evolutionary synthesis model,
PEGASE (Fioc & Rocca-Volmerange [CITE], 2000), we compute
theoretical SEDs for three kinds of star formation rate (SFR)
histories. From best fits on templates of PEGASE and observed SEDs
by the
-minimization procedure, we find that both the
constant and exponentially decreasing (hereafter Exp,
Gyr) SFR give good agreement between models and observations. We
then obtain age distributions (when the observed stellar
population formation began) and evolutionary histories of M 33 for
the two models. For the constant SFR, an age gradient is clearly
found between stellar populations of the central regions and of
the outer regions. The stellar populations in its central regions
are older than 10 Gyr; stars in the outer regions are younger,
about 7 Gyr and the youngest components in the spiral arms are less
than 5 Gyr. The Exp SFR gives a similar age distribution, but with
absolute ages that are smaller by
Gyr. We conclude that
M 33 has been forming stars continuously for most of its lifetime,
with the interior having built up its stellar populations several
Gyr earlier than the outer parts.
Key words: galaxies: individual: M 33 / galaxies: evolution / galaxies: stellar content
© ESO, 2004
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