| Abstract
| - We present CCD spectrophotometry of the nearby spiral galaxy M 33 using images obtained with National Astronomical Observatories of China (NAOC) $60/90$ 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 $\chi^{2}$-minimization procedure, we find that both the constant and exponentially decreasing (hereafter Exp, $\tau=12$ 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 ${\sim} 2$ 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.
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