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| Paper IPM / Astronomy / 14579 |
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We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). On the basis of their variability, we have identified stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. In this fifth paper of the series, we construct the birth mass function and hence derive the star formation history across the galactic disc of M33. The star formation rate has varied between ∼ 0.010±0.001 ( ∼ 0.012±0.007) and 0.060±0.005 (0.052±0.009)M\odot yr−1 kpc−2 statistically (systematically) in the central square kiloparsec of M33, comparable with the values derived previously with another camera. The total star formation rate in M33 within a galactocentric radius of 14 kpc has varied between ∼ 0.110±0.005 ( ∼ 0.174±0.060) and ∼ 0.560±0.028 ( ∼ 0.503±0.100) M\odot yr−1 statistically (systematically). We find evidence of two epochs during which the star formation rate was enhanced by a factor of a few - one that started ∼ 6 Gyr ago and lasted ∼ 3 Gyr and produced ≥ 71
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