The Role of AGB Stars Feedback in Sustaining Galaxy Evolution

Abstract

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope, of the Local Group spiral galaxy M33. The main aim was to identify 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 first instance, only the central square kiloparsec were monitored and analysed, with the UIST camera. Photometry was obtained for 18,398 stars; of these 812 stars were found to be variable, most of which are asymptotic giant branch (AGB) stars. We constructed the birth mass function and hence derived the star formation history. These stars are also important dust factories. We measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry. The mass loss rates are seen to increase with increasing strength of pulsation and with increasing bolometric luminosity. We construct a 2D map of the mass return rate, showing a radial decline but also local enhancements due to the concentration of red supergiants. We conclude that star formation in the central region of M33 can only be sustained if gas is accreted from further out in the disc or from circum--galactic regions. By using data of wide-field camera (WFCAM), the campaign was expanded to cover two orders of magnitude larger area, comprising the disc of M33 and its spiral arms. Photometry was obtained for 403,734 stars; of these 4643 stars were found to be variable. We here present the star formation history across the disc of M33.