The UK Infrared Telescope M33 monitoring project - II. The star formation history in the central square kiloparsec

Abstract

We have conducted a near-infrared monitoring campaign at the UK Infrared Telescope, of the Local Group spiral galaxy M33 (Triangulum). 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 this second paper of the series, we construct the birth mass function and hence derive the star formation history. The star formation rate has varied between ∼0.002 and 0.007 M yr −1 kpc −2 . We give evidence of two epochs of a star formation rate enhanced by a factor of a few -one that happened ≥6 Gyr ago and produced ≥80 per cent of the total mass in stars, and one around 250 Myr ago that lasted ∼200 Myr and formed ≤6 per cent of the mass in stars. We construct radial and azimuthal distributions in the image plane and in the galaxy plane for populations associated with old first-ascent red giant branch (RGB) stars, intermediate-age asymptotic giant branch (AGB) stars and young (massive) blue and red supergiants. We find that the RGB stars follow an spheroidal distribution, while younger stars follow a flat-disc distribution. The intermediate-age population displays signs of a pseudo-bulge or possibly a bar. The inner spiral arm pattern as recorded in mid-19th-century drawings is confirmed. We interpret our findings as evidence for an old, pressure-supported component and a younger disc formed 6 Gyr ago, with an accretion event occurring 250 Myr ago giving rise to the compact nucleus in M33. Our study provides support for recent Padova stellar evolution models except that super-AGB stars likely reach low temperatures and thus high mass-loss rates, supporting the super-AGB nature of the progenitors of dust-enshrouded supernovae such as SN 2008S.

DUSTiNGS. III. DISTRIBUTION OF INTERMEDIATE-AGE AND OLD STELLAR POPULATIONS IN DISKS AND OUTER EXTREMITIES OF DWARF GALAXIES

“…The Sérsic function is a generalized exponential profile more often used to fit surfacebrightness profiles. However, it has also been applied to stellar surface-density profiles with consistent results (e.g., Battinelli et al 2007;McConnachie et al 2007;Javadi et al 2011). …”

The UK Infrared Telescope M33 monitoring project – IV. Variable red giant stars across the galactic disc

“…Paper I in the series presented the photometric catalogue of stars in the inner square kpc (Javadi et al 2011a), with Paper II presenting the galactic structure and star formation history (Javadi, van Loon & Mirtorabi 2011b), and Paper III presenting the mass-loss mechanism and dust production rate (Javadi et al 2013). This paper describes the extension of the survey to a nearly square degree area covering much of the M 33 optical disc.…”

Monitoring survey of pulsating giant stars in the Local Group galaxies: survey description, science goals, target selection

“…They trace stellar populations as young as ~ 30 Myr to as old as the oldest globular clusters. The most evolved AGB stars are long period variables (LPVs), and identifying them is one of the best ways to reconstruct the star formation history [5]. LPVs reach the largest amplitudes of their brightness variations at optical wavelengths, due to the changing temperature.…”