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Stellar structures in the outer regions of M 33

M. Grossi, N. Hwang, E. Corbelli, C. Giovanardi, S. Okamoto, N. Arimoto

Abstract
Aims: We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the outer regions of M 33. Our aim is to search for stellar structures corresponding to extended Hi clouds found in a recent 21-cm survey of the galaxy. Three fields probe a large Hi complex to the southeastern(SE) side of the galaxy. An additional three fields cover the northwestern(NW) side of the galaxy along the Hi warp. A final target field was chosen further north, at a projected distance of approximately 25 kpc, to study part of the large stellar plume recently discovered around M 33. Methods: We analyse the stellar population at R > 10 kpc by means of V, I colour magnitude diagrams reaching the red clump. We constrain the age and metallicity of the different stellar populations, search for density enhancements that correspond to the Hi features, and investigate the radial surface distribution of the stars. Results: We find evolved stellar populations in all fields out to 120'(~30 kpc),while a diffuse population of young stars(~200 Myr) is detected out to a galactocentric radius of 15 kpc. The mean metallicity in the southern fields remains approximately constant at [M/H] = -0.7 beyond the edge of the optical disc, from 40'out to 80'. Along the northern fields probing the outer Hi disc, we also find a metallicity of [M/H] = -0.7 between 35'and 70'from the centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In the northernmost field, outside the disc extent, the stellar population of the large stellar plume possibly related to a M 33-M 31 interaction is on average more metal-poor([M/H] = -1.3) and older(?6 Gyr). Conclusions: An exponential disc with a large scale-length(~7 kpc) fits well the average distribution of stars detected in both the SE and NW regions from a galactocentric distance of 11 kpc out to 30 kpc. The stellar disc extends beyond the Hi disc. The stellar distribution at large radii is disturbed and, although there is no clear correlation between the stellar substructures and the location of the Hi clouds, this gives evidence of tidal interaction or accretion events. Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.Photometry is only available in electronic form at at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/533/A91

Keywords
galaxies: evolution - galaxies: stellar: content - galaxies: halos - galaxies: individual: M 33

Astronomy and Astrophysics
Volume 533, Number of pages A91_1
2011 September

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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