On the Radial Extent of the Dwarf Irregular Galaxy IC10

We present new deep and accurate space (Advanced Camera for Surveys-Wide Field Planetary Camera 2 on board the Hubble Space Telescope) and ground-based (Suprime-Cam at Subaru Telescope, Mega-Cam at Canada-France-Hawaii Telescope) photometric and astrometric data for the Local Group dwarf irregular IC10. We confirm the significant decrease of the young stellar population when moving from the center toward the outermost regions. We find that the tidal radius of IC10 is significantly larger than previous estimates of r<SUB>t</SUB> <~ 10'. By using the I, V-I color-magnitude diagram based on the Suprime-Cam data, we detect sizable samples of red giant (RG) stars up to radial distances of 18'-23' from the galactic center. The ratio between observed star counts (Mega-Cam data) across the tip of the RG branch and star counts predicted by Galactic models indicates a star count excess at least at a 3sigma level up to 34'-42' from the center. This finding supports the hypothesis that the huge H I cloud covering more than 1 across the galaxy is associated with IC10. We also provide new estimates of the total luminosity (L<SUB>V</SUB> ~ 9 10<SUP>7</SUP> L <SUB>sun</SUB>, M<SUB>V</SUB> ~ -15.1 mag) that agree with similar estimates available in the literature. If we restrict our study to the regions where rotational velocity measurements are available (r ≈ 13'), we find a mass-to-light ratio (~10 M <SUB>sun</SUB>/L <SUB>sun</SUB>) that is at least one order of magnitude larger than previous estimates. The new estimate should be cautiously treated, since it is based on a minimal fraction of the body of the galaxy. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. This research is based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.