Volume 544, August 2012
|Number of page(s)||13|
|Published online||24 August 2012|
INAF Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
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Received: 30 March 2012
Accepted: 12 July 2012
Context. Thanks to the outstanding capabilites of the HST, our current knowledge about the M 31 globular clusters (GCs) is similar to our knowledge of the Milky Way GCs in the 1960s − 1970s, which set the basis for studying the halo and galaxy formation using these objects as tracers, and established their importance in defining the cosmic distance scale.
Aims. We intend to derive a new calibration of the MV(HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M 31 in the HST archive.
Methods. We collected the BVI data for 48 old GCs in M 31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour–magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, horizontal branch (HB) luminosity and distance were determined self-consistently for each cluster.
Results. There are three main results of this study: i) the relation MV(HB) = 0.25( ± 0.02)[Fe/H] + 0.89( ± 0.03), which is obtained from the above parameters and is calibrated on the distances of the template Galactic GCs; ii) the distance modulus to M 31 of (m − M)0 = 24.42 ± 0.06 mag, obtained by normalising this relation at the reference value of [Fe/H] = −1.5 to a similar relation using V0(HB). This is the first determination of the distance to M 31 based on the characteristics of its GC system, which is calibrated on Galactic GCs, iii) the distance to the Large Magellanic Cloud (LMC), which is estimated to be 18.54 ± 0.07 mag as a consequence of the previous results. These values agree excellently with the most recent estimate based on HST parallaxes of Galactic Cepheid and RR Lyrae stars, as well as with recent methods.
Key words: galaxies: individual: M 31 / galaxies: star clusters: general / catalogs / Local Group / techniques: photometric
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
Full Fig. 2 is available in electronic form at http://www.aanda.org
Photometric catalogs are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/A155 and at http://www.bo.astro.it/M31/hstcatalog/
© ESO, 2012
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