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Register a new userGlobular clusters in the outer halo of M31: the survey
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We report the discovery of 40 new globular clusters (GCs) that have been found in surveys of the halo of M31 based on INT/WFC and CHFT/Megacam imagery. A subset of these these new GCs are of an extended, diffuse nature, and include those already found in Huxor et al. (2005). The search strategy is described and basic positional and V and I photometric data are presented for each cluster. For a subset of these clusters, K-band photometry is also given. The new clusters continue to be found to the limit of the survey area (~100 kpc), revealing that the GC system of M31 is much more extended than previously realised. The new clusters increase the total number of confirmed GCs in M31 by approximately 10% and the number of confirmed GCs beyond 1 degree (~14 kpc) by more than 75%. We have also used the survey imagery as well recent HST archival data to update the Revised Bologna Catalogue (RBC) of M31 globular clusters.
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We present Keck/HIRES spectra of 3 globular clusters in the outer halo of M31, at projected distances beyond ~80 kpc from M31. The measured recession velocities for all 3 globular clusters confirm their association with the globular cluster system of M31. We find evidence for a declining velocity dispersion with radius for the globular cluster system. Their measured internal velocity dispersions, derived virial masses and mass-to-light ratios are consistent with those for the bulk of the M31 globular cluster system. We derive old ages and metallicities which indicate that all 3 belong to the metal-poor halo globular cluster subpopulation. We find indications that the radial gradient of the mean metallicity of the globular cluster system interior to 50 kpc flattens in the outer regions, however it is still more metal-poor than the corresponding field stars at the same (projected) radius.
We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.
We present nine newly discovered globular clusters in the outer halo of M31, found by a semi-automated procedure from a INT Wide Field Camera survey of the region. The sample includes a candidate at the yet largest known projected galactocentric radius from M31.
We report the first results from deep ACS imaging of ten classical globular clusters in the far outer regions (15 < R_p < 100 kpc) of M31. Eight of the clusters, including two of the most remote M31 globular clusters presently known, are described for the first time. Our F606W, F814W colour-magnitude diagrams extend ~ 3 magnitudes below the horizontal branch and clearly demonstrate that the majority of these objects are old (> 10 Gyr), metal-poor clusters. Five have [Fe/H] ~ -2.1, while an additional four have -1.9 < [Fe/H] < -1.5. The remaining object is more metal-rich, with [Fe/H] ~ -0.70. Several clusters exhibit the second parameter effect. Using aperture photometry, we estimate integrated luminosities and structural parameters for all clusters. Many, including all four clusters with projected radii greater than 45 kpc, are compact and very luminous, with -8.9 < M_V < -8.3. These four outermost clusters are thus quite unlike their Milky Way counterparts, which are typically diffuse, sub-luminous (-6.0 < M_V < -4.7) and more metal-rich (-1.8 < [Fe/H] < -1.3).
We use deep, high quality colour magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index [SMI; B/(B+R)] to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M31 galaxy (Sample-A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SMI vs. [Fe/H] plane. We find that the majority of the considered M31 clusters lie in a significantly different locus, in this plane, with respect to Galactic clusters lying at any distance from the center of the Milky Way. In particular they have redder HB morphologies at a given metallicity, or, in other words, clusters with the same SMI value are ~0.4 dex more metal rich in the Milky Way than in M31. We discuss the possible origin of this difference and we conclude that the most likely explanation is that many globular clusters in the outer halo of M31 formed ~1-2 Gyr later than their counterparts in the outer halo of the Milky Way, while differences in the cluster-to-cluster distribution of He abundance of individual stars may also play a role. The analysis of another sample of 25 bright M31 clusters (eighteen of them with M_V<= -9.0, Sample-B), whose SMI estimates are much more uncertain as they are computed on shallow colour magnitude diagrams, suggests that extended blue HB tails can be relatively frequent among the most massive M31 globular clusters, possibly hinting at the presence of multiple populations.
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