A new population of extended, luminous star clusters in the halo of M31

Huxor, Avon; Tanvir, N. R.; Irwin, M. J.; Ibata, Rodrigo; Collett, J. L.; Ferguson, Annette M. N.; Bridges, Terry J.; Lewis, Geraint F.

doi:10.1111/j.1365-2966.2005.09086.x

Cited by 145 publications

(229 citation statements)

References 39 publications

(68 reference statements)

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“…It is worth recalling that Mackey et al (2006) and Mackey et al (2007) noted a strong second parameter effect among the recently discovered group of remote (and/or extended) clusters of M 31 (Huxor et al 2004(Huxor et al , 2005Galleti et al 2006). However, a quantitative and systematic comparison with MW clusters is attempted here for the first time.…”

Section: An Obvious Selection Biasmentioning

confidence: 99%

The horizontal branch morphology of M 31 globular clusters

Bellazzini

Buzzoni

Cacciari

et al. 2012

A&A

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We use deep, high quality color 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 M 31 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 M 31 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 M 31. 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 M 31 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 M 31 clusters (eighteen of them with M V ≤ −9.0, Sample B), whose SMI estimates are much more uncertain as they are computed on shallow color magnitude diagrams, suggests that extended blue HB tails can be relatively frequent among the most massive M 31 globular clusters, possibly hinting at the presence of multiple populations.

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Section: An Obvious Selection Biasmentioning

confidence: 99%

The horizontal branch morphology of M 31 globular clusters

Bellazzini

Buzzoni

Cacciari

et al. 2012

A&A

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“…Huxor et al (2005) found three ECs around M 31, which have very large radii above 30 pc. These clusters were detected by chance as the automatic detection algorithms of the MegaCam Survey discarded such extended objects as likely background contaminations.…”

Section: Introductionmentioning

confidence: 99%

A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

Brüns

Kroupa

Fellhauer

et al. 2011

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Context. In the last decade, very extended old stellar clusters with masses in the range from a few 10 4 to 10 8 M have been found in various types of galaxies in different environments. Objects with masses comparable to normal globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). In heavily interacting galaxies star clusters tend to form in larger conglomerations called star cluster complexes (CCs). The individual star clusters in a CC can merge and form a variety of spheroidal stellar objects. Aims. The parametric study aims to analyze how the structural parameters of the final merger objects correlate with the underlying CC parameter space. Methods. In this work we systematically scan a suitable parameter space for CCs and perform numerical simulations to study their further fate. The varied sizes and masses of the CCs cover a matrix of 5 × 6 values with CC Plummer radii between 10-160 pc and CC masses between 10 5.5 -10 8 M , which are consistent with observed CC parameters. The CCs of the parametric study are on orbits with galactocentric distances between 20 kpc and 60 kpc. In addition, we studied also the evolution of CCs on a circular orbit at a galactocentric distance of 60 kpc to verify that also extremely extended ECs and UCDs can be explained by our formation scenario. Results. All 54 simulations end up with stable merger objects, wherein 26 to 97% of the initial CC mass is bound. The objects show a general trend of increasing effective radii with increasing mass. Despite the large range of input Plummer radii of the CCs (10 to 160 pc) the effective radii of the merger objects are constrained to values between 10 and 20 pc at the low mass end and to values between 15 and 55 pc at the high mass end. The structural parameters of the models are comparable to those of the observed ECs and UCDs. The results of the circular orbits demonstrate that even very extended objects like the M 31 ECs found by Huxor in 2005 and the very extended (r eff > 80 pc), high-mass UCDs can be explained by merged cluster complexes in regions with low gravitational fields at large galactocentric radii. Conclusions. We conclude that the observed ECs and UCDs can be well explained as evolved star cluster complexes.

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“…In general, typical CMDs of M 31 GCs obtained by HST data barely reach the base of the red giant branch (RGB), including most horizontal branch (HB) stars, except the bluest/faintest ones, when present (Ajhar et al 1996;Rich et al 1996;Fusi Pecci et al 1996;Holland et al 1997;Jablonka et al 2000;Meylan et al 2001;Rich et al 2005;Perina et al 2009b;Huxor et al 2004Huxor et al , 2005Huxor et al , 2008Galleti et al 2006;Mackey et al 2006Mackey et al , 2007. In this magnitude range the HB morphology can be used as a rough age indicator, because the presence of RR Lyrae variables and/or blue HB stars implies low-mass progenitors, hence old ages.…”

Section: Introductionmentioning

confidence: 99%

HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters

Perina¹,

Galleti²,

Pecci³

et al. 2011

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We present deep (V 28.0) BV photometry obtained with the wide field channel of the Advanced Camera for Surveys on board HST for four M 31 globular clusters that were identified as candidate intermediate-age (age ∼ 1−9 Gyr) by various authors, based on their integrated spectra and/or broad/intermediate-band colors. Two of them (B292 and B350) display an obvious blue horizontal branch, indicating that they are as old as the oldest Galactic globulars. On the other hand, for the other two (B058 and B337), which display red horizontal branches, it was not possible either to confirm or disconfirm the age estimate from integrated spectra. The analysis of the distribution in the spectral indices Mg2 and Hβ of the M 31 and Milky Way clusters whose horizontal branch can be classified as red or blue based on existing CMDs, strongly suggests that classical age diagnostics from integrated spectra may be significantly influenced by the HB morphology of the clusters and can lead to erroneous age-classifications. We also provide the CMD for another two clusters that fall into the field of the main targets, B336, an old and metal-poor globular with a significant population of RR Lyrae variables, and the newly discovered B531, a cluster with a very red red giant branch.

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A new population of extended, luminous star clusters in the halo of M31

Cited by 145 publications

References 39 publications

The horizontal branch morphology of M 31 globular clusters

The horizontal branch morphology of M 31 globular clusters

A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters

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