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The Newly-Discovered Remote Globular Cluster System of M31

The Newly-Discovered Remote Globular Cluster System of M31. Dougal Mackey (RSAA, ANU). In collaboration with: Avon Huxor (Bristol) Annette Ferguson (Edinburgh) Mike Irwin (Cambridge) Nial Tanvir (Leicester) Alan McConnachie (HIA ) Nicolas Martin (MPIA) and many more ….

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The Newly-Discovered Remote Globular Cluster System of M31

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  1. The Newly-Discovered Remote Globular Cluster System of M31 Dougal Mackey (RSAA, ANU) • In collaboration with: • Avon Huxor (Bristol) • Annette Ferguson (Edinburgh) • Mike Irwin (Cambridge) • NialTanvir (Leicester) • Alan McConnachie (HIA) • Nicolas Martin (MPIA) • and many more…

  2. GCs – why should we care? No dark energy or (non-baryonic) dark matter… Boring! • Actually, GCs are central to many fields of astrophysics. • They are relatively simple astrophysical objects (ie, not galaxies). • Roughly spherical, single(?) stellar populations. • Ubiquitous in galaxies above a certain (low) mass limit. • Most are compact and luminous (ie, visible from a distance!). • Testing grounds for stellar evolution models (binaries, exotica). • Laboratories for gravitational dynamics. • A fossil record of host galaxy formation & evolution. • Trace star formation, chemical evolution, mergers, kinematics. • Local probes of the early Universe (cosmological significance?).

  3. Milky Way globular cluster system: There are ~ 150 known globular clusters associated with our Galaxy. • These extend from Galactic centre to ~ 120 kpc in the halo. • Metallicities ranging from [Fe/H] ~ -2.4 – 0.0. • Comprise an inhomogeneous group in terms of masses, densities, spatial locations, ages, orbits, horizontal branch morphologies… • Following the original suggestion by Searle & Zinn (1978), there • has been accumulation of evidence for many GCs having been • accreted into the Milky Way (particularly remote clusters). • See Zinn (1993), Mackey & Gilmore (2004), Forbes & Bridges (2010). • Verification of this scenario with discovery of Sagittarius dwarf. • However, still the only concrete, unambiguous example.

  4. Sagittarius dwarf

  5. Globular clusters in M31: • M31 provides a contrasting system to the Milky Way – what can • we learn from its GCs about the similarities and differences • between the two galaxies… is the MW typical? Also, an extended parameter space for interesting objects. (M31 has at least 450 GCs..!) PAndAS (and earlier surveys) are opening a new window on the outer halo globular cluster system of M31.

  6. The Pan-Andromeda Archaeological Survey (PAndAS) CFHT / MegaCam Large program aimed at surveying entire M31 halo to ~150 kpc (and M33 halo to ~50 kpc). • Running 2008B – 2010B • Limiting depth of g = 25.5 • and i = 24.5 (~3-4 mag RGB) • Detect structures to SB of • ~32-33 mag/arcsec2 • Allowing discovery ofmany • new globular clusters…

  7. The Pan-Andromeda Archaeological Survey PAndAS A. McConnachie et al. 2009, Nature, 461, 66 Status after 2008B

  8. PAndAS is notable for its fantastic data quality. • Seeing is typically better than ~ 0.7 arcsec (!) • Removes (for the first time) almost all the ambiguity involved • when identifying globular clusters.

  9. The “new” remote GC system of M31: • Until very recently only GCs in the (very) inner regions (~disk) • of M31 had been catalogued and studied. • With PAndAS we are now characterising the remote, outer halo • globular cluster population for the first time. • The Revised Bologna Catalogue (RBC) lists 34 M31 GCs outside • 15 kpc, of which only 3 lie outside 30 kpc. • We have discovered ~85 new globular clusters lying outside • 15 kpc in M31, nearly 65 of which are outside 30 kpc (to >120 kpc). • (very many compared to Milky Way!) • Huxor et al. (2005, 2008, 2009, 2010); Mackey et al (2006, 2007).

  10. The colours of outer M31 globulars are very similar to Galactic counterparts. Suggests similar ages and metallicities. However… noticeable difference between the luminosities of clusters. Outer MW globulars are typically sub-luminous; M31 globulars are bright. (Huxor et al. 2008, MNRAS, 385, 1989)

  11. MGC1 in M31 (Gemini/GMOS) – Rp = 117 kpc

  12. MGC1is ancient, metal-poor and significantly closer than M31. Combined with large projected radius  MGC1 is ~ 200 kpc from M31 (!) It has an extended structure consistent with models of isolated GCs. (Mackey et al. 2010, MNRAS, 401, 533)

  13. Extended Clusters in M31: • Cluster-like objects with unusually extended structures. • Originally discovered by Huxor et al. (2005) from INT / WFC. • Followed by many more fromPAndAS… • Of the ~ 85 new GCs outside 15 kpc, ~ 25 – 30 are extended. • Also seen in M33 and NGC 6822 but maybe not in our Galaxy. • Populate an unusual region of the size-luminosity plane.

  14. From HST/ACS imaging (GO-10394), see Mackey et al. (2006, 2007) (Mackey et al. 2007, ApJ, 655, L85)

  15. From HST/ACS imaging (GO-10394), see Mackey et al. (2006, 2007) (Mackey et al. 2006, ApJ, 653, L105)

  16. Dwarf galaxies Star clusters Complex stellar populations; DM dominated. “Simple” stellar populations; No dark matter. (Huxoret al. 2010, MNRAS, nearly submitted)

  17. Spatial distribution of remote M31 GCs: • Azimuthally averaged density profile shows a striking break. • This is also seen in the metal-poor halo population. • Evidence for the accretion of GCs into the outer halo… (Huxor et al. 2010, MNRAS, nearly submitted)

  18. If the clusters were smoothly distributed azimuthally, what would the distribution of surrounding stellar densities look like? Quite different from what we observe which is biased to higher densities. The two distributions drawn from same parent with only ~ 1.3% probability. (Mackey et al., 2010, ApJ, 717, L11)

  19. Streams without clusters are in the minority. This may suggest that most streams in the M31 halo are due to the accretion of a few larger progenitors. What can we learn about these galaxies from the clusters? Future work will fold in radial velocity info (in progress)… (Mackey et al., 2010, ApJ, 717, L11)

  20. Summary & Conclusions: • M31 has many more remote GCs than the Milky Way, extending • further out and with a wider variety of properties. • So far, their colours appear consistent with being ancient and • metal-poor (more work in progress). • Their radial profile shows a clear break at ~30 kpc, mirrored in • the metal-poor field population. • Their spatial distribution is not smooth, and shows a statistically • significant correlation with underlying field substructures. • Our results are strong evidence that the majority of the remote • M31 GC system has been assembled from the accretion of • cluster-bearing satellite galaxies (cf. Milky Way). (Mackey et al., 2010, ApJ, 717, L11)

  21. Rp = 125 kpc!

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