Original paper can be found at: http://www.astrosociety.org/pubs/cs/309.html--Copyright Astronomical Society of the PacificWe present nine newly discovered globular clusters in the outer halo of M31, found by a semi-automated procedure from an INT Wide Field Camera survey of the region. The sample includes a candidate at the largest known projected galactocentric radius yet from M31

Bridges, T.

Ferguson, A.M.N.

Huxor, A.P.

Ibata, R.A.

Irwin, M.J.

Lewis, G.F.

Tanvir, N.

English

arXiv

Huxor, A.

Tanvir, N. R.

Irwin, M.

Ferguson, A.

Ibata, R.

Lewis, G.

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Newly discovered globular clusters in the outer halo of M31

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Satellites and Tidal StreamsASP Conference Series, Vol. 327, 2004F. Prada, D. Mart´ınez Delgado, and T. J. MahoneyNewly Discovered Globular Clusters in the Outer Halo ofM31A. Huxor and N. R. TanvirUniversity of Hertfordshire, College Lane, Hatfield, Herts, AL10 9AB,UKM. IrwinInstitute of Astronomy, University of Cambridge, Madingley Road,Cambridge CB3 0HA, UKA. FergusonMax Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1,Postfach 1317, D-85741 Garching, GermanyR. IbataObservatiore de Strasbourg, 11 rue de l’Universite´, F-67000 Strasbourg,FranceG. LewisSchool of Physics A28, University of Sydney, NSW 2006, AustraliaT. BridgesAnglo–Australian Observatory, P.O. Box 296, Epping NSW 1710,AustraliaAbstract. We present nine newly discovered globular clusters in the outerhalo of M31, found by a semi-automated procedure from an INT Wide FieldCamera survey of the region. The sample includes a candidate at the largestknown projected galactocentric radius yet from M31.1. IntroductionΛCDM cosmological models, and their semi-analytical extensions, have had con-siderable success in describing hierarchical structure formation and galaxy evo-lution. However, important questions still remain. For example, where are thepredicted large number of low mass dark halos? What are the profiles of thedark halos? How important are major and minor mergers in building galax-ies and star formation? One way to address such issues is by detailed studyof nearby resolved galaxies, to deduce evolutionary history from their presentday structure. The galaxies of the local group are ideal for such investigations,and a wide area CCD survey of the halo of M31 has been undertaken by ourcollaboration. This survey has already revealed previously unknown detail andstructure (Ferguson et al. 2002), including the discovery of a large stellar stream,118Newly Discovered Globular Clusters in M31 119presumably the relic of dwarf galaxy destruction (Ibata et al. 2001). We havenow used this survey to complete a search for previously undiscovered globularclusters, which are important tracers of the dynamics and chemistry of the outerhalo .Figure 1. The area of our imaging survey (roughly outlined by the largeellipse) in relation to the visual extent of the M31 disk (small ellipse). Thenew GCs (asterisks) contribute additional dynamical probes, which, althoughfew in number, extend well beyond the region for which radial velocities havebeen published (points) by Perrett et al. (2002).2. GCs as Dynamical and Chemical TracersRecent modeling by Evans et al. (2000) suggests that, contrary to accepted wis-dom, the mass of the halo of M31 is comparable to, or less than, that of theMilky Way. However the uncertainties are large. For example, test particlesat large galactocentric radius are important to constrain the total mass, butare currently limited to dwarf satellites and other Local Group galaxies whosepast interactions with, for example, Milky Way are not known. Thus Evans& Wilkinson (2000) emphasize the need for more test particles at an appropri-ate distance. Globular clusters (GCs) are particularly useful, being relatively120 Huxor et al.Figure 2. Eight of the globular cluster candidates. Images are from theINT Wide Field Camera survey, taken in the i band; each is 30′′×30′′. Northis up and East is left.Figure 3. i and V band images of the GC candidate that is at the largestknown project galactocentric distance, some 55 kpc. Images also from theINT/WFC survey, and have same size and orientation as in Figure 2.bright, they enable good spectroscopy, and are found far from the host galaxy(e.g., Figure 1), hence our motivation in seeking new examples in the outer halo.Furthermore, there is evidence that they occur in distinct sub-populations, ascharacterized by position, velocity, and color (Perrett et al. 2003). These pop-ulations provide a fossil record of the formation history of the galaxy. Variousroutes have been proposed for the production of GCs. Each produces specificNewly Discovered Globular Clusters in M31 121signatures being found in that GC population, which will help to unravel thehistory of the M31 system. Hence, we are also interested in the metallicity ofour candidate globular clusters, in addition to their kinematics.3. Candidate IdentificationInitial candidates were found from the survey database, produced by the stan-dard INT Wide Field Survey pipeline and provided by the Cambridge Astronom-ical Survey Unit. Candidate selection was based on magnitude, color, ellipticity,object classification (stellar/non-stellar), and image width (sigma). These valueswere iteratively refined, by ensuring that known good quality cluster candidateswere accepted by the cuts. Few GCs are expected at such large galactocentricradii, and selecting candidates based solely on the criteria listed above wouldproduce too many false positives for spectroscopic follow-up. Hence, the initiallist of candidates were visually inspected to identify final candidates, and thesewere finally cross-checked against existing catalogues (especially the online cat-alogues maintained by Barmby and by Huchra) giving the final nine, previouslyunknown, GCs presented here.4. Future WorkOne of the candidate GCs, at the extreme south-west of the M31 survey field, liesat the largest known projected galactocentric radius from M31. Spectroscopy ofthis object was obtained with WHT/ISIS in service time at the end of 2002. Thedata is currently being reduced, and the results will presented in a later paper.Further time for radial velocity and metallicity determination of the other eightcandidates is currently being sought.ReferencesEvans, N. W., et al. 2000, ApJ, 540, L9Evans, N. W., & Wilkinson, M. I. 2000, MNRAS, 316, 929Ferguson, A. M. N., et al. 2002, AJ, 124, 1452Ibata, R., et al. 2001, Nat, 412, 49Perrett, K. M., et al. 2003, ApJ, 589, 790Perrett, K. M., et al. 2002, AJ, 123, 2490

Newly Discovered Globular Clusters in the Outer Halo of M31

Newly Discovered Globular Clusters in the Outer Halo of M31

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