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THE YOUNG GLOBULAR CLUSTERS of the MILKY WAY and the LOCAL GROUP GALAXIES: PLAYING with GREAT CIRCLES F. Fusi Pecci1, M. Bellazz

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THE YOUNG GLOBULAR CLUSTERS of the MILKY WAY and the LOCAL GROUP GALAXIES: PLAYING with GREAT CIRCLES F. Fusi Pecci1, M. Bellazz View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server THE YOUNG GLOBULAR CLUSTERS OF THE MILKY WAY AND THE LOCAL GROUP GALAXIES: PLAYING WITH GREAT CIRCLES 1 2 1 1 F. Fusi Pecci , M. Bellazzi ni , C. Cacciari , F.R. Ferraro 1 Osservatorio Astronomico , Bologna, Italy 2 Dipartimento di Astronomia , Bologna, Italy V. Zamboni 33, I-40126, Bologna, Italy BAP 07-1995-032-OAB The Astronomical Journal, in press. Send proofs to: M. Bellazzini E-mail: b ellazzini@astb o3.b o.astro.it 1 ABSTRACT The small group of Galactic Globular Clusters (GGC) (Pal 12, Terzan 7, Ruprecht 106, Arp 2) recently discovered to b e signi cantly younger (by 3 4 Gyr) than the average cluster p opulation of the Galaxy are shown to lie near planes passing in the vicinityof some satellite galaxies of the Milky Way and through the Galactic Centre itself. Assuming that these con gurations represent a fossil record of interactions b etween the Galaxy and its companions from which these clusters originated, we identi ed, along one of them, another candidate \young" GGC, i.e. IC4499, whose Color-Magnitude Diagram is presented. Various hyp otheses on the p ossible origin of \young" GGC are also brie y discussed within a framework where the lo cation on preferential planes may b e seen as a general characteristic for the Lo cal Group memb ers. 2 1. INTRODUCTION The evidence that several satellites of the Milky Way app ear to b e situated along a few great circles (streams), and that this may b e somehow related to their origin and dynamical and chemical evolution, has b een a sort of unsettled but p ersistent theme in the last twentyyears or so for most studies of the stellar systems (galaxies and clusters) p opulating the Lo cal Group. In particular, Ho dge and Michie (1969), Kunkel and Demers (1977), Kunkel (1979), Lynden-Bell (1976, 1982) and others (see the reviews by Ma jewski 1993a,b) p ointed out the existence of two remarkable great circles which account for most of the dwarf satellites of the Milky Way. The rst, the Magel lanic o Plane (MP, Kunkel and Demers 1977), at an angle of ab out 40 with resp ect to the plane de ned by the Magellanic Stream (MS), roughly passes through the Magellanic Clouds, the Galactic Centre and the Draco-Ursa Minor region. The second, called by Lynden-Bell (1982) the Fornax - Leo - Sculptor Stream (FLS), ideally contains in a plane Fornax, Leo I and I I, and Sculptor. Concerning the Galactic globular clusters (GGCs), as recently reviewed and dis- cussed for instance by Zinn (1993), van den Bergh (1993), and Ma jewski (1993b, 1994), there are now many indications suggesting the existence of di erent p opulations (e.g. disk, old and young halo, etc.) p ossibly originated via di erent mechanisms. Moreover, the p ossibility of an early history of the Galaxy a ected by tidal interactions, mergers or captures, involving satellite galaxies and outer halo globular clusters has recently received a noticeable supp ort by the latest discovery of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) currently b eing disrupted and absorb ed by the Milky Way (Ibata et al. 1994, Mateo et al. 1994). Our sp eci c interest in this sub ject originates from the early detections of a small (but growing) set of globular clusters which are signi cantly younger (by 3-4 Gyr) than the bulk of the cluster p opulation studied so far in our own Galaxy, i.e. Pal 12 (Gratton and Ortolani 1988, Stetson et al. 1989), Ruprecht 106 (Buonanno et al. 1990, 1993), Arp 2 (Buonanno et al. 1994a,b), and Terzan 7 (Buonanno et al. 1994a,c). As stressed in a previous pap er (Buonanno et al. 1994a), we noted that these four clusters app ear to lie nearly along a great circle in the sky and, following an early suggestion by Lin and Richer (1992), wewere led to conclude that they could b e on similar orbits and mayhave b een captured by the Milky Way. Our present analysis of the available data on lo cations and kinematics of the known stellar systems in the Lo cal Group is strongly centered on the problem of the origin of the young globulars of the Galaxy and has two aims: (1) Search for additional candidate young Galactic globular clusters among those which are suitably lo cated with resp ect to the most attractive con gurations identi ed b etween the four known young globulars and the satellite galaxies. 3 (2) Analyze the con gurations somehow linking young GGCs to the other satellites of the Milky Way to see if and how they could t into the picture of a few great circles (planes) representing the signature of recent or old connections. This is only a rst step towards a comprehensive study of the dynamics, kinematics and chemical enrichment history of these stellar comp onents, which will require more data (in particular prop er motions and detailed chemical abundances) b efore it can pro duce conclusive results. However we b elieveitisworth rep orting some preliminary results and sp eculations as a p ossible guide for further observations and mo dels, esp e- cially since these have led us to pick up, mayb e fortuitously, another \young candidate" (i.e. IC 4499, Ferraro et al. 1994), which con rms the p otentialityofsuch a pro cedure. 2. DATA-SET AND PROCEDURE Our database consists of all the Lo cal Group galaxies, and the GGCs more distant than 10 Kp c from the Galactic Centre. The main reason for this choice is that since the Galactic Centre is the pivot of any plane, the ob jects to o close to that p oint do not provide any signi cant information to distinguish b etween di erent solutions. The basic data-sources for the GGCs are from Thomas (1989, co ordinates and radial velo cities), Djorgovski (1993, metallicities), Armandro (1989) and Peterson (1993) for V and HB E (B V ). If missing from these sources, radial velo cities and metallicities have b een taken from other lists (Zinn 1985, Webbink 1985, Armandro and Da Costa 1991, Freeman et al. 1983). The helio centric and galacto centric distances have b een calculated with the same assumptions as Armandro (1989). The data on Lo cal Group galaxies are drawn from van den Bergh (1994a) and Zaritsky (1994). It is imp ortant to stress that the use of any other reference source for these parameters would make substantially no di erence since we are just lo oking at overall con gurations hardly a ected by the uncertainties asso ciated to these observables. Conversely, it is imp ortant to note that the adopted distance scale is strictly homogeneous, so the description of the GGC system is fully self-consistent as far as standard candles and zero-p oints are concerned. To carry out our tests wehave selected various groups of interesting ob jects (for instance LMC, SMC, all dSph's, Arp 2, Terzan 7, Ruprecht 106, Pal 12 and many other ob jects in some way connected to them in previous studies) and haveinteractively checked their lo cations with resp ect to planes passing through any pair of them and the Galactic Center. When an interesting con guration was found we simply checked the characteristics of the p ossible memb ers and then optimized the matching of the param- eters involved in the description. The criteria that guide our judgement in recognising an \interesting" con guration are that a) the con guration is clearly de ned, i.e. the distances of each ob ject from the de ned plane are as small as p ossible; and b) the max- imum numb er of ob jects, in particular of young GGCs and Galaxy satellites, b elong to the con guration. The p osition of the pro jection in the sky of the considered plane with resp ect to the Magellanic Stream was also considered. See Sect. 3.1 for more details. 4 The diagrams presented in Figure 1 and 2 are made adopting the approach of Lynden- Bell (1982, Table I), i.e. we plot (a) the co ordinates l , b at which each ob ject would G G b e seen if viewed from the Galactic Centre, assumed to b e at 8 Kp c from the sun and (b) the Galactic Great Circles which are the pro jection on the sky of the main planes discussed in text. The approximate lo cation and extension of the Magellanic Stream has also b een rep orted as derived from Kunkel (1979, Fig. 1). 3. RESULTS Figure 1 shows a diagram where al l the ob jects (clusters and galaxies) considered in the present analysis are plotted in the adopted reference frame (l , b ) together with G G the Galactic Great Circles representing the quoted Magellanic Plane and Fornax-Leo- Sculptor Plane. If one considers that these planes are also passing through the Galactic Centre, some alignments are at least curious. For instance, the newly discovered Sgr dSph is not far from the Magellanic Plane in the sky, and the FLS-plane accounts fairly consistently for Sextans and even Pho enix (at ab out 400 Kp c indeed, see Ma jewski 1994). However, one has to recall that the Magellanic Clouds and the pair Draco-Ursa Minor lie nearly at the antip o des on the sky, and keeping these two nodes xed it is p ossible to obtain a family of Magellanic Planes.
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