Mon. Not. R. Astron. Soc. 000, 000–000 (2004) Printed 15 October 2018 (MN LaTEX style file v1.4) Globular clusters and the formation of the outer Galactic halo Sidney van den Bergh1 and A. D. Mackey2 1Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, British Columbia, V9E 2E7, Canada. E-mail: [email protected] 2Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK. E-mail: [email protected] Accepted –. Received – ABSTRACT Globular clusters in the outer halo (Rgc > 15 kpc) are found to be systematically fainter than those at smaller Galactocentric distances. Within the outer halo the compact clusters with half- light radii Rh < 10 pc are only found at Rgc < 40 kpc, while on the other hand the larger clusters with Rh > 10 pc are encountered at all Galactocentric distances. Among the compact clusters with Rh < 10 pc that have Rgc > 15 kpc, there are two objects with surprisingly high metallicities. One of these is Terzan 7, which is a companion of the Sagittarius dwarf. The other is Palomar 1. The data on these two objects suggests that they might have had similar evolutionary histories. It is also noted that, with one exception, luminous globular clusters in the outer halo are all compact whereas faint ones may have any radius. This also holds for globular clusters in the LMC, SMC and Fornax dwarf. The lone exception is the large luminous globular NGC 2419. Possibly this object is not a normal globular cluster, but the stripped core of a former dwarf spheroidal. In this respect it may resemble ω Centauri. Key words: Galaxy: halo, formation – globular clusters: general – Magellanic Clouds 1 INTRODUCTION the present time and have become the Magellanic Clouds and the dwarf spheroidal galaxies”. For additional detailed information the Between 1962 and 1977 it was generally believed that the Galaxy reader is referred to van den Bergh (1995; 2004). had formed by rapid dissipative collapse of a single massive proto- Galaxy. Faith in this paradigm was severely shaken by two papers presented at the 1977 Yale conference on The Evolution of Galax- ies and Stellar Populations (Tinsley & Larson 1977). In one of Presently available data appear to favour the view that the these Searle (1977) showed that, contrary to expectations from the main body of the Milky Way system, i.e. the region with Rgc < 10 arXiv:astro-ph/0407346v1 16 Jul 2004 Eggen, Lynden-Bell & Sandage (1962) (= ELS) model, globular kpc, was formed more-or-less ´ala ELS (as modified by Sandage clusters in the outer halo of the Galaxy did not exhibit a metallic- (1990)), whereas the region with Rgc > 15 kpc was probably ity gradient (although we note that it is not necessarily expected mainly assembled by infall and capture of lesser fragments as en- that a radial abundance gradient be set up in the very outer part of visioned in the SZ model. Zinn (1993) first pointed out that the the Galaxy if the initial phase of the ELS collapse is in free-fall observed data for the halo globular clusters was best explained in (Sandage 1990)). Furthermore Toomre (1977) pointed out that “It terms of both ELS and SZ. He split the halo globular clusters into seems almost inconceivable that there wasn’t a great deal of merg- two groups according to their horizontal branch morphologies. The ing of sizable bits and pieces (including quite a few lesser galaxies) properties of Zinn’s “old halo” (blue horizontal branch) sub-system early in the career of every major galaxy”. These ideas were elab- are consistent with the majority of its members having been formed orated upon by Searle & Zinn (1978) (= SZ) who argued that the as part of an ELS-type collapse, while the properties of his “young lack of an abundance gradient in the outer Galactic halo, along with halo” (red horizontal branch, or second parameter) sub-system are anomalies in the colour-magnitude diagrams of outer halo clusters more in line with its members having been formed in ancestral frag- suggested that the outer Galactic halo was assembled over an ex- ments and accreted into the outer halo at later epochs. Additional tended duration via the infall of transient proto-Galactic fragments. support for this view is provided by the observation (van den Bergh Zinn (1980) describes this process of accretion of proto-Galactic 1993) that a significant fraction of the globular clusters at Rgc > 15 gas clouds in more detail: “The clouds in the central zone [of the kpc are on plunging orbits, and that a number of outer halo objects Galaxy] merged to form a large gas cloud that later evolved into with relatively well-determined orbits have retrograde motions (Di- the Galactic disk. The clouds in the second zone evolved as isolated nescu et al. 1999). It is noted in passing (van den Bergh 1995) that systems for various lengths of time up to ∼ 5 Gyr, but eventually cluster metallicity appears to correlate more strongly with the peri- all of these clouds collided with the disk and were destroyed. The Galactic distances of globular clusters than it does with the present clouds of the outermost zone have evolved in relative isolation until Galactic distance Rgc. c 2004 RAS 2 Sidney van den Bergh & A. D. Mackey (see below). Of the 35 objects with Rgc > 15 kpc, the following Table 1. Globular clusters with Rgc > 15 kpc seven appear to be associated with the Sagittarius dwarf galaxy: M54, Terzan 7 and 8, and Arp 2 (Ibata et al. 1994; Da Costa & Name Rgc MV [Fe/H] Rh Armandroff 1995); Pal. 12 and NGC 4147 (Dinescu et al. 2000; (kpc) (pc) Mart´ınez-Delgado et al. 2002; Bellazzini et al. 2003a); and Pal. 2 (Majewski et al. 2004). Also note the possible physical association NGC 1261 18.2 −7.81 −1.35 3.6 Pal. 1 17.0 −2.47 −0.60 2.2 of the globular clusters NGC 1851, NGC 1904, NGC 2298 and AM 1 123.2 −4.71 −1.80 17.7 NGC 2808, plus a number of old open clusters, with the recently Eridanus 95.2 −5.14 −1.46 10.5 discovered Canis Major dwarf (Martin et al. 2004; Bellazzini et Pal. 2 35.4∗ −8.01 −1.30 5.4 al. 2003b; Frinchaboy et al. 2004). In particular, Bellazzini et al NGC 1851 16.7+ −8.33 −1.22 1.8 (2003b) find strong evidence that the clusters AM 2 and Tombaugh NGC 1904 M79 18.8+ −7.86 −1.57 3.0 2 [which are not cataloged as globulars by Harris] are associated NGC 2298 15.7+ −6.30 −1.85 2.4 with the Canis Major system. In fact, these authors suggest that NGC 2419 91.5 −9.58 −2.12 17.9 Tombaugh 2 may actually represent an over-density in the CMa 1 + NGC 2808 11.1 −9.39 −1.15 2.1 field itself, similar to those observed in the Sagittarius and Ursa Pyxis 41.7 −5.75 −1.30 15.6 Minor dwarf galaxies. Finally, Carraro et al. (2004) have shown that Pal. 3 95.9 −5.70 −1.66 17.8 Pal. 4 111.8 −6.02 −1.48 17.2 the cluster Berkeley 29 is associated with the Monoceros stream, NGC 4147 21.3∗ −6.16 −1.83 2.4 which is thought (Martin et al. 2004) to be part of the disrupted Rup. 106 18.5 −6.35 −1.67 6.8 CMa dwarf. However, this cluster has an age of ∼ 5 Gyr, which NGC 5024 M53 18.3 −8.70 −1.99 5.7 makes it somewhat too young to be of interest for the present study NGC 5053 16.9 −6.72 −2.29 16.7 of globular clusters. AM 4 25.5 −1.60 −2.00 3.7 Information on the globular clusters in the LMC, the SMC, NGC 5466 16.2 −6.96 −2.22 10.4 and the Fornax dwarf spheroidal is collected in Table 2 using the NGC 5634 21.2 −7.69 −1.88 4.0 data from Mackey & Gilmore (2003a; 2003b; 2003c). The total lu- NGC 5694 29.1 −7.81 −1.86 3.3 minosities and half-light radii in this Table have been newly derived IC 4499 15.7 −7.33 −1.60 8.2 for the present work. The total luminosities (MV ) were obtained NGC 5824 25.8 −8.84 −1.85 3.4 Pal. 5 18.6 −5.17 −1.41 20.0 by integrating these authors’ radial brightness profiles to appropri- Pal. 14 AvdB 69.0 −4.73 −1.52 24.7 ate limiting radii (∼ 50 pc) using Eq. 12 of Mackey & Gilmore NGC 6229 29.7 −8.05 −1.43 3.3 (2003a). Rearranging this equation then allows the subsequent de- Pal. 15 37.9 −5.49 −1.90 15.7 termination of the half-light radii. Distance moduli of 18.50, 18.90, IC 1257 17.9 −6.15 −1.70 ... and 20.68 have been adopted for the LMC, SMC, and Fornax sys- ∗ NGC 6715 M54 19.2 −10.01 −1.58 3.8 tems, respectively. ∗ Ter. 7 16.0 −5.05 −0.58 6.6 The LMC sample of Mackey & Gilmore (2003a) omits four Arp 2 21.4∗ −5.29 −1.76 15.9 ∗ known globulars (NGC 1928, 1939, Reticulum, and ESO121- Ter. 8 19.1 −5.05 −2.00 7.6 SC03). A recent Hubble Space Telescope program has obtained NGC 7006 38.8 −7.68 −1.63 4.6 ∗ images of these four objects using the Advanced Camera for Sur- Pal.