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TheGaia Sausage is an elongated structure in velocity space discovered by Belokurov et al. using the kinematics of metal-rich halo stars. They showed that it could be created by a massive dwarf galaxy (∼5 × 10¹⁰) on a strongly radial orbit that merged with the Milky Way at a redshiftz ≲ 3. This merger would also have brought in globular clusters (GCs). We seek evidence for the associated Sausage Globular Clusters (GCs) by analyzing the structure of 91 Milky Way GCs in action space using theGaia Data Release 2 catalog, complemented withHubble Space Telescope proper motions. There is a characteristic energy that separates the in situ objects, such as the bulge/disk clusters, from the accreted objects, such as the young halo clusters. There are 15 old halo GCs that haveE > . Eight of the high-energy, old halo GCs are strongly clumped in azimuthal and vertical action, yet strung out like beads on a chain at extreme radial action. They are very radially anisotropic (β ∼ 0.95) and move on orbits that are all highly eccentric (e ≳ 0.80). They also form a track in the age–metallicity plane compatible with a dwarf galaxy origin. These properties are consistent with GCs associated with the merger event that gave rise to theGaia Sausage.

There are multiple and striking pieces of evidence for the existence of a massive ancient merger that provides the bulk of the stars in the inner halo of the Milky Way galaxy. For example, the radial density profile of the stellar halo shows a dramatic break at around 30 kpc in tracers such as RR Lyrae and blue horizontal branch stars (e.g., Watkins et al.2009; Deason et al.2011). Deason et al. (2013) argued that this could be interpreted as the last apocenter of a massive progenitor galaxy accreted between 8 and 10 Gyr ago. Myeong et al. (2018a) showed that the kinematics of metal-rich halo stars (−1.9 < [Fe/H] < −1.1) betray extensive evidence of recent accretion using the Sloan Digital Sky Survey (SDSS)-Gaia catalog. The variation in Oosterhoff classes of RR Lyraes with radius (Belokurov et al.2018a) similarly shows evidence that the bulk of the field RRab is provided by a single massive progenitor. Finally, Belokurov et al. (2018b) demonstrated that the shape of the velocity ellipsoid of the inner metal-rich stellar halo is highly non-Gaussian and sausage-shaped. They interpreted thisGaiaSausage as evidence that two-thirds of the local stellar halo could have been deposited via the disruption of a massive (≳10¹⁰) galaxy on a strongly radial orbit between redshiftz = 3 andz = 1. Although identified in the SDSS-Gaia catalog, recent investigations by Haywood et al. (2018) with the newGaia Data Release 2 (DR2) catalog (Gaia Collaboration et al.2018) support the original hypothesis. If so, then this beast must have brought with it a population of globular clusters (GCs), now dispersed in the inner halo. After all, the similarly massive Sagittarius galaxy (Sgr) is now known to have brought at least four and possibly seven GCs with it (e.g., Forbes & Bridges2010; Sohn et al.2018).

The main aim of this Letter is to search for the Sausage Globular Clusters. The identification of objects accreted in the same merger event is easiest in action space. Actions have the property of adiabatic invariance, so that they stay approximately constant when changes in the potential occur slowly (e.g., Goldstein1980; Binney & Spergel1982). GCs accreted in the same event are identifiable as clumped and compact substructures in action space (as is indeed the case for the 4 Sgr GCs—Terzan 7, Terzan 8, Arp 2, and Pal 12). Historically, actions were cumbersome to calculate, but recent theoretical advances have transformed the situation (e.g., Binney2012; Sanders & Binney2016). The power of actions has recently been demonstrated by the identification of the tidal disgorgements ofω Centauri (Myeong et al.2018b). Here, we display the Milky Way GCs in action space using a realistic Galactic potential comprising flattened stellar and gas disks, halo, and bulge (McMillan2017) with the specific aim of identifying the Sausage GCs.

The Milky Way GCs are a disparate group: some were formed in situ in the Milky Way, some were acquired by the engulfment of dwarf galaxies. A classification was introduced by Zinn (1993), in which GCs are divided into bulge/disk, old halo, and young halo on the basis of cluster metallicity and horizontal branch morphology. The bulge/disk (BD) systems are concentrated in the Galactic bulge and inner disk, while the old halo (OH) clusters are predominantly in the inner halo. They are mostly believed to have been formed in the Milky Way, though ∼15%–17% might have been accreted. The young halo (YH) clusters can extend to large radii and are all believed to have been accreted (see e.g., Mackey & Gilmore2004; Mackey & van den Bergh2005).

The combination of observables from the Gaia Collaboration et al. (2018), Sohn et al. (2018), and Harris (1996, 2010 edition) allows us to obtain full six-dimensional information for 91 GCs (out of a total of ∼150 in the Galaxy). To convert from observables to the Galactic rest-frame, we use the circular speed of 232.8 km s⁻¹ at the Sun’s position of 8.2 kpc, consistent with the McMillan (2017) potential, while for the Solar peculiar motion we use the most recent value from Schönrich et al. (2010), namely (U,V,W) = (11.1, 12.24, 7.25) km s⁻¹. These values differ from those used by the Gaia Collaboration et al. (2018) or Posti & Helmi (2018), so there are small differences in quantities such as apocenters and eccentricities. We use the numerical method of Binney (2012) and Sanders & Binney (2016) to compute the action variables of each GC (). GCs associated with theGaia Sausage must lie on highly radial orbits, and so have lowJ_ϕ andJ_z, but very largeJ_R. The uncertainty in proper motions is the main contributor to the median (total) velocity error of ∼9 km s⁻¹. This leads to median errors in the actions of ∼10%, and so features in action space are robust against uncertainties. Figure1 presents the distribution of GCs in energy and action space, while Figure2 presents the projections onto the principal planes of action space. In both cases, we also show as gray pixels the distribution of main-sequence turn-off stars (MSTOs) from Myeong et al. (2018a). This is to give an idea of the range in action at any energy level occupied by the stellar halo. Both plots are color coded according to the conventional classification from Mackey & van den Bergh (2005): red circles mark the OH GCs, blue triangles the YH GCs, yellow triangles the BD GCs, and green diamonds the Sagittarius GCs. The Sagittarius dwarf (Sgr) is also marked as a black filled square. The YH GCs all lie above a critical energy of = −1.6 × 10⁵ km² s⁻². The BD globulars all lie below this critical energy. We regard the identification of this critical energy as a reference level. Though the value of does depend on potential, the existence of a critical energy level is robust—it is the value of the most bound YH cluster. We argue that GCs with comparable or higher energy are all accreted from dwarf galaxies.

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The BD and OH clusters form tracks in Figures1 and2. We can see that the BD clusters branch out toward positiveJ_ϕ, while maintaining lowJ_R and lowJ_z values, as befit disk orbits. They are entirely limited toE ≤ . For the OH clusters, we can see a similar branching toward positiveJ_ϕ at low energy (E <). The low-energy OH clusters are all concentrated at lowJ_R. There are similarities in the morphology of the (J_R,E) distribution for the low-energy OH clusters and the the MSTOs as illustrated in Myeong et al. (2018a). In the (J_z,E) plane, the OH clusters seemingly break up into two separate branches at low energy, though it is unclear whether this is caused by dynamical or selection effects.

There are 15 OH clusters above the critical energy (). Their azimuthal actionJ_ϕ distribution is narrower than the low-energy ones. It resembles the tips of the “diamond-like” contours seen in the distribution of MSTOs in the metal-rich halo (Myeong et al.2018a). Also, the radial actionJ_R distribution of high-energy OH clusters is extremely distended. Most of them have high radial action, tracing out a structure similar to the picture of the metal-rich halo.

Of the 15 high-energy OH clusters, there are 6 with high vertical action (J_z ≳ 1000 km s⁻¹ kpc). They lie well apart from the main group. They have a wide spread in azimuthal (J_ϕ) and radial (J_R) actions similar to the YHs, suggesting an accretion origin. The main group are concentrated at largeJ_R, lowJ_z and lowJ_ϕ region in the action space, indicating radial orbits. They show surprisingly low vertical action ( km s⁻¹)—they are actually less extended inJ_z than the low-energy OH clusters and much less extended than the MSTO stars with similar energy. This tight concentration, especially inJ_z, is interesting because the range ofJ_z becomes wider as we move to higher energy, as is demonstrated by the MSTO sample. The eight high-energy OHs forming this main group (NGCs 1851, 1904, 2298, 2808, 5286, 6864, 6779, and 7089) are marked with black circles in Figures1 and2. For this group of eight, the maximumJ_z is ∼360 km s⁻¹ kpc, the maximum is ∼500 km s⁻¹ kpc, while the minimumJ_R is ∼700 km s⁻¹ kpc. In action space, their distribution is highly flattened and sausage-like. Interestingly, there are no OH clusters with comparable energy that have high vertical actionJ_z (see e.g., the middle panel of Figure2). Mackey & Gilmore (2004) suggest that 15%–17% of the OH clusters might have been accreted. In our picture, at least 8 Sausage GCs (or 14 including those with very highJ_z) out of 53 are accreted, in rough accord with the estimate.

The YH GCs all haveE > , and show a broad spread in all actions. They include extreme prograde and retrograde members in the sample, as well as the ones with largest radialJ_R and verticalJ_z actions (excluding the Sgr GCs). The two black open squares in Figures1 and2 provide an extended selection to the Sausage GCs. They are two YH GCs (NGC 362, and NGC 1261) with a rather similar horizontal branch morphology to OH clusters (see Section3) that also have similar actions and energy to the Sausage GCs. These are possibles rather than probables.

The upper panel of Figure3 shows the apocenters and pericenters of the sample, with lines of constant eccentricity superposed. The Gaia Collaboration et al. (2018) already noted the tendency for GCs with larger apocenters to have larger eccentricities. The eight probable and two possible Sausage GCs are denoted by black open circles and open squares. They form a clump concentrated at highJ_R, lowJ_z, and lowJ_ϕ, and they all have high orbital eccentricity ≳0.80. We can also see that most of the BD clusters have low eccentricity. There are also many OH clusters with comparably low eccentricity. The YH clusters are again widely dispersed, as they have high energy and highly spread actions.

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Finally, we must consider whether selection effects could cause this. As the Gaia Collaboration et al. (2018) point out, GCs with high energy are more likely to be observed if they are on eccentric orbits. Even so, the middle panel of Figure2 demonstrates that there are no OH clusters in this energy range that have highJ_z. By taking the positions of GCs in our sample, and sampling their velocities from a Gaussian withσ = 130 km s⁻¹, we show the expected distribution in action space in the lower-left panel of Figure3, Notice that there is only a very mild bias toward lowJ_z, soGaia should have seen any highJ_z GCs at this energy range if they existed. However, if the cluster velocity distribution is highly anisotropic, as shown in the lower-right panel of Figure3, this distribution will become more biased toward lowJ_z, though still the tips of the Sausage are not expected.

The properties of the eight probable and two possible Sausage GCs in energy and action space are listed in Table1.

Table 1. Kinematic Properties of the Eight Probable and Two Possible Sausage GCs

Name	(vr,vθ,vφ)	e		E
(NGC)	(km s−1)		(km s−1 kpc)	(km2 s−2)
1851	(134.8, 11.6, 28.6)	0.91	(1493, −178, 230)	−134706
1904	(46.5, −2.9, −21.5)	0.93	(1477, 51, 155)	−137390
2298	(−96.1, 41.3, −57.7)	0.79	(949, −648, 317)	−140391
2808	(−152.9, −35.5, −3.7)	0.86	(1038, 394, 35)	−152947
5286	(−202.3, 42.4, −58.3)	0.84	(856, −366, 148)	−153940
6864	(−113.0, −27.6, 24.1)	0.83	(1144, 316, 324)	−143397
6779	(159.4, 19.9, −76.9)	0.86	(677, −182, 199)	−159799
7089	(231.3, 24.1, 28.0)	0.88	(1368, −192, 309)	−139217
362	(147.1, 7.9, −33.5)	0.85	(837, −57, 317)	−159510
1261	(−113.8, 30.5, 7.2)	0.86	(1474, −393, 351)	−132973

Note. The galactic rest-frame velocity in spherical polars, the actions in cylindrical polars, the energy, and orbital eccentricity are all given.

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The identification of theGaia Sausage in MSTOs is most evident in velocity space. Belokurov et al. (2018b) showed that the velocity anisotropy parameterβ_MSTO is very extreme,

Here,v_φ is the azimuthal velocity in the direction of the Milky Way’s rotation,v_θ is increasing toward the Milky Way’s north pole, andv_r is the radial velocity in spherical coordinates. Given thatβ = 1 implies that all orbits are linear straight lines through the Galactic Center, the metal-rich local halo stars are very radially anisotropic. This gives the Sausage its name, as the structure (which is also highly non-Gaussian) looks sausage-shaped in velocity space. Figure4 shows the velocities of the GCs resolved with respect to spherical polar coordinates. The Sausage GCs have an even more extreme value of the anisotropy parameter than the Sausage MSTOs, with. Of course, both here and in Belokurov et al. (2018b), cuts have been used to remove stars and GCs to isolate the Sausage component.

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The upper panel of Figure5 shows age versus metallicity for the Sausage GCs, as well as seven GCs that have been claimed as associates of Sgr, specifically Terzan 7, Terzan 8, Arp 2, Pal 12, NGC 4147, NGC 6715, and Whiting 1 (Forbes & Bridges2010). As noted by Forbes & Bridges (2010), the age–metallicity relation for the Milky Way’s GCs reveals two distinct tracks. There is broad swathe of BD and OH GCs with a roughly constant old age of ∼12.8 Gyr. This comprises the bulk of the sample. However, Forbes & Bridges (2010) pointed out that the Sgr GCs form a separate track that branches to younger ages, and is shown as open diamonds in Figure5. We find that the Sausage GCs similarly follow a track that is very different from the bulk of the Milky Way’s in situ GCs. It is similar to, but vertically offset from, the Sgr track. The lower panel of Figure5 shows the horizontal branch index versus metallicity using data from Mackey & van den Bergh (2005). The plot emphasizes the ambiguous nature of the two clusters, NGC 362 and NGC 1261. Although Mackey & van den Bergh (2005) classified them as YH clusters based on their horizontal branch morphology, they are in fact close to the dividing line. We therefore suggest that this classification can be debatable. They are kinematically close to the Sausage GCs, who may well be their true brethren.

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This Letter argues that there are at least 8, and possibly 10, halo GCs that belong to a single, ancient massive merger event identified by Belokurov et al. (2018b) and responsible for theGaia Sausage in velocity space. The evidence is threefold. First, there is a strong prior expectation of finding a population of radially anisotropic GCs. Evidence for a major accretion event is provided by studies of the kinematics of halo MSTOs in the SDSS-Gaia catalog (Belokurov et al.2018b; Myeong et al.2018a), as well as inGaia DR2 (Haywood et al.2018). It explains the peculiar, highly non-Gaussian, radial anisotropic local velocity distribution of halo stars (hence the “Gaia Sausage”). The existence of the Sausage GCs supports the idea of a single event and allows us to put estimates on the mass of the progenitor. Judging from GC numbers, it must have been more massive than Fornax and comparable to the Sgr progenitor, which Gibbons et al. (2017) estimated as 5 × 10¹⁰ in total mass. This is in good agreement with the mass estimate provided in Belokurov et al. (2018b).

Second, just as the GCs associated with Sgr can be identified by their agglomeration in action space, so can the GCs associated with theGaia Sausage. A critical energy separates the YH clusters (which have all been accreted) from the BD clusters (which are all formed in situ). The OH clusters are mainly formed in situ, though Mackey & Gilmore (2004) suggested that 15%–17% were accreted. They straddle the critical energy. Eight of the OH clusters withE >  form a narrow, clumped, and compact distribution in action space. They have characteristic low vertical (J_z) and high radial (J_R) action. They show strong radial anisotropy (β ≈ 0.95) and highly radial, eccentric orbits (e ≳ 0.80). These are exactly the characteristics expected for the Sausage GCs. There may even be two further members—if we, for example, permit the inclusion of YH clusters.

Third, the eight GCs identified as belonging to theGaia Sausage were chosen without any regard to their age or metallicity. However, these eight clusters show the typical age–metallicity trend expected from dwarf galaxies, which is additional evidence supporting their extragalactic origin. The time of infall can also be roughly reckoned from the tracks in age–metallicity space as ∼10 Gyr orz ∼ 3, in accord with the estimate in Belokurov et al. (2018b).

Could this peculiarity of the data be due to a selection effect, against which the Gaia Collaboration et al. (2018) already caution? High-energy GCs are more likely to be observed if they are on eccentric orbits. We have demonstrated that there is a weak preference for GCs in the Sausage GC energy and angular momentum range to have largerJ_R thanJ_z. However, the Sausage GCs are a significantly more radially anisotropic population than expected purely from selection effects. This indicates that the selection effects have limited impact on our conclusions.