Identifying true satellites of the Magellanic Clouds
Laura V. Sales, Julio F. Navarro, Nitya Kallivayalil, Carlos S. Frenk
MMon. Not. R. Astron. Soc. , 000–000 (0000) Printed 13 May 2016 (MN L A TEX style file v2.2)
Identifying true satellites of the Magellanic Clouds
Laura V. Sales (cid:63) , Julio F. Navarro , Nitya Kallivayalil and Carlos S. Frenk Department of Physics and Astronomy, University of California Riverside, 900 University Ave., CA92507, US Senior CIfAR Fellow. Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
13 May 2016
ABSTRACT
The hierarchical nature of Λ CDM suggests that the Magellanic Clouds must have been sur-rounded by a number of satellites before their infall into the Milky Way. Many of thosesatellites should still be in close proximity to the Clouds, but some could have dispersedahead/behind the Clouds along their Galactic orbit. Either way, prior association with theClouds results in strong restrictions on the present-day positions and velocities of candidateMagellanic satellites: they must lie close to the nearly-polar orbital plane of the Magellanicstream, and their distances and radial velocities must follow the latitude dependence expectedfor a tidal stream with the Clouds at pericenter. We use a cosmological numerical simulationof the disruption of a massive subhalo in a Milky Way-sized Λ CDM halo to test whether anyof the dwarfs recently-discovered in the DES, SMASH, Pan-STARRS, and ATLAS sur-veys are truly associated with the Clouds. Of the systems with kinematic data, only Hydra IIand Hor 1 have distances and radial velocities consistent with a Magellanic origin. Of theremaining dwarfs, six (Hor 2, Eri 3, Ret 3, Tuc 4, Tuc 5, and Phx 2) have positions and dis-tances consistent with a Magellanic origin, but kinematic data are needed to substantiate thatpossibility. Conclusive evidence for association would require proper motions to constrain theorbital angular momentum direction, which, for true Magellanic satellites, must coincide withthat of the Clouds. We use this result to predict radial velocities and proper motions for all newdwarfs. Our results are relatively insensitive to the assumption of first or second pericenter forthe Clouds. Key words: galaxies: haloes - galaxies: formation - galaxies: evolution - galaxies: kinematicsand dynamics.
The Large and Small Magellanic Clouds (LMC and SMC, respec-tively) are a galaxy pair orbiting together in the halo of the MilkyWay and provide a prime example of the nested hierarchy of struc-tures expected in the Λ CDM galaxy formation paradigm (Springelet al. 2008). Their physical association seems beyond doubt, giventheir relative proximity, correlated kinematics, and abundant evi-dence of past interaction (for a recent review, see, e.g., D’Onghia& Fox 2015).The path of the Clouds around the Galaxy is well constrainedby precise estimates of their distances, positions, radial veloci-ties and proper motions, which indicate a nearly-polar orbit ona plane closely aligned with the Magellanic Stream (Kallivayalilet al. 2006). The Clouds are just past pericenter, since their Galac-tocentric radial velocities are positive and much smaller than their (cid:63)
E-mail: [email protected] tangential velocities ( V t ∼ , V r ∼ +64 km/s for the LMC, see,e.g., Kallivayalil et al. 2013). Their orbit must also have a fairlylarge apocentric radius, since their total speed ( | V LMC | ∼ km/s) exceeds the circular velocity of the Milky Way ( ∼ km/s)by a substantial amount. A large apocenter implies a long orbitalperiod, which has led to the suggestion that the Clouds might be ontheir first pericentric passage.This conclusion depends on the total mass assumed for theMilky Way halo, as well as on its assumed outer radial profile(Besla et al. 2007), but it would explain naturally why the LMCand SMC are still so tightly bound. Indeed, if the Clouds were atfirst pericenter then the Galactic tide would not have yet had timeto disrupt the pair nor to disperse fully the common (sub)halo theyinhabit. As a result, most other dwarf companions of the Cloudsshould still lie in their close vicinity. Such “Magellanic satellites”have long been speculated (see, e.g., Lynden-Bell & Lynden-Bell1995), and their existence would be consistent with the relativelycommon occurrence of dwarf galaxy associations in the nearby c (cid:13) a r X i v : . [ a s t r o - ph . GA ] M a y Sales et al.
Universe (Tully et al. 2006). The immediate surroundings of theClouds should thus be a fertile ground to search for new dwarfs, asproposed by Sales et al. (2011, S11 hereafter).A full search for satellites around the Clouds would be ex-tremely valuable. One reason is that, in Λ CDM, the satellite lumi-nosity function is expected to be a nearly scale-free function whenexpressed in units of the luminosity of the primary (Sales et al.2013). In other words, to first order, the Galactic satellite abun-dance should be simply a scaled-up version of that of the Clouds.A complete catalogue of Magellanic faint and ultra-faint satelliteswould be easier to compile (the relevant survey volume is muchsmaller than the full Galactic halo) and could therefore help to con-strain the incompleteness of all-sky surveys of Galactic satellites.In general, the surrounding of dwarf galaxies, especially those inthe field, are promising sites for the discovery of new faint galaxies(Sales et al. 2013; Wheeler et al. 2015).A second application would be to clarify the effects of en-vironment on the star formation history of dwarfs (D’Onghia &Lake 2008; Wetzel et al. 2015). An unambiguous identification ofMagellanic origin would enable a direct comparison with Galacticsatellites of similar stellar mass that have evolved in a rather differ-ent environment. Finally, Magellanic satellites might also provideclues to the nature of dark matter: indeed, fewer satellites are ex-pected around the Milky Way in general, and the LMC in particular,if dark matter was “warm” rather than cold (see, e.g., Kennedy et al.2014).Given this context, it is not surprising that the recent discoveryof a number of candidate dwarfs in southern surveys targeting theClouds’ vicinity, such as the Dark Energy Survey (DES; Bechtolet al. 2015; Koposov et al. 2015; Drlica-Wagner et al. 2015; Kim& Jerjen 2015; Kim et al. 2015), the Survey of the MAgellanicStellar History (SMASH; Martin et al. 2015), as well as in otherlarge surveys, such as PAN-STARRS (Laevens et al. 2015), andATLAS (Torrealba et al. 2016), have attracted much attention.While not all of these candidates have follow-up spectroscopyconfirming that they are dark matter-dominated dwarf galaxiesrather than star clusters—six have spectra thus far (Walker et al.2016; Kirby et al. 2015; Martin et al. 2016)—they do occupy thesame region in the size-luminosity plane as ultra-faint dwarf galax-ies (M V between -2.0 and -7.8 and half-light radii, r h , between ∼ and ∼ pc). It is not clear either which of these dwarfs,if any, have a Magellanic origin.On that point, Deason et al. (2015) cite a statistical argumentbased on abundance-matching models applied to massive subhalosin the ELVIS simulations (Garrison-Kimmel et al. 2014) to suggestthat - of the then known DES candidates might have come intothe Milky Way with the LMC. Yozin & Bekki (2015), on the otherhand, conclude, on the basis of orbit models, that the majority ofthe DES dwarfs could have been at least loosely associated with theClouds. Yet another analysis suggests, using tailor-made numericalsimulations, that only about half of the DES new dwarf galaxiesare very likely to have been associated with the LMC in the past(Jethwa et al. 2016) .Here, we take a complementary and targeted approach, usingan LMC analog subhalo identified in a fully cosmological simula-tion of a Milky Way-sized halo in Λ CDM. We track the positionsand velocities of subhalo particles to constrain the likely locationin phase space of systems with prior association with the Clouds.This is an extension of the analysis previously presented in S11,who concluded that none of the Milky Way satellites known atthe time were convincingly associated with the Clouds. The maingoal of the present work is to assess the likelihood of association with the Clouds of the recently-discovered dwarfs, as well as topredict the radial velocities and proper motions required for thatassociation to be true.In § § § We use the Aquarius Project (Springel et al. 2008), a suite ofzoomed-in cosmological simulations that follow the formation ofof Milky Way-sized halos with virial masses in the range . - . × M (cid:12) . These halos were selected from a large scale simu-lation of a cosmologically representative volume (the Millennium-II Simulation, see Boylan-Kolchin et al. 2009). We focus in this pa-per on the properties of an “LMC analog” system (hereafter iden-tified as LMCa, for short) which was identified and presented inS11. LMCa was chosen because it is a fairly massive subhalo with apericentric distance ( ∼ kpc ) and velocity ( ∼ km/s) in goodagreement with that of the LMC (Kallivayalil et al. 2006, hereafterK06). Identified before infall, at z id = 0 . , LMCa has a virial massof M = 3 . × M (cid:12) , which corresponds to a circular veloc-ity of ∼ km/s.LMCa first crosses the virial boundary of the main Aquariushalo (Aq-A) at z = 0 . ( t = 8 . Gyr), reaches first pericenterat t = 9 . Gyr, and is able to complete a second pericentricpassage at t = 13 . Gyr. The host halo has a virial mass of M = 1 . × M (cid:12) at z = 0 . (These times are actually slightlypast actual pericenter, thus chosen so as to best accommodate thefact that the LMC has a slight positive radial velocity and is itselfjust past pericenter at present.)At t p and t p , the distances, radial velocities, and tangentialvelocities are, respectively, r p = 65 kpc, r p = 69 kpc, V r, p =78 km/s, V r, p = 89 km/s; V t, p = 345 km/s; and V t, p = 302 km/s. These values are in reasonable agreement with the K06 LMCmeasurements (see Fig. 1 in S11), although the tangential velocitieswere a bit below the observed values. The revised proper motionsfor the LMC from Kallivayalil et al. (2013) suggest a slightly lowertotal velocity than previously determined, ± km/s comparedto ± km/s, resulting from the combination of an added third-epoch of observations, the adoption of a different local standard ofrest, and a new determination of the LMC’s dynamical center. Thisdecrease in velocity accommodates the tangential motion of LMCamore comfortably at both pericenters.It is still a matter of debate whether the Clouds are on first orsecond pericentric passage (see, e.g., Shattow & Loeb 2009; Saleset al. 2011), although indirect evidence favour a first infall scenario,including: a ) their large tangential velocity, b ) their blue colors and We define the virial mass, M , as that enclosed by a sphere of meandensity times the critical density of the Universe, ρ crit = 3 H / πG .Virial quantities are defined at that radius, and are identified by a “200”subscript. c (cid:13)000
Universe (Tully et al. 2006). The immediate surroundings of theClouds should thus be a fertile ground to search for new dwarfs, asproposed by Sales et al. (2011, S11 hereafter).A full search for satellites around the Clouds would be ex-tremely valuable. One reason is that, in Λ CDM, the satellite lumi-nosity function is expected to be a nearly scale-free function whenexpressed in units of the luminosity of the primary (Sales et al.2013). In other words, to first order, the Galactic satellite abun-dance should be simply a scaled-up version of that of the Clouds.A complete catalogue of Magellanic faint and ultra-faint satelliteswould be easier to compile (the relevant survey volume is muchsmaller than the full Galactic halo) and could therefore help to con-strain the incompleteness of all-sky surveys of Galactic satellites.In general, the surrounding of dwarf galaxies, especially those inthe field, are promising sites for the discovery of new faint galaxies(Sales et al. 2013; Wheeler et al. 2015).A second application would be to clarify the effects of en-vironment on the star formation history of dwarfs (D’Onghia &Lake 2008; Wetzel et al. 2015). An unambiguous identification ofMagellanic origin would enable a direct comparison with Galacticsatellites of similar stellar mass that have evolved in a rather differ-ent environment. Finally, Magellanic satellites might also provideclues to the nature of dark matter: indeed, fewer satellites are ex-pected around the Milky Way in general, and the LMC in particular,if dark matter was “warm” rather than cold (see, e.g., Kennedy et al.2014).Given this context, it is not surprising that the recent discoveryof a number of candidate dwarfs in southern surveys targeting theClouds’ vicinity, such as the Dark Energy Survey (DES; Bechtolet al. 2015; Koposov et al. 2015; Drlica-Wagner et al. 2015; Kim& Jerjen 2015; Kim et al. 2015), the Survey of the MAgellanicStellar History (SMASH; Martin et al. 2015), as well as in otherlarge surveys, such as PAN-STARRS (Laevens et al. 2015), andATLAS (Torrealba et al. 2016), have attracted much attention.While not all of these candidates have follow-up spectroscopyconfirming that they are dark matter-dominated dwarf galaxiesrather than star clusters—six have spectra thus far (Walker et al.2016; Kirby et al. 2015; Martin et al. 2016)—they do occupy thesame region in the size-luminosity plane as ultra-faint dwarf galax-ies (M V between -2.0 and -7.8 and half-light radii, r h , between ∼ and ∼ pc). It is not clear either which of these dwarfs,if any, have a Magellanic origin.On that point, Deason et al. (2015) cite a statistical argumentbased on abundance-matching models applied to massive subhalosin the ELVIS simulations (Garrison-Kimmel et al. 2014) to suggestthat - of the then known DES candidates might have come intothe Milky Way with the LMC. Yozin & Bekki (2015), on the otherhand, conclude, on the basis of orbit models, that the majority ofthe DES dwarfs could have been at least loosely associated with theClouds. Yet another analysis suggests, using tailor-made numericalsimulations, that only about half of the DES new dwarf galaxiesare very likely to have been associated with the LMC in the past(Jethwa et al. 2016) .Here, we take a complementary and targeted approach, usingan LMC analog subhalo identified in a fully cosmological simula-tion of a Milky Way-sized halo in Λ CDM. We track the positionsand velocities of subhalo particles to constrain the likely locationin phase space of systems with prior association with the Clouds.This is an extension of the analysis previously presented in S11,who concluded that none of the Milky Way satellites known atthe time were convincingly associated with the Clouds. The maingoal of the present work is to assess the likelihood of association with the Clouds of the recently-discovered dwarfs, as well as topredict the radial velocities and proper motions required for thatassociation to be true.In § § § We use the Aquarius Project (Springel et al. 2008), a suite ofzoomed-in cosmological simulations that follow the formation ofof Milky Way-sized halos with virial masses in the range . - . × M (cid:12) . These halos were selected from a large scale simu-lation of a cosmologically representative volume (the Millennium-II Simulation, see Boylan-Kolchin et al. 2009). We focus in this pa-per on the properties of an “LMC analog” system (hereafter iden-tified as LMCa, for short) which was identified and presented inS11. LMCa was chosen because it is a fairly massive subhalo with apericentric distance ( ∼ kpc ) and velocity ( ∼ km/s) in goodagreement with that of the LMC (Kallivayalil et al. 2006, hereafterK06). Identified before infall, at z id = 0 . , LMCa has a virial massof M = 3 . × M (cid:12) , which corresponds to a circular veloc-ity of ∼ km/s.LMCa first crosses the virial boundary of the main Aquariushalo (Aq-A) at z = 0 . ( t = 8 . Gyr), reaches first pericenterat t = 9 . Gyr, and is able to complete a second pericentricpassage at t = 13 . Gyr. The host halo has a virial mass of M = 1 . × M (cid:12) at z = 0 . (These times are actually slightlypast actual pericenter, thus chosen so as to best accommodate thefact that the LMC has a slight positive radial velocity and is itselfjust past pericenter at present.)At t p and t p , the distances, radial velocities, and tangentialvelocities are, respectively, r p = 65 kpc, r p = 69 kpc, V r, p =78 km/s, V r, p = 89 km/s; V t, p = 345 km/s; and V t, p = 302 km/s. These values are in reasonable agreement with the K06 LMCmeasurements (see Fig. 1 in S11), although the tangential velocitieswere a bit below the observed values. The revised proper motionsfor the LMC from Kallivayalil et al. (2013) suggest a slightly lowertotal velocity than previously determined, ± km/s comparedto ± km/s, resulting from the combination of an added third-epoch of observations, the adoption of a different local standard ofrest, and a new determination of the LMC’s dynamical center. Thisdecrease in velocity accommodates the tangential motion of LMCamore comfortably at both pericenters.It is still a matter of debate whether the Clouds are on first orsecond pericentric passage (see, e.g., Shattow & Loeb 2009; Saleset al. 2011), although indirect evidence favour a first infall scenario,including: a ) their large tangential velocity, b ) their blue colors and We define the virial mass, M , as that enclosed by a sphere of meandensity times the critical density of the Universe, ρ crit = 3 H / πG .Virial quantities are defined at that radius, and are identified by a “200”subscript. c (cid:13)000 , 000–000 he satellites of the Magellanic Clouds -150° -120° -90° -60° -30° 0° 30° 60° 90° 120° 150°-75°-60°-45°-30°-15°0°15°30° 45° 60° 75° Scl For Car LeoI Sex LeoII UMin Dra Sag UMajI UMajII CVenI PiscII Seg2 Seg3 Com Her Boo LeoV
50 100 150 200 250 300 350 400 450 r GC [ kpc ] LMCSMC
HyIICol1 Sag2Cra2 Dra2Peg3 M . S t r e a m Figure 1.
Aitoff projection of particles associated with the LMC analog subhalo (LMCa), shown just after first pericentric approach, when its pericentricdistance and velocity closely matches that of the Large Magellanic Cloud. The LMCa center is chosen to coincide with the LMC and coordinates are chosenso that the direction of its orbital angular momentum matches that of the LMC. This results in a nearly-polar orbital pane, which roughly aligns with theMagellanic Stream (grey line). Particles of the LMC analog (identified before infall) are colored by their average Galactocentric distance. Red circles indicatethe position of known Milky Way satellites. Filled circles indicate “classical” dwarf spheroidals (i.e., brighter than M V = − ); open circles denote fainterobjects. Newly discovered dwarfs (the subject of this paper) are shown as black starred symbols. large gas content and c ) the requirement that the LMC and SMChave been a long-lived binary (which favors a low-mass Milky Way,or a high-mass LMC, see discussion in Kallivayalil et al. 2013).Therefore in what follows we analyze in detail a first infall scenariobut include a brief discussion about how our conclusions would beaffected if the LMC is in its second pericenter passage (Sec. 3.5).Following S11, we use the Aquarius “A” halo at level 3 resolu-tion, or Aq-A-3 in the notation of Springel et al. (2008), which hasa mass per particle m p = 4 . × M (cid:12) . We identify and followall particles that were associated with the LMCa friends-of-friendsgroup at the time of infall, and evaluate their positions and veloci-ties at the time of first and second pericenter passages.Using SUBFIND (Springel et al. 2001), we have identifiedmore than 200 subhalos associated with LMCa at infall time (seeFig. 1 in S11 for their individual orbits), suggesting that a largesatellite such as the LMC should bring along its own population ofsatellites (Springel et al. 2008). We use for our analysis all parti-cles (and not just the subhalos) initially bound to LMCa in order toprovide a more complete sampling of the positions and velocitiesof any potential companion associated with the LMC.
We transform the coordinate system of the simulation into “Galac-tic coordinates” by requiring that the orientation of the orbital an-gular momentum of LMCa coincides with that measured for theLMC’s orbit, and that its position on the sky coincides with the LMC. For consistency with S11, we use throughout this paper theLMC proper motion as given by K06 . After the rotation, we alsorescale sightly all Galactocentric distances so that LMCa is, at eachpericenter, at the measured distance of the LMC: kpc. We first examine the sky distribution of particles associated at in-fall with the LMC analog subhalo (hereafter “LMCa debris”, forshort). We use this footprint, as well as their radial and tangentialvelocities, to compare with available data for the newly-discovereddwarfs. As mentioned above, we shall interpret coincidence in skyposition, radial velocity and distance between debris particles andobserved dwarfs as evidence of a possible association with theLMC. We note however that the change in the direction of the orbit given by thenew updated measurements from Kallivayalil et al. (2013) is very small: ( j x , j y , j z ) = ( − . , . , − . versus ( − . , . , − . forthe 2006 and 2013 determinations, respectively. These numbers correspondto a unit vector in a Cartesian system aligned with the disk of the galaxy, asdescribed in Sec. 3.c (cid:13) , 000–000 Sales et al.
220 240 260 280 300 320 340Galactic longitude [deg]706560555045403530 G a l a c t i c l a t i t u d e [ d e g ] SMC LMC For
Dra2 Cra2 HyII Sag2 Pic1 Peg3 Ind1 Ret3 Ret2 Eri2 Tuc5 Gru2 Tuc2 Hor2 Hor1 Tuc4 Tuc3 Gru1 Eri3 Phx2
40 48 56 64 72 80 88 96 r GC [ kpc ] Figure 2.
Zoom-in of the area just south of the Clouds outlined by the dashed magenta box in Fig. 1. This area samples the trailing arm of the LMCa tidaldebris, and contains the new dwarfs discovered in the Dark Energy Survey (DES). Col 1 is the only DES dwarf located far away from the stream (not shown).Note also that Ind 1 has now been shown to be a star cluster (Kim et al. 2015). The LMC and SMC are shown as grey squares; red circles are previouslyknown Galactic satellites; new dwarfs are shown by starred symbols. The arrows indicate the expected tangential motion of those satellites, assuming that theywere associated with the Clouds (see Sec. 3.6). Arrows are only shown for systems deemed likely Magellanic satellite candidates in a first pericenter passagescenario (see text for more details).
At the time of the first pericenter, tidal disruption due to the hosthalo has already set in, but most particles are still bound and closeto the subhalo center. The rest of the material is distributed alonga thick but well-defined tidal stream that follows the projection ofthe subhalo’s orbital path on the sky. A leading and trailing armextend towards more positive and negative latitudes, respectively.The distribution of this debris roughly agrees with the position ofthe HI Magellanic Stream, sketched here by a line that traces thehigh-density HI in the sky maps of Nidever et al. (2010).Most of the debris, however, is close to the current positionof the Clouds (grey squares indicate the observed positions of theLMC and SMC). Particles are colour coded in Fig. 1 by their Galac-tocentric distance (see color bar), which shows a clear gradientalong the stream with distances reaching up to kpc, well be-yond the virial radius of the main host.For reference, we indicate the positions of all known MilkyWay satellites in the figure as well. Red filled circles correspondto the “classical” (i.e., brighter than M v = − ) dwarf spheroidal(dSph) companions of the Milky Way; open circles indicate the po-sition of previously known, fainter satellites. We refer the interestedreader to S11 for a discussion of the probability of association withthe LMC of those satellites.The recently-discovered dwarfs that are the focus of this paperare shown using black starred symbols in Fig. 1. We include inthis sample: (i) the dwarfs reported by Koposov et al. (2015) fromyear-1 DES data (see also Bechtol et al. 2015), (ii) the certaindetections from year-2 DES data (Drlica-Wagner et al. 2015), and(iii) additional individual discoveries such as Hydra II (Martin et al.2015, Hy II), Horologium 2 (Kim & Jerjen 2015, Hor 2), Pegasus 3 (Kim et al. 2015, Peg 3), Draco 2 and Sagittarius 2 (Laevens et al.2015, Dra 2 and Sag 2) and Crater 2 (Torrealba et al. 2016, Cra 2).Table 1 lists all the “new dwarfs” considered in what follows (i.e.,black stars in Fig. 1). With the exception of Hy II, Cra 2 and Dra 2,all other dwarfs are in the region of the sky occupied by the trailingarm of the stream.Fig. 1 shows that position on the sky and distance provide ontheir own powerful constraints on a potential Magellanic origin fora dwarf. Those satellites must be close to the orbital plane (tracedby the debris and the Magellanic Stream), ruling out satellites likeSagittarius, Hercules, and Seg 2. In addition, the farther a satelliteis from the LMC the larger, on average, its Galactocentric distanceshould be, a fact that rules out many of the satellites in the Galac-tic northern cap. Indeed, the latter are typically much closer to theGalactic centre than the leading arm of the LMCa debris, whichreaches a distance of ∼ kpc at b = +45 ◦ .Fig. 2 zooms in on the vicinity of the LMC (the region high-lighted by the magenta box in Fig. 1) and shows in more detail theposition of individual dwarfs as well as the distance gradient ex-pected for this section of the stream. This figure also shows thatCol 1 lies outside of the LMCa debris footprint. This, combinedwith its large distance ( ∼ kpc) makes a Magellanic associationrather unlikely (see also Drlica-Wagner et al. 2015). We thereforeexclude Col 1 from the rest of our analysis, together with Sag 2,whose position in the sky is not favorable either. Furthermore, wealso remove Indus 1 from our analysis since it has now been classi-fied as a stellar cluster rather than a dwarf galaxy (Kim et al. 2015).The distance gradients with Galactic latitude shown in Figs. 1and 2 result from the fact that LMCa is close to pericenter and, c (cid:13)000
At the time of the first pericenter, tidal disruption due to the hosthalo has already set in, but most particles are still bound and closeto the subhalo center. The rest of the material is distributed alonga thick but well-defined tidal stream that follows the projection ofthe subhalo’s orbital path on the sky. A leading and trailing armextend towards more positive and negative latitudes, respectively.The distribution of this debris roughly agrees with the position ofthe HI Magellanic Stream, sketched here by a line that traces thehigh-density HI in the sky maps of Nidever et al. (2010).Most of the debris, however, is close to the current positionof the Clouds (grey squares indicate the observed positions of theLMC and SMC). Particles are colour coded in Fig. 1 by their Galac-tocentric distance (see color bar), which shows a clear gradientalong the stream with distances reaching up to kpc, well be-yond the virial radius of the main host.For reference, we indicate the positions of all known MilkyWay satellites in the figure as well. Red filled circles correspondto the “classical” (i.e., brighter than M v = − ) dwarf spheroidal(dSph) companions of the Milky Way; open circles indicate the po-sition of previously known, fainter satellites. We refer the interestedreader to S11 for a discussion of the probability of association withthe LMC of those satellites.The recently-discovered dwarfs that are the focus of this paperare shown using black starred symbols in Fig. 1. We include inthis sample: (i) the dwarfs reported by Koposov et al. (2015) fromyear-1 DES data (see also Bechtol et al. 2015), (ii) the certaindetections from year-2 DES data (Drlica-Wagner et al. 2015), and(iii) additional individual discoveries such as Hydra II (Martin et al.2015, Hy II), Horologium 2 (Kim & Jerjen 2015, Hor 2), Pegasus 3 (Kim et al. 2015, Peg 3), Draco 2 and Sagittarius 2 (Laevens et al.2015, Dra 2 and Sag 2) and Crater 2 (Torrealba et al. 2016, Cra 2).Table 1 lists all the “new dwarfs” considered in what follows (i.e.,black stars in Fig. 1). With the exception of Hy II, Cra 2 and Dra 2,all other dwarfs are in the region of the sky occupied by the trailingarm of the stream.Fig. 1 shows that position on the sky and distance provide ontheir own powerful constraints on a potential Magellanic origin fora dwarf. Those satellites must be close to the orbital plane (tracedby the debris and the Magellanic Stream), ruling out satellites likeSagittarius, Hercules, and Seg 2. In addition, the farther a satelliteis from the LMC the larger, on average, its Galactocentric distanceshould be, a fact that rules out many of the satellites in the Galac-tic northern cap. Indeed, the latter are typically much closer to theGalactic centre than the leading arm of the LMCa debris, whichreaches a distance of ∼ kpc at b = +45 ◦ .Fig. 2 zooms in on the vicinity of the LMC (the region high-lighted by the magenta box in Fig. 1) and shows in more detail theposition of individual dwarfs as well as the distance gradient ex-pected for this section of the stream. This figure also shows thatCol 1 lies outside of the LMCa debris footprint. This, combinedwith its large distance ( ∼ kpc) makes a Magellanic associationrather unlikely (see also Drlica-Wagner et al. 2015). We thereforeexclude Col 1 from the rest of our analysis, together with Sag 2,whose position in the sky is not favorable either. Furthermore, wealso remove Indus 1 from our analysis since it has now been classi-fied as a stellar cluster rather than a dwarf galaxy (Kim et al. 2015).The distance gradients with Galactic latitude shown in Figs. 1and 2 result from the fact that LMCa is close to pericenter and, c (cid:13)000 , 000–000 he satellites of the Magellanic Clouds Figure 3.
Galactocentric distance r GC vs. radial velocity V r for LMCaparticles at first pericenter, color-coded by Galactic latitude b ( − ◦ < b < ◦ ; see color bar on right). For clarity, we only show the Galactic longituderange l = [210 ◦ , ◦ ] , which encompasses most of the LMCa materialin Fig. 1. Note the correlation between latitude and radial velocity, with theleading arm having already passed through pericenter (positive V r ) and thetrailing material still approaching the Galaxy with V r < . As before, theLMC and SMC are shown with grey squares and other previously knowndwarfs in this region of the sky are marked with black squares; new dwarfswith measured kinematics are shown with black starred symbols. The littleoverlap between Fornax, Gru 1 and Tuc 2 and the LMCa debris impliesa low probability of prior association between these dwarfs and the LMC,assuming first infall. Hor 1 is the dwarf most likely to have had a Magellanicassociation. therefore, at roughly the minimum distance of all associated debris.Debris north of the LMC is farther away and moving out (alreadypast pericenter), whereas debris to the south is also farther away butmoving in (has yet to reach pericenter). This induces a correlatedsignature in the radial velocities, which we explore next. We explore the correlation between radial velocity and Galactic lat-itude in Fig. 3. This figure shows the Galactocentric radial velocity V r as a function of distance r GC for LMCa debris in the Galacticlongitude range l = [210 ◦ − ◦ ] , which encloses the stream andthe positions of the DES dwarfs.Particles are colored according to their Galactic latitude, inthe range − ◦ < b < ◦ (see color bar). Fig. 3 shows a clear gra-dient in radial velocity with Galactic latitude, showing generallypositive values (outward moving) for particles north of the posi-tion of the LMC (i.e., b LMC > − . ◦ ) and negative values (in-falling) for those south of that. Although the latitude trend is clear,the dispersion about the mean trend is quite large. This is becausethe velocity dispersion of LMCa before infall was quite substantial(at z id = 0 . , σ = V / √ . km/s), making the tidally-induced stream quite thick. As a result, the constraints on a possibleMagellanic origin provided by b , r GC and V r alone are relativelylax, and serve mainly to rule out the most unlikely candidates.For example, Fig. 3 shows that Fornax (even though it is close to the stream in sky projection) has a distance that is too large tobe associated with the LMC, whereas the SMC, as expected, lieswell within the velocity-distance range spanned by the LMCa de-bris. Starred symbols show the “new dwarfs” that fall in this re-gion of the sky and for which kinematic measurements are avail-able (Walker et al. 2016; Koposov et al. 2015): Hor 1, Ret 2 areclear candidates, whereas Tuc 2 and Gru 1 seem only marginallyconsistent with a Magellanic origin.More stringent constraints may be obtained by combining theresults from Fig. 1 and Fig. 3, since membership to the LMC groupis only likely for systems in narrow regions of the four dimensionalspace drawn by (i) position on the sky ( l, b ) ; (ii) radial velocity V r ,and (iii) Galactocentric distance r GC . We illustrate this in Fig. 4,where we plot the distance and radial velocity of all LMCa particleswhose positions on the sky fall within ◦ of each individual dwarf.The top two panels on the left of Fig. 4 are meant to illustratethe analysis procedure. For the case of the LMC (top left) mostparticles in the LMCa subhalo are, by construction (Sec. 2.2), atthe observed location and radial velocity of the LMC (shown witha blue square). The SMC panel illustrates that most LMCa particlesselected in that direction of the sky ( b = − . ◦ , l = 302 . ◦ ) areat ∼ kpc from the Galactic center and have, on average, a radialvelocity of ∼ km/s, which is in excellent agreement with theobserved SMC values (blue square).The red vertical bands in the panels of Fig. 4 indicate a (gen-erous) uncertainty in the distance estimate to each dwarf; itsintersection with LMCa particles is used to draw the velocity his-tograms in the right-hand side of each panel. Coincidence betweenthe velocity of the blue square and the histogram indicates that theobserved velocity is not unexpected in a scenario where the dwarforiginates from a disrupted LMC group. The velocity histogramsmay therefore be used to “predict” the radial velocity of dwarfs forwhich kinematic data is not yet available, assuming a Magellanicorigin.As may be seen from Fig. 4, and not surprisingly, the SMCpasses these tests handily, making its association with the LMCquite likely. On the other hand, the probability of association of adwarf like Fornax is quite remote. Most debris in that direction ofthe sky are at much closer distances, and the little that overlaps indistance with Fornax (two particles) has a rather high positive radialvelocity, quite unlike that observed. This illustrates the argumentsused by S11 to exclude an LMC association not only for Fornaxbut also for all other Galactic satellites known at that time in caseof first infall.The “new dwarfs” with kinematic data are shown in the bot-tom two rows of Fig. 4. From this comparison we conclude thatDra 2 has little chance of LMC association. Likewise, Ret 2, Tuc 2and Gru 1 have velocities only marginally consistent with a Mag-ellanic relation. Hy II, on the other hand, has the correct radial ve-locity for its distance, despite its large angular separation from theLMC, at the far northern edge region of the leading stream. Theonly clear candidate for Magellanic association is Hor 1, which iswell within the expected velocity-distance range at its location. We can use the procedure described in the previous subsection topredict the radial velocities that the remaining “new dwarfs” would c (cid:13) , 000–000 Sales et al.
Figure 4.
Galactocentric distance vs. radial velocity for LMCa particleswithin ◦ from each observed dwarf (blue squares), as labeled. Particleswith r GC within of the observed distance fall within the red shadedarea, and are used to “predict” the radial velocity expected for LMC asso-ciation (see black velocity histograms on the right of each panel). The topthree panels are meant to illustrate the procedure for well studied systems.The LMC sits at the middle of the distribution by construction . The SMC isa likely LMC satellite; Fornax is not. The bottom two rows show the newlydiscovered dwarfs for which kinematic measurements are available. OnlyHy II and Hor 1 show velocities consistent with those expected for priorassociation with the LMC. have if they were truly Magellanic satellites. We show this in Fig. 5,which lists dwarfs in order of decreasing Galactic latitude. Inspec-tion of individual panels suggests some preliminary conclusions.The Galactocentric distances measured for Pic 1 and Eri 2 seeminconsistent with previous association with the LMC. Cra 2 is inthe same category, given the very little overlap with the edge of theleading arm of the stream. Aside from those three cases, all otherdwarfs show some degree of overlap in the ( l, b ) - r GC plane withthe LMCa debris. For the latter, the black histograms in Fig. 5 showtheir expected radial velocities for a Magellanic origin. We summa-rize these predictions in Table 3, together with uncertainties derivedfrom the interquartile velocity range of the histograms in Fig. 5. The discussion of Figs. 4 and 5 suggests that, for each dwarf, theprobability of prior LMC association scales with the total numberof LMCa particles that match its sky position, distance, and radialvelocity. We emphasize that these are not probabilities in the sta-tistical “likelihood” sense, but nevertheless provide a simple wayto rank order the dwarfs in terms of their potential association withthe LMC and to weed out unrelated systems.We adopt the following procedure, which attempts to quan-tify how likely the observed position (and velocity, when available)of a dwarf is, assuming LMCa membership. To this aim, we firstcompute, for each LMCa particle, the radius of a sphere, r p ,that contains the nearest debris particles, and rank them by thismetric . The smaller r p the more closely associated a particle We have checked that none of our conclusions change when selecting,instead, or particles for this exercise. Figure 5.
Same as Fig. 4 but for the new dwarfs with no measured V r . TheGalactocentric distances for Pic 1, Eri 2 and Tuc 3 seem inconsistent withthe distances measured for the LMCa debris around their positions on thesky. On the other hand, Ret 3, Hor 2 and several of the Tucanas show highchance of association, at least based on their positions and distances alone. is to the stream, which suggests that we may use r p to definea probability of association. In other words, we assign a dwarf a“probability of association” equal to the fraction of LMCa particleswith r p values greater than that computed using the dwarf’s po-sition. Probabilities assigned in this manner are listed in Table 1 forall newly-discovered dwarfs’.For dwarfs with measured radial velocities, we compute a fur-ther probability by comparing its radial velocity with that of thenearest LMCa particles. In practice, we use the mean and dis-persion of those radial velocities to compute the probabilitythat the observed velocity of the dwarf was drawn at random fromthat distribution, assuming Gaussian statistics. The probabilitieslisted in columns 10 and 11 of Table 1 are computed by multiply-ing this value by that estimated using the position alone (columns 8and 9, respectively).The results are shown in Table 1, where we list all “newdwarfs”, as well as their assigned probability, with and without ve-locity information. As discussed before, aside from the SMC, the c (cid:13)000
Same as Fig. 4 but for the new dwarfs with no measured V r . TheGalactocentric distances for Pic 1, Eri 2 and Tuc 3 seem inconsistent withthe distances measured for the LMCa debris around their positions on thesky. On the other hand, Ret 3, Hor 2 and several of the Tucanas show highchance of association, at least based on their positions and distances alone. is to the stream, which suggests that we may use r p to definea probability of association. In other words, we assign a dwarf a“probability of association” equal to the fraction of LMCa particleswith r p values greater than that computed using the dwarf’s po-sition. Probabilities assigned in this manner are listed in Table 1 forall newly-discovered dwarfs’.For dwarfs with measured radial velocities, we compute a fur-ther probability by comparing its radial velocity with that of thenearest LMCa particles. In practice, we use the mean and dis-persion of those radial velocities to compute the probabilitythat the observed velocity of the dwarf was drawn at random fromthat distribution, assuming Gaussian statistics. The probabilitieslisted in columns 10 and 11 of Table 1 are computed by multiply-ing this value by that estimated using the position alone (columns 8and 9, respectively).The results are shown in Table 1, where we list all “newdwarfs”, as well as their assigned probability, with and without ve-locity information. As discussed before, aside from the SMC, the c (cid:13)000 , 000–000 he satellites of the Magellanic Clouds procedure ranks Hor 1 as the best candidate for a true Magellanicsatellite when considering satellites with or without radial veloci-ties. Of systems without kinematic data, Hor 2, Eri 3 and Ret 3 arethe best candidates, but this could certainly change when radial ve-locities become available. An example of the importance of kine-matic information is provided by Ret 2, whose probability dropssubstantially (from ∼ . to . ) when adding its radial velocityto the analysis (assuming first pericenter).We shall hereafter retain as “Magellanic candidates” systemswhose probabilities exceed that obtained for the SMC, withoutvelocity information. The list of candidates is quite short: only systems of the new dwarfs make the cut in the case of position-only information: Hor 1, Hor 2, Eri 3, Ret 3, Tuc 5, Tuc 4 andPhx 2. The above procedure also allows us to explore the sensitivity of ourfindings to our assumption that the LMC is on first approach. Wedo this by performing the same analysis but using the LMCa dataat second pericenter ( t p ), after updating the Galactic coordinatesystem transformation described in Sec. 2.2. The new probabilitiesare also listed in Table 1.Because of the procedure, probabilities are nominally higher,on average, at second pericenter. This is because the LMCa debrisspreads out further in phase space at second pericenter, thus gen-erally boosting the probability values computed for most systems.In general, however, there is a strong correlation between the prob-abilities at both pericenters, so our conclusions seem only weaklydependent on the assumption of first infall.The increase in probability is most notable in the cases ofDra 2 and Hy II, whose probabilities jump from ∼ . and . ina first pericenter passage to . , and . , respectively, when con-sidering the second pericentric passage. The jump in probability iseven more remarkable when including velocities, reaching . inthe case of Dra 2 at second pericenter, more than for the SMC.The reason for this, in the case of Dra 2, is that it sits at thevery far edge of the “trailing arm” of the tidal stream. Although itsdistance and velocity are consistent with an LMC association, atfirst infall there are only a few particles at that sky location and itsprobability is quite low. When the Clouds are in a second passage,several particles accumulate near the apocenter of the LMCa orbit,not far from where Dra 2 is, increasing substantially its probabilityof association. Similarly, Hy II is at the tip of the leading arm ofthe stream, a position that is much more heavily populated after theClouds have completed one full orbit around the Galaxy. Ultimately, the most compelling evidence for LMC association willcome from the proper motions of the new dwarfs. This is becauseall material associated with the LMC before infall is expected to re-tain the direction of its orbital angular momentum. In other words,to first order, Galactic tides are not expected to torque the LMC orits debris away from their original orbital plane.We show this in Fig. 6, where we plot the direction of theorbital angular momentum of LMCa particles at first pericenter.The innermost and outermost isodensity contours enclose , and of all LMCa particles, respectively, and are centered at thelocation of the LMC orbital pole (central grey square). The othergrey square (at b = − ◦ and l = 175 ◦ ) corresponds to the SMC,and is consistent with its assumed association with the LMC.
120 140 160 180 200 220Galactic longitude [deg]60402002040 G a l a c t i c l a t i t u d e [ d e g ] LMC Ret2 Hor1 Phx2 Tuc2 Tuc4 Ret3 Tuc5 Hor2
Figure 6.
Sky coordinates of the orbital poles (i.e., the direction of theorbital angular momentum) of particles associated with LMCa. Contoursshow constant density lines for the distribution of all LMCa. Symbols cor-respond to the orbital poles estimated for new dwarf galaxies deemed likelycandidate Magellanic satellites at first pericenter, as labeled. These esti-mates are based on the particles in the stream that are close in the sky andthat lie at distances within of the measured values (see shaded red re-gions in Fig. 4 and 5). The common infall scenario preserves the coherenceof the orbital plane, resulting in a tight distribution of the orbital poles in awell-defined region of the sky.
We also show with starred symbols in Fig. 6 the orbital angu-lar momentum direction predicted for each of the “candidate Mag-ellanic satellites” (i.e., those that exceed probability comparedto the SMC) at first pericenter assuming that they were associatedwith the LMC. This is computed as the median l and b of the orbitalpoles of all LMCa particles with matching sky position and Galac-tocentric distance (i.e., the particles that fall into the red shadedregions of each panel in Figs. 4 and 5).We list in Table 2, for each Magellanic candidate, the coordi-nates of the predicted orbital angular momentum unit vector, in aCartesian system where the Z -axis is perpendicular to the Galacticdisk, the X -axis points away from the sun and the Y -axis is definedsuch that we get a right-handed system. Uncertainties correspond tothe r.m.s. values from the individual LMCa particles used for eachdwarf.Assuming a radial velocity (for those without a measurement),this is equivalent to predicting the tangential motion of each dwarf,which we also list in Table 3. The predicted projected velocitiesare shown with arrows in Fig. 2. Table 2 may therefore be used toevaluate the hypothesis of prior LMC association for these dwarfsonce proper motions for these objects become available. We have used a Λ CDM cosmological N-body simulation of theformation of a Milky Way-sized halo to investigate which of the c (cid:13) , 000–000 Sales et al. newly-discovered Galactic satellites in the DES, Pan-STARRS,SMASH and ATLAS surveys might have been associated with theMagellanic Clouds before infall. Our study extends that of Saleset al. (2011), which used a massive subhalo with orbital parametersthat closely match those of the LMC (an LMC analog: LMCa) andtracked the position and velocity of its constituent particles at firstand second pericentric passages. This enables the probability ofLMC association to be assessed by checking whether individualdwarfs lie in a region of phase space populated by debris from thedisrupting LMC subhalo.On the basis of that analysis, Sales et al. (2011) concludedthat, except for the SMC, none of the other Galactic satellitesknown at the time had positions and velocities consistent with aMagellanic origin. That was the first study to investigate a possibleMagellanic association by using all of the available phase-spaceinformation in a fully cosmological context. Extending this studyto the newly-discovered dwarfs yields the following conclusions,assuming that the LMC is at first pericentric passage. • We eliminate four systems from our analysis. Sag 2, Eri 2, andCol 1 lie too far outside the LMCa footprint for their associationwith the LMC to be plausible. In addition, Ind 1 has recently beenreclassified as a star cluster. • For the rest of the systems, a quantitative “probability” of as-sociation has been computed using the positions and velocities ofthe LMCa particles closest to each dwarf. We deemed likely can-didate Magellanic satellites dwarfs whose probability exceeds halfthe value assigned to the SMC. • Of the six systems with available distances and radial veloci-ties, only Hor 1 is clearly consistent with a Magellanic origin. Ret 2,Tuc 2, and Gru 1 have radial velocities which are only marginallyconsistent with LMC association. Dra 2 is too far off the LMCafirst-pericenter footprint. Hy II has the right distance and radial ve-locity, but its probability is small, given its position at the thinly-populated, very far end of the LMCa leading tidal arm. • Of the remaining systems with only sky positions and dis-tances, our analysis retains of them at higher than the prob-ability of the SMC (Hor 2, Eri 3, Ret 3, Tuc 5, Tuc 4, and Phx 2).For these candidates, the nearest LMCa particles are used to predict their radial velocities, assuming a Magellanic origin. • Aside from radial velocities, the most telling evidence of apotential LMC association would be provided by proper motions.These constrain the direction of the orbital angular momentum ofeach dwarf, which must roughly coincide with that of the LMC. Weuse this result to predict proper motions for all newly-discoveredsatellites, again assuming a Magellanic origin. The radial and tan-gential velocity predictions could be used to reassess the hypothesisof a possible Magellanic association once kinematic data becomeavailable.Our conclusions are insensitive to our choice of first or secondpericenter for the LMC, in the sense that the association probabili-ties of most dwarfs computed at each time show strong correlation.Because the LMCa debris spreads out to cover a larger volume inphase-space at second pericenter, the probabilities of four extra sys-tems, computed using positions alone, are lifted above that ofthe SMC: Tuc 2, Dra 2, Cra 2, and Peg 3. Of these, Tuc 2 seemsquite unlikely given its radial velocity. Dra 2, on the other hand,has position and velocity consistent with being at the far end of thetrailing stream during a second pericenter.Our main conclusion is therefore that few of the newly dis-covered dwarfs are definitely associated with the LMC. This is notentirely unexpected. The simple scaling argument of Sales et al. (2013) suggests that the fraction of all Galactic satellites associatedwith the Clouds should be close to the ratio of the stellar mass ofthe LMC and the Milky Way, i.e., ∼ . Given that we now haveidentified a total ∼ dwarfs within kpc from the Galacticcenter (excluding the LMC/SMC pair), only to should, in prin-ciple, be associated with the Clouds. So far our analysis seems con-sistent with this expectation. Accurate radial velocities and propermotions are needed to accept/reject the hypothesis of associationbetween these dwarfs and the LMC. Confirming the existence ofmultiple Magellanic satellites would provide a wonderful confir-mation of the hierarchical nature of galaxy formation predicted bythe current cosmological paradigm. NK is supported by the NSF CAREER award 1455260. This re-search was supported in part by the National Science Foundationunder Grant No. NSF PHY11-25915 and by the hospitality of theKavli Institute for Theoretical Physics at the University of Califor-nia, Santa Barbara.
REFERENCES
Bechtol K., Drlica-Wagner A., Balbinot E., Pieres A., Simon J. D.,Yanny B., et al. 2015, ApJ, 807, 50Besla G., Kallivayalil N., Hernquist L., Robertson B., Cox T. J.,van der Marel R. P., Alcock C., 2007, ApJ, 668, 949Boylan-Kolchin M., Springel V., White S. D. M., Jenkins A.,Lemson G., 2009, MNRAS, 398, 1150Deason A. J., Wetzel A. R., Garrison-Kimmel S., Belokurov V.,2015, MNRAS, 453, 3568D’Onghia E., Fox A. J., 2015, ArXiv e-prints 1511.05853D’Onghia E., Lake G., 2008, ApJL, 686, L61Drlica-Wagner A., Bechtol K., Rykoff E. S., Luque E., QueirozA., et al. 2015, ApJ, 813, 109Garrison-Kimmel S., Boylan-Kolchin M., Bullock J. S., Lee K.,2014, MNRAS, 438, 2578Jethwa P., Erkal D., Belokurov V., 2016, ArXiv e-prints1603.04420Kallivayalil N., van der Marel R. P., Alcock C., Axelrod T., CookK. H., Drake A. J., Geha M., 2006, ApJ, 638, 772Kallivayalil N., van der Marel R. P., Besla G., Anderson J., AlcockC., 2013, ApJ, 764, 161Kennedy R., Frenk C., Cole S., Benson A., 2014, MNRAS, 442,2487Kim D., Jerjen H., 2015, ApJL, 808, L39Kim D., Jerjen H., Mackey D., Da Costa G. S., Milone A. P., 2015,ApJL, 804, L44Kim D., Jerjen H., Milone A. P., Mackey D., Da Costa G. S., 2015,ApJ, 803, 63Kirby E. N., Simon J. D., Cohen J. G., 2015, ApJ, 810, 56Koposov S. E., Belokurov V., Torrealba G., Evans N. W., 2015,ApJ, 805, 130Koposov S. E., Casey A. R., Belokurov V., et al. 2015, ApJ, 811,62 c (cid:13) , 000–000 he satellites of the Magellanic Clouds Table 1.
Parameters of the newly-discovered dwarfs considered in this paper, together with their probability of association with the LMC in either first orsecond pericenter passage, as defined in Sec. 3.4. Cols. 8 and 9 list probabilities computed using positions alone; cols. 10 and 11 also include also radialvelocity data. We list the V -band absolute magnitude, stellar mass, galactocentric coordinates ( l, b ) , measured heliocentric velocity V (cid:12) and heliocentricdistance D (cid:12) of each satellite, taken from the following references: [1] Koposov et al. 2015, [2] Drlica-Wagner et al. 2015, [3] Martin et al. 2015, [4] Laevenset al. 2015, [5] Kim & Jerjen 2015, [6] Kim et al. 2015, [7] Torrealba et al. 2016, [8] McConnachie 2012, [9] Simon et al. 2015, [10] Koposov et al. 2015,[11] Kirby et al. 2015, [12] Walker et al. 2016,[13] Martin et al. 2016). For cases without estimates of M ∗ we derive it from their listed V-band magnitudesassuming a mass-to-light ratio γ = 2 in solar units. Dwarfs are grouped according to their probability of association with the Clouds at first pericenter, usingonly their distance and position on the sky (col. 8). The main two groups include “likely candidates” and “unlikely candidates”, according to whether theirprobabilities are above or below the probability assigned to the SMC. The final group lists those that were discarded from the analysis, either becausetheir position on the sky is such that the probability of LMC association is remote, or because they are considered star clusters, and not dwarf galaxies.Name M V M ∗ l b V (cid:12) D (cid:12) Prob st per Prob nd per Prob st per Prob nd per Refs.[mag] [10 M (cid:12) ] [deg] [deg] [km/s] [kpc] ( l , b , r ) ( l , b , r ) ( l , b , r , V r ) ( l , b , r , V r )LMC − . . × . − . . .
63 0 .
71 0 .
52 0 . [8]SMC − . . × . − . . .
28 0 .
65 0 .
21 0 . [8]Hor 1 − . .
96 270 . − . . ± . .
30 0 .
66 0 .
17 0 . [1],[10]Hor 2 − . .
47 262 . − . .
27 0 . [5]Eri 3 − . .
54 274 . − . .
25 0 . [1]Ret 3 − . . . − . .
25 0 . [2]Tuc 5 − . . . − . .
15 0 . [2]Tuc 4 − . . . − . .
15 0 . [2]Phx 2 − . .
13 323 . − . .
15 0 . [1]Tuc 2 − . . . − . − . ± . .
11 0 .
47 0 .
06 0 . [1],[12]Gru 2 − . . . − . .
09 0 . [2]Ret 2 − . . . − . . ± . .
09 0 .
10 0 .
01 0 . [1],[9],[10]Tuc 3 − . . . − . .
08 0 . [2]Gru 1 − . .
96 338 . − . − . ± . .
06 0 . < .
01 0 . [1],[12]Pic 1 − . . . − . .
06 0 . [1]Peg 3 − . .
46 69 . − . .
05 0 . [6]Cra 2 − . .
25 283 . . .
02 0 . [7]Dra 2 − . .
47 98 . . − . ± . .
01 0 .
50 0 0 . [4],[13]Hy II − . . . . . ± . .
01 0 .
16 0 .
01 0 . [3],[11]Eri 2** − . . . . . [1]Sag 2** − . .
47 189 . − . – – [4]Ind 1** − . . . − . – – [1]Col 1** − . . . − . – – [2] Laevens B. P. M., Martin N. F., Bernard E. J., et al. 2015, ApJ,813, 44Lynden-Bell D., Lynden-Bell R. M., 1995, MNRAS, 275, 429Martin N. F., Geha M., Ibata R. A., Collins M. L. M., LaevensB. P. M., Bell E. F., Rix H.-W., Ferguson A. M. N., ChambersK. C., Wainscoat R. J., Waters C., 2016, MNRAS, 458, L59Martin N. F., Nidever D. L., Besla G., Olsen K., et al. 2015, ApJL,804, L5McConnachie A. W., 2012, AJ, 144, 4Nidever D. L., Majewski S. R., Butler Burton W., Nigra L., 2010,ApJ, 723, 1618Sales L. V., Navarro J. F., Cooper A. P., White S. D. M., FrenkC. S., Helmi A., 2011, MNRAS, 418, 648Sales L. V., Wang W., White S. D. M., Navarro J. F., 2013, MN-RAS, 428, 573Shattow G., Loeb A., 2009, MNRAS, 392, L21Simon J. D., Drlica-Wagner A., Li T. S., Nord B., Geha M., Bech-tol K., et al. 2015, ApJ, 808, 95Springel V., Wang J., Vogelsberger M., Ludlow A., Jenkins A.,Helmi A., Navarro J. F., Frenk C. S., White S. D. M., 2008, MN-RAS, 391, 1685Springel V., White S. D. M., Frenk C. S., Navarro J. F., Jenkins A.,Vogelsberger M., Wang J., Ludlow A., Helmi A., 2008, Nature,456, 73 Springel V., Yoshida N., White S. D. M., 2001, New Astronomy,6, 79Torrealba G., Koposov S. E., Belokurov V., Irwin M., 2016, ArXive-prints 1601.07178Tully R. B., Rizzi L., Dolphin A. E., Karachentsev I. D.,Karachentseva V. E., Makarov D. I., Makarova L., Sakai S.,Shaya E. J., 2006, AJ, 132, 729Walker M. G., Mateo M., Olszewski E. W., Koposov S., Be-lokurov V., Jethwa P., Nidever D. L., Bonnivard V., Bailey IIIJ. I., Bell E. F., Loebman S. R., 2016, ApJ, 819, 53Wetzel A. R., Deason A. J., Garrison-Kimmel S., 2015, ApJ, 807,49Wheeler C., O˜norbe J., Bullock J. S., Boylan-Kolchin M., ElbertO. D., Garrison-Kimmel S., Hopkins P. F., Kereˇs D., 2015, MN-RAS, 453, 1305Yozin C., Bekki K., 2015, MNRAS, 453, 2302 c (cid:13) , 000–000 Sales et al.
Table 2.
Cartesian components of the direction (average) of the angular momentum of the LMCa particles near each Magellanic candidate dwarf, accordingto the discussion of Sec. 3.4. All vectors are normalized to have modulus unity. For each dwarf, we list the results for the first (top row) and/or second (bottomrow) pericenter passage. The bottom group includes dwarfs that are only likely Magellanic candidates at second pericenter. Because the LMC is in a nearlypolar orbit, the angular momentum of all material associated with it points in all cases in the − X direction (i.e., to the Sun from the Galactic center).Name time j X j Y j Z LMC t p − . ± .
03 0 . ± . − . ± . t p − . ± .
03 0 . ± . − . ± . observed − . ± .
01 0 . ± . − . ± . SMC t p − . ± .
05 0 . ± . − . ± . t p − . ± .
05 0 . ± . − . ± . observed − . ± .
05 0 . ± . − . ± . Hor 1 t p − . ± .
05 0 . ± . − . ± . t p − . ± .
19 0 . ± . − . ± . Hor 2 t p − . ± .
02 0 . ± . − . ± . t p − . ± . − . ± .
46 0 . ± . Eri 3 t p − . ± .
07 0 . ± . − . ± . t p − . ± .
20 0 . ± . − . ± . Ret 3 t p − . ± .
02 0 . ± . − . ± . t p − . ± .
19 0 . ± . − . ± . Tuc 5 t p − . ± .
03 0 . ± . − . ± . t p − . ± .
04 0 . ± . − . ± . Tuc 4 t p − . ± . − . ± . − . ± . t p − . ± .
03 0 . ± . − . ± . Phx 2 t p − . ± .
02 0 . ± . − . ± . t p − . ± .
02 0 . ± . − . ± . Tuc 2 t p − . ± .
03 0 . ± . − . ± . Peg 3 t p − . ± .
02 0 . ± . − . ± . Cra 2 t p − . ± .
08 0 . ± .
17 0 . ± . Dra 2 t p − . ± . − . ± .
19 0 . ± . Table 3.
Predicted Galactocentric radial and tangential velocity for Magellanic candidate dwarfs under the assumption of association with the Clouds. We showthe median and - percentiles in the case of first (columns 2-4) and second (columns 5-7) pericenter passage. The last column shows the galactocentricradial velocity for the 6 dwarfs with measured kinematics. The bottom group includes dwarfs that are only likely Magellanic candidates at second pericenter.Name V pred r V pred l V pred b V pred r V pred l V pred b V obs r [km/s] 1st per. 1st per. 2nd per. 2nd per. 2nd per. [km/s]Hor 1 − − − − − − − . Hor 2 − − − − − − Eri 3 − − − − − − − Ret 3 − − − − − − − Tuc 5 − − − − − − − − − − Tuc 4 − − − − − − − − − − Phx 2 − − − − − − − − − − Tuc 2 – – – − − − − − − . Peg 3 – – – − − − − − − Cra 2 – – – − − − − Dra 2 – – – − − − − − − . c (cid:13)000