A&A 623, A129 (2019) Astronomy https://doi.org/10.1051/0004-6361/201833458 & c ESO 2019 Astrophysics Gaia DR 2 and VLT/FLAMES search for new satellites of the LMC?,?? T. K. Fritz1,2, R. Carrera3, G. Battaglia1,2, and S. Taibi1,2 1 Instituto de Astrofisica de Canarias, Calle Via Lactea s/n, 38205 La Laguna, Tenerife, Spain e-mail: [email protected] 2 Universidad de La Laguna, Dpto. Astrofisica, 38206 La Laguna, Tenerife, Spain 3 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy Received 20 May 2018 / Accepted 13 February 2019 ABSTRACT A wealth of tiny galactic systems populates the surroundings of the Milky Way. However, some of these objects might have origi- nated as former satellites of the Magellanic Clouds, in particular of the Large Magellanic Cloud (LMC). Examples of the importance of understanding how many systems are genuine satellites of the Milky Way or the LMC are the implications that the number and luminosity-mass function of satellites around hosts of different mass have for dark matter theories and the treatment of baryonic physics in simulations of structure formation. Here we aim at deriving the bulk motions and estimates of the internal velocity dis- persion and metallicity properties in four recently discovered distant southern dwarf galaxy candidates, Columba I, Reticulum III, Phoenix II, and Horologium II. We combined Gaia DR2 astrometric measurements, photometry, and new FLAMES/GIRAFFE intermediate-resolution spectroscopic data in the region of the near-IR Ca II triplet lines; this combination is essential for finding potential member stars in these low-luminosity systems. We find very likely member stars in all four satellites and are able to deter- mine (or place limits on) the bulk motions and average internal properties of the systems. The systems are found to be very metal poor, in agreement with dwarf galaxies and dwarf galaxy candidates of similar luminosity. Of these four objects, we can only firmly place +6:8 −1 Phoenix II in the category of dwarf galaxies because of its resolved high velocity dispersion (9:5−4:4 km s ) and intrinsic metallicity spread (0.33 dex). For Columba I we also measure a clear metallicity spread. The orbital pole of Phoenix II is well constrained and close to that of the LMC, suggesting a prior association. The uncertainty on the orbital poles of the other systems is currently very large, so that an association cannot be excluded, except for Columba I. Using the numbers of potential former satellites of the LMC +1:3 11 identified here and in the literature, we obtain for the LMC a dark matter mass of M200 = 1:9−0:9 × 10 M . Key words. proper motions – stars: abundances – galaxies: dwarf – galaxies: kinematics and dynamics 1. Introduction This might be interpreted as a qualitative confirmation of one of the predictions of the ΛCDM hierarchical formation framework. In the Λ cold dark matter (ΛCDM) framework, not only large Recently, about two dozen low-luminosity candidate dwarf galaxies, but also low-mass halos are expected to host their own galaxies were discovered at projected locations in the sky close systems of satellite sub-halos. How many of these will con- to the Magellanic Clouds (Drlica-Wagner et al. 2015; Koposov tain a luminous component depends on several variables, among et al. 2015a, 2018; Martin et al. 2015; Bechtol et al. 2015; which the mass of the host halo, the mass and build-up history Laevens et al. 2015; Torrealba et al. 2018a; Kim & Jerjen 2015; of the sub-halos themselves, and various environmental factors, Kim et al. 2015). This has of course raised the question whether including the strength of the UV-ionizing background (see, e.g., some of these systems might be, or were before infall, part of the review by Bullock & Boylan-Kolchin 2017 and references a satellite system of the Clouds rather than of the Milky Way therein). (MW). Determining how many and which of these dwarf satel- Several low-luminosity galaxies have been detected that lites might have been brought in by the Clouds would give might be physically associated with stellar systems with masses insights into several aspects of galaxy formation in a cosmolog- similar to or lower than that of the LMC (e.g., Antlia A and ical context: in addition to improving the current observational the recently discovered Antlia B around NGC 3109, Sand et al. information on the properties of satellite systems around galax- 2015; Scl-MM-Dw1 around NGC 253, Sand et al. 2014); in ies of lower mass than the MW, it would allow us to further some cases, the “status” of satellite galaxy is guaranteed by the consider the efficiency of galaxy formation at the low-mass end ongoing tidal disruption of such systems (e.g., Rich et al. 2012; (see, e.g., Dooley et al. 2017), and might imply a revision of our Amorisco et al. 2014; Annibali et al. 2016; Toloba et al. 2016). understanding of the properties of the MW satellite system itself in terms of the number of its members as well as its luminosity ? Full Table 2 is only available at the CDS via anonymous ftp and circular velocity function. Identifying which of these dwarf to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. galaxies in particular might be or have been associated with the u-strasbg.fr/viz-bin/qcat?J/A+A/623/A129 Clouds also provides a direct way to start addressing the effects ?? Based on ESO programs 096.B-0785(A) and 098.B-0419(A). of group preprocessing onto the observed properties of dwarf Article published by EDP Sciences A129, page 1 of 15 A&A 623, A129 (2019) galaxies from the specific perspective of resolved stellar popula- An important general conclusion from these works is that tion studies. knowledge of the systemic radial velocities, combined with sky The number of satellites that could be associated with sys- position and distance, can greatly aid in the identification of pre- tems with stellar masses similar to the Magellanic Clouds has vious Magellanic Cloud satellites. In particular, the most com- been predicted, and in which stellar mass range they should be pelling evidence for association is expected to be provided by found (see, e.g., Dooley et al. 2017). Dooley and collaborators the additional information afforded by knowledge of systemic concluded that there is a dearth of “massive” satellites around proper motions because the accreted galaxies are expected to the Large and Small Magellanic Clouds (LMC and SMC); this share a similar direction of the orbital angular momentum of the could imply a Magellanic Cloud “missing satellite problem”, LMC. although other solutions are possible, such as strong modifica- With the second Gaia mission (Gaia Collaboration 2016) tions to abundance-matching relations (which at the low-mass data release (GDR2; Gaia Collaboration 2018a), the situation end are very uncertain, see, e.g., Garrison-Kimmel et al. 2017; has dramatically improved: not only has the accuracy of the Revaz & Jablonka 2018, and references therein) and strong tidal systemic proper motions of the classical MW dwarf spheroidal stripping. galaxies (dSphs)1 been significantly improved in several cases Several studies have instead focused on predicting which (Gaia Collaboration 2018b), but such determination has finally of the dwarf galaxies found in the surroundings of the MW become possible for dozens of the ultra-faint dwarf galaxies could have been brought in by the Magellanic system (Sales (UFDs; Fritz et al. 2018a; Kallivayalil et al. 2018; Massari & et al. 2011, 2017; Deason et al. 2015; Yozin & Bekki 2015; Helmi 2018; Simon 2018), while before only Segue 1 had a sys- Jethwa et al. 2016). Of these, Deason et al.(2015) used the temic proper motion measurement (Fritz et al. 2018b). All of the ELVIS N-body simulations to identify LMC-mass sub-haloes of above is in a common, absolute reference system. MW/M31-like systems (considering virial masses in the range Kallivayalil et al.(2018, hereafter K18) used the Sales et al. 11 1–3 × 10 M ) and showed that the system of satellites rapidly (2011, 2017) LMC analog to test a possible association with the disperses in phase-space unless the group has infallen recently. LMC for 32 UFDs with MV & −8. For the systems for which The sample of 25 LMC analogs included three dynamical 3D velocities could be obtained, given the additional availability analogs (with similar radial and tangential velocity as observed of published spectroscopic data, they concluded that four (Hor I, for the LMC): the expectations in these cases are that the sub- Car II, Car III, and Hyd I) were former satellites of the Clouds, halos found at z = 0 within ∼50 kpc, or with a 3D velocity dif- while Hyd II and Dra II could be reconciled with a model allow- fering by less ∼50 km s−1, from the original host have a chance ing for a larger dispersion of the tidal debris properties in veloc- higher than 50% to have been part of an LMC-mass group. ity and distance or sky location. Nonetheless, for the whole sample considered together, systems For the systems that lacked either systemic proper motion within 50 kpc and 50 km s−1 of an LMC-mass dwarf at z = 0 and/or radial velocity measurements at the times of the stud- would have a >90% probability of having been former group ies, predictions are provided in several of the works cited above members. under the assumption of a prior physical association to the Mag- Sales et al.(2017) used the LMC analog identified in Sales ellanic system.