Astronomy & Astrophysics manuscript no. spec_v1_ref1_arx c ESO 2019 February 14, 2019 A 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, E-38205 La Laguna, Tenerife, Spain e-mail: [email protected] 2 Universidad de La Laguna, Dpto. Astrofisica, E-38206 La Laguna, Tenerife, Spain 3 INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy ABSTRACT A wealth of tiny galactic systems populates the surroundings of the Milky Way. However, some of these objects might actually have their origin as former satellites of the Magellanic Clouds, in particular of the 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 dispersion and metallicity properties in four recently discovered distant southern dwarf galaxy candidates, Columba I, Reticulum III, Phoenix II and Horologium II. We combine Gaia DR2 astrometric measurements, photometry and new FLAMES/GIRAFFE intermediate resolution spectroscopic data in the region of the near-IR Ca II triplet lines; such 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 determine (or place limits on) the systems bulk motions and average internal properties. The systems are found to be very metal-poor, in agreement with dwarf galaxies and dwarf galaxy candidates of similar luminosity. Among the four objects, the only one that we can place firmly in the category of dwarf galaxies is +6.8 Phoenix II given its resolved large velocity dispersion (9.5−4.4 km/s) and intrinsic metallicity spread (0.33 dex). Also for Columba I we 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 are presently very large, so that an association cannot be excluded, apart from Columba I. Using the numbers of potential former satellites of the LMC identified here and in the literature, we +1.3 11 obtain for the LMC a dark matter mass of M200 = 1.9−0.9 × 10 M⊙. Key words. Astrometry - Proper motions - Galaxies: dwarf - Galaxies: kinematics and dynamics - Local Group - Galaxies: evolution 1. Introduction 2018a; Kim & Jerjen 2015; Kim et al. 2015; Koposovet al. 2018). In the ΛCold Dark Matter (ΛCDM) framework, not only large galaxies, but also low mass halos are expected to host their own This has of course raised the question of whether some of systems of satellite sub-halos. How many of these will contain a these systems might be, or were before infall, part of a satel- luminous component depends on several variables, among which lite system of the Clouds rather than of the Milky Way (MW). the mass of the host halo, the mass and build-up history of the Tackling how many, and which ones, of these dwarf satellites sub-halos themselves and various environmental factors, includ- might have been brought in by the Clouds would give insights ing the strength of the UV-ionizing background (see e.g. the re- into several aspects of galaxy formation in a cosmological con- view by Bullock & Boylan-Kolchin 2017 and references there- text: besides improving the current observational information on in). the properties of satellite systems around galaxies of lower mass Observationally, there have already been several detections than the MW, it would allow to make further considerations into ffi of low-luminosity galaxies possibly physically associated to the e ciency of galaxy formation at the low-mass end (see e.g. stellar systems of similar or lower stellar mass than the LMC Dooley et al. 2017), and might imply a revision of our under- (e.g. Antlia A and the recently discovered Antlia B around standing of the properties of the MW satellite system itself, in terms of the number of its members, as well as its luminosity arXiv:1805.07350v4 [astro-ph.GA] 13 Feb 2019 NGC 3109, Sand et al. 2015; Scl-MM-Dw1 around NGC 253, which ones Sand et al. 2014); in some cases, the "status" of satellite galaxy and circular velocity function. Identifying of these / is guaranteed by the on-going tidal disruption of such sys- dwarf galaxies in particular might be have been associated to the ff tems (e.g. Rich et al. 2012; Amorisco et al. 2014; Annibali et al. Clouds gives also a direct avenue to start addressing the e ects 2016; Toloba et al. 2016). This could in principle be interpreted of group pre-processing onto the observed properties of dwarf as a qualitative confirmation of one of the predictions of the galaxies from the detailed perspective of resolved stellar popu- ΛCDM hierarchical formation framework. lation studies. Recently, about two dozens low-luminosity, candidate dwarf There have been predictions on the number of satellites that galaxies were discovered at projected locations in the sky could be associated to systems with stellar masses similar to the close to the Magellanic Clouds (Drlica-Wagner et al. 2015a; Magellanic Clouds, and in what stellar mass range they should Koposov et al. 2015a; Martin et al. 2015; Bechtol et al. 2015; be found (see e.g. Dooley et al. 2017). A conclusion from the Laevens et al. 2015; Drlica-Wagner et al. 2015b; Torrealba et al. aforementioned study is that there is a dearth of “massive” satel- lites around the LMC and SMC; this could imply a Magellanic- ⋆ Based on ESO programs 096.B-0785(A) and 098.B-0419(A). Clouds “missing satellite problem”, although other solutions are Article number, page 1 of 16 A&A proofs: manuscript no. spec_v1_ref1_arx possible, such as strong modifications to abundance-matching With the second Gaia mission (Gaia Collaboration et al. relations (which at the low mass end are very uncertain, see e.g. 2016a) data release (GDR2; Gaia Collaboration et al. 2018a), Garrison-Kimmel et al. 2017; Revaz & Jablonka 2018, and ref- the situation has dramatically improved: not only have the ac- erences therein), strong tidal-stripping etc. curacy of the systemic proper motions of the classical MW 1 Several studies have instead focused on predicting which dwarf spheroidal galaxies (dSphs) been significantly improved ones of the dwarf galaxies found in the surroundings of the in several cases (Gaia Collaboration et al. 2018b), but such de- Milky Way could have been brought in by the Magellanic sys- termination has finally become possible for dozens of the ultra- tem (Sales et al. 2011; Deason et al. 2015; Yozin & Bekki 2015; faint dwarf galaxies (Fritz et al. 2018a; Kallivayalil et al. 2018; Jethwa et al. 2016; Sales et al. 2017). Massari & Helmi 2018; Simon 2018), while before only Segue 1 had a systemic proper motion measurement (Fritz et al. 2018b). Among these, Deason et al. (2015) used the ELVIS N-body All of the above, being in a common, absolute reference system. simulations to identify LMC-mass sub-haloes of MW/M31-like 11 Kallivayalil et al. (2018, hereafter K18) used the Sales et al. systems (considering virial masses in the range 1-3×10 M⊙) and showed that the system of satellites disperse rapidly in (2011, 2017) LMC-analog to test a possible association to the phase-space, unless the group has infallen recently. The sample LMC for 32 UFDs with MV & −8. For the systems for which of 25 LMC-analogs included three dynamical analogs (with sim- 3D velocities could be obtained, given the additional availabil- ilar radial and tangential velocity as observed for the LMC): the ity of published spectroscopic data, they conclude that four of expectations in these cases are that the sub-halos found at z = 0 those (Hor I, Car II, Car III and Hyd I) were former satellites of within ∼50kpc, or with a 3D velocity differing less ∼50km/s, the Clouds, while Hyd II and Dra II could be reconciled with a from the original host have more than 50% chance to have been model allowing for a larger dispersion of the tidal debris proper- part of a LMC-mass group. Nonetheless, for the whole sam- ties in velocity and distance/sky location. ple considered together, systems within 50kpc and 50km/s of For the systems that were lacking either systemic proper mo- a LMC-mass dwarf at z = 0 would have >90% probability of tion and/or radial velocity measurements at the times of the stud- having been former group members. ies, predictions are provided in several of the works cited above, under the assumption of a prior physical association to the Mag- Salesetal. (2017) used the LMC-analog identified in ellanic system. In particular, Jethwa et al. (2016) and K18 pro- Sales et al. (2011) in the Aquarius simulations, which has a peri- vide such predictions in the observable space of proper motion center and velocity in good agreement with the measurements, and/or radial velocity measurements, which has the advantage of to test whether any of the 20 dwarfs known at the time in the not carrying the error propagation in the conversion to Galacto- vicinity of the LMC/SMC are/were associated to the Clouds. centric velocities.