Astronomy & Astrophysics manuscript no. eGDR3_ppm_arxiv ©ESO 2021 June 17, 2021 Gaia early DR3 systemic motions of Local Group dwarf galaxies and orbital properties with a massive Large Magellanic Cloud. G. Battaglia1; 2,e-mail: [email protected], S. Taibi1; 2, G. F. Thomas1; 2, and T. K. Fritz1; 2 1 Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38206 La Laguna, Tenerife, Spain 2 Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, E-38205 La Laguna, Tenerife, Spain Received: ; accepted: ABSTRACT Aims. We perform a comprehensive determination of the systemic proper motions of 74 dwarf galaxies and dwarf galaxy candidates in the Local Group based on Gaia early data release 3. The outputs of the analysis for each galaxy, including probabilities of membership, will be made publicly available. The analysis is augmented by a determination of the orbital properties of galaxies within 500 kpc. Methods. We adopt the flexible Bayesian methodology presented by McConnachie & Venn (2020), which takes into account the location of the stars on the sky, on the colour-magnitude diagram and on the proper motion plane. We apply some modifications, in particular to the way the colour-magnitude diagram and spectroscopic information are factored in, e.g. by including stars in several evolution phases. The bulk motions are integrated in three gravitational potentials: two where the Milky Way is treated in isolation 12 and has a mass 0.9 & 1.6 ×10 M and the time-varying potential by Vasiliev et al. (2021), which includes the infall of a massive Large Magellanic Cloud (LMC). Results. We are able to determine bulk proper motions for 73 systems, and we consider reliable 66 of these measurements. For the first time, systemic motions are presented for galaxies out to a distance of 1.4 Mpc, in the NGC 3109 association. The inclusion of the infall of a massive LMC significantly modifies the orbital trajectories of the objects, with respect to orbit integration in static Milky Way-only potentials, and leads to 6 galaxies being likely associated to the LMC, 3 possibly associated and 1 recently captured object. We discuss the results of the orbit integration in the context of the relation of the galaxies to the system of Milky Way satellites, implications for the too-big-to-fail problem, impact on star formation histories, and tidal disruption. Key words. Methods: statistical – Astrometry – Galaxies: dwarf – Galaxies: evolution – Galaxies: kinematics and dynamics – Local Group 1. Introduction (e.g. Mayer et al. 2006; Muñoz et al. 2008; Kazantzidis et al. 2011; Battaglia et al. 2015; Hausammann et al. 2019; Iorio et al. Knowledge of the bulk motions of galaxies residing in the Local 2019; Miyoshi & Chiba 2020; Ruiz-Lara et al. 2021; Rusakov Group (LG) is a precious resource for a wealth of galaxy evolu- et al. 2021; Di Cintio et al. 2021; Genina et al. 2020, and refer- tion and near-field cosmology investigations, e.g. inferences of ences therein). the mass, barycenter position and velocity of the LG (e.g. Kahn & Woltjer 1959; Peebles et al. 2001; Li & White 2008; van der Before the second data release of the Gaia mission (GDR2) Marel et al. 2012b; González et al. 2014; Peñarrubia et al. 2014, (Gaia Collaboration et al. 2016, 2018a), measurements of the to mention a few), studies of the possible history of past interac- systemic proper motions (PMs) of galaxies in the LG were es- tions between the Milky Way (MW) and the M31 system and its sentially limited to the Magellanic Clouds, the so-called classical future fate (e.g. Loeb et al. 2005; van der Marel et al. 2012a; Sa- MW dwarf spheroidal galaxies (dSphs), one "ultra faint dwarf" lomon et al. 2020, and references therein); determinations of the (UFD), M31, M33 and IC 10, mostly from HST observations mass of the MW through dynamical modelling of tracers of its and a few VLBI observations of OH masers (see references in gravitational potential, such as its satellite galaxies (e.g. Wilkin- Sect.6); some of them having become available surprisingly re- son & Evans 1999; Battaglia et al. 2005; Boylan-Kolchin et al. cently (e.g. in the case of the Sextans MW dSph the first such arXiv:2106.08819v1 [astro-ph.GA] 16 Jun 2021 2013; Patel et al. 2018; Callingham et al. 2019; Fritz et al. 2020, measurement was published only in 2018 by Casetti-Dinescu see references in Fritz et al. 2020 for an overview on the works et al. 2018). on this topic); group infall as well as the significance and stabil- Since April 2018, the situation has seen a dramatic improve- ity of the Vast Polar Structure (e.g. Metz et al. 2008; Pawlowski ment, starting with the Gaia science verification article (Gaia & Kroupa 2013; Fritz et al. 2018; Kallivayalil et al. 2018; Li Collaboration et al. 2018b). Multiple determinations of the sys- et al. 2021a); considerations on the missing satellite problem temic PM of a large number of MW satellite galaxies and galaxy (e.g. Simon 2018; Fritz et al. 2018). The orbital history of MW candidates blossomed in a matter of weeks after GDR2, led by satellite galaxies is also very likely to influence several aspects several groups in the community, and using a variety of tech- of their evolution, through e.g. the impact of ram-pressure strip- niques, e.g. focusing only on stars with prior spectroscopic in- ping and tidal effects onto their gas content, star formation his- formation (Simon 2018; Fritz et al. 2018) or including the full tory (SFH), morphology and dark matter (DM) halo properties set of stars with astrometric information (Kallivayalil et al. 2018; Article number, page 1 of 47 A&A proofs: manuscript no. eGDR3_ppm_arxiv Massari & Helmi 2018). It is now becoming routine to use Gaia galaxies and its distribution on the colour-magnitude plane ac- astrometric data also to remove contaminants, as well as to at- counting also for the photometric completeness of eGDR3 data; tempt systemic proper motions determinations along with the this allows us also to determine probability of memberships for study of other properties of the systems (e.g. Longeard et al. stars in different evolutionary phases, which we will make avail- 2018; Torrealba et al. 2019). Surveys of the MW stellar halo and able to the community 2, together with several other outputs of sub-structures within are and will make plentiful use of Gaia as- our analysis. trometry to boost the success rate in target selection (e.g. Conroy Finally, we study the orbital properties of the galaxies sur- et al. 2019; Li et al. 2019; Allende Prieto et al. 2020, but also the roundings of the MW by integrating their bulk motions in a set 12 surveys to be carried out with 4MOST and WEAVE, to mention of gravitational potentials, bracketing a 0.9-1.6×10 M range some of the upcoming ones). for the mass of the MW. Motivated by the work by Patel et al. The methodologies applied in the early GDR2 works men- (2020), who showed that the orbits of MW satellites can differ tioned above were rather simple ones, being based on iterative significantly between a gravitational potential including only the cleanings of the data-sets via σ-clipping and no statistical treat- MW and one where the gravitational influence of the LMC (and ment of the foreground/background contamination. Later on, SMC) are taken into account (and all galaxies are free to move in more sophisticated methods were used, e.g. with simultaneous response), we integrate the bulk motions also in the triaxial time- statistical modelling of the properties of the dwarf galaxy and the varying MW potential made available by Vasiliev et al.(2021), contamination. For instance, Pace & Li(2019) used the spatial where the infall of a massive LMC and the response of the MW and PM information of stars preselected to have magnitude and to this infall are modelled. In this context, we also revisit the color lying on an isochrone and adopted a multi-variate Gaussian association of dwarfs surrounding the MW to the LMC system. in proper motion for both the dwarf galaxy and the MW, while In Sect.2 we introduce the sample of galaxies analysed and McConnachie & Venn(2020a) used all the observables at once in Sect.3 the data-sets used and the quality selection criteria ap- and adopted the empirical distribution of the contaminant stars plied. In Sect.4 we present the methodology for the systemic in the PM and the colour-magnitude (CM) planes. proper motion determinations, and discuss the output and the Interestingly, the use of GDR2 data has been also pushed tests performed to tackle the robustness of the method; in Sect.5 beyond the MW system, with determinations of the tangential we complement the results with a determination of the zero- motions of M31 & M33 (van der Marel et al. 2019), as well as points and additional errors due to systematics in the Gaia eDR3 of a few LG dwarf galaxies such as NGC 6822, IC 1613, WLM data, using QSO. Our systemic proper motions are compared to and Leo A (McConnachie et al. 2021). those in the literature in Sect.6. In Sect.7 the bulk motions are The early third data release of Gaia data, hereafter eGDR3, integrated in the three gravitational potentials and the resulting (Gaia Collaboration et al. 2021a) has implied more precise and orbital trajectories and parameters are then used to address the accurate astrometric measurements; in particular, for PMs the impact of the LMC on the reconstructed orbital history, make precision has increased of a factor of two and systematic er- considerations on the too-big-to-fail problem, on the system of rors decreased by a factor ∼2.5 (Lindegren et al.