The Missing Satellites of the Magellanic Clouds? Gaia Proper Motions of the Recently Discovered Ultra-faint Galaxies

Abstract

According to LCDM theory, hierarchical evolution occurs on all mass scales, implying that satellites of the Milky Way should also have companions. The recent discovery of ultra-faint dwarf galaxy candidates in close proximity to the Magellanic Clouds provides an opportunity to test this theory. We present proper motion (PM) measurements for 13 of the 32 new dwarf galaxy candidates using Gaia data release 2. All 13 also have radial velocity measurements. We compare the measured 3D velocities of these dwarfs to those expected at the corresponding distance and location for the debris of an LMC analog in a cosmological numerical simulation. We conclude that 4 of these galaxies (Hor1, Car2, Car3 and Hyi1) have come in with the Magellanic Clouds, constituting the first confirmation of the type of satellite infall predicted by LCDM. Ret2, Tuc2 and Gru1 have velocity components that are not consistent within 3 sigma of our predictions and are therefore less favorable. Hya2 and Dra2 could be associated with the LMC and merit further attention. We rule out Tuc3, Cra2, Tri2 and Aqu2 as potential members. Of the dwarfs without measured PMs, 5 of them are deemed unlikely on the basis of their positions and distances alone as being too far from the orbital plane expected for LMC debris (Eri2, Ind2, Cet2, Cet3 and Vir1). For the remaining sample, we use the simulation to predict PMs and radial velocities, finding that Phx2 has an overdensity of stars in DR2 consistent with this PM prediction.

The Orbital Histories of Magellanic Satellites Using Gaia DR2 Proper Motions

“…In Kallivayalil et al (2018), it was found that Hydra 2, Draco 2 and Phoenix 2 may also be associated with the MCs. Thus, we additionally include these three UFDs in our sample.…”

“…Nearly 30 new dwarf galaxies have recently been discovered in the vicinity of the Magellanic Clouds (MCs; Bechtol et al 2015;Drlica-Wagner et al 2015;Koposov et al 2015b;Martin et al 2015;Laevens et al 2015;Drlica-Wagner et al 2016;Torrealba et al 2016aTorrealba et al ,b, 2018Koposov et al 2018;Homma et al 2018). Furthermore, the timely second data release from the Gaia mission (Gaia Collaboration et al 2018a) has enabled proper motion (PM) measurements for these ultra-faint satellites (Simon 2018;Fritz et al 2018;Kallivayalil et al 2018;Pace & Li 2019;Massari & Helmi 2018), now making it possible to study their 3D kinematics and orbital histories in unprecedented detail. With this new data from Gaia DR2, several authors have aimed to identify the subset of known UFDs and classical dwarf spheroidals in the MW's halo that were originally satellites of the MCs.…”

“…With this new data from Gaia DR2, several authors have aimed to identify the subset of known UFDs and classical dwarf spheroidals in the MW's halo that were originally satellites of the MCs. Kallivayalil et al (2018) measured the PMs of 13 UFDs that also had radial velocity measurements using Gaia DR2. They compared the new 3D kinematics of UFDs to the tidal debris of a cosmological analog of the LMC to determine which UFDs have coincident kinematics with the LMC debris, and found that four UFDs (Carina 2, Carina 3, Horologium 1, Hydrus 1) are likely members of the Magellanic system.…”

“…In some studies (i.e. Kallivayalil et al 2018;Jahn et al 2019), the inclusion of the SMC is inhibited by the simulations in that finding a reasonable cosmological match to the MW+LMC+SMC system is rare (e.g. Boylan-Kolchin et al 2011).…”

Revealing the tidal scars of the Small Magellanic Cloud

“…From an observational perspective, the past decade has witnessed the discovery of many new structures around the MCs, including a slew of 'ultra-faint' dwarfs likely to be associated to the MCs (Drlica-Wagner et al 2015;Koposov et al 2015;Deason et al 2015;Drlica-Wagner et al 2016;Jethwa, Erkal & Belokurov 2016;Sales et al 2016;Conn et al 2018;Kallivayalil et al 2018;); a 'stellar cloud' within the LMC tidal radius (Mackey et al 2016); SMC structures such as the SMCNOD that could owe to a primordial SMC satellite in advanced stage of disruption (Pieres et al 2017) and the shell-like overdensity studied by Martinez-Delgado et al (2019) that could owe to a recent star formation event triggered by interactions between the MCs and/or the MW; and the very extended, low-density, envelope of stellar material with a disturbed shape around the LMC (Nidever et al 2019). All of these structures can be used as tracers of the gravitational interaction history of the MCs .…”