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

TL;DR

This study uses Gaia data to measure proper motions of ultra-faint dwarf galaxy candidates near the Magellanic Clouds, testing LCDM predictions about satellite infall and identifying confirmed and potential members of the LMC debris.

Contribution

First proper motion measurements for 13 ultra-faint dwarf candidates near the Magellanic Clouds, confirming some as LMC satellites and providing insights into satellite infall predictions.

Findings

4 galaxies confirmed as LMC satellites with proper motions.

Some candidates are unlikely to be LMC debris based on position and velocity.

Predicted proper motions for remaining candidates aid future identification.

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.