# Antlia2's role in driving the ripples in the outer gas disk of the Galaxy

## TL;DR

This study investigates how the Antlia 2 dwarf galaxy influences the outer gas disk of the Milky Way, suggesting it may be the primary driver of observed perturbations based on orbital calculations and simulations.

## Contribution

It provides new orbital estimates for Antlia 2 and demonstrates its potential role in causing outer disk ripples, contrasting with the Sagittarius dwarf galaxy.

## Key findings

- Antlia 2's orbit with low pericenters matches observed disk disturbances.
- Proper motion estimates of Antlia 2 are consistent with previous data but suggest closer pericenters.
- Antlia 2's orbit is nearly coplanar with the Galactic disk, supporting its influence on disk ripples.

## Abstract

We employ the earlier published proper motions of the newly discovered Antlia 2 dwarf galaxy derived from Gaia data to calculate its orbital distribution in the cosmologically recent past. Using these observationally motivated orbits, we calculate the effect of the Antlia 2 dwarf galaxy on the outer HI disk of the Milky Way, using both test particle and Smoothed Particle Hydrodynamics simulations. We find that orbits with low pericenters, $\sim$ 10 kpc, produce disturbances that match the observed outer HI disk perturbations. We have independently recalculated the proper motion of the Antlia 2 dwarf from Gaia data and found a proper motion of $(\mu_{\alpha}cos\delta, \mu_{\delta}) = (-0.068,0.032) \pm (0.023,-0.031)~\rm mas/yr$, which agrees with results from Torrealba et al. (2019) within the errors, but gives lower mean pericenters, e.g., $\sim$ 15 kpc for our fiducial model of the Milky Way. We also show that the Sagittarius dwarf galaxy interaction does not match the observed perturbations in the outer gas disk. Thus, Antlia 2 may be the driver of the observed large perturbations in the outer gas disk of the Galaxy. The current location of the Antlia 2 dwarf galaxy closely matches that predicted by an earlier dynamical analysis (Chakrabarti \& Blitz 2009) of the dwarf that drove ripples in the outer Galaxy, and, in particular, its orbit is nearly coplanar to the Galactic disk. If the Antlia 2 dwarf galaxy is responsible for the perturbations in the outer Galactic disk, it would have a specific range of proper motions that we predict here; this can be tested soon with Gaia DR-3 and Gaia DR-4 data.