Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall

TL;DR

This paper reports the discovery of AndXII, a dwarf galaxy rapidly falling into the Local Group for the first time, providing direct evidence of late satellite infall predicted by cosmological models.

Contribution

It presents spectroscopic and observational evidence of a dwarf galaxy's first infall into the Local Group, supporting cosmological simulation predictions.

Findings

AndXII has a heliocentric velocity of -556 km/s.

It is likely falling into the Local Group for the first time.

The galaxy has a low H I gas mass limit, indicating prior gas removal.

Abstract

We present spectroscopic observations of the AndXII dwarf spheroidal galaxy using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group (-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW), falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. AndXII therefore represents a dwarf galaxy plausibly falling into the Local Group for the first time, and never having experienced a dense galactic environment. From Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun suggests that AndXII's gas could have been removed prior to experiencing the tides of the Local Group galaxies. Orbit models suggest the dwarf is close to the escape velocity of M31 for published mass models. AndXII is our best direct evidence for the late infall of satellite galaxies, a prediction of cosmological simulations.