Unravelling the Andromeda Galaxy's most important merger

Richard D'Souza (UMich)
Friday, February 22, 2019 - 12:15pm to 1:00pm

The Andromeda Galaxy (M31), our nearest large galactic neighbour, offers a unique opportunity to 

test how mergers affect galaxy properties. M31's stellar halo caused by the tidal disruption of 

satellite galaxies is the best tracer of the galaxy's accretion history. Despite a decade of 

effort in mapping out M31's large stellar halo, we are unable to convert M31's stellar halo into

a merger history. Here we use cosmological models of galaxy formation to show that M31's massive 

and metal-rich stellar halo containing intermediate age stars implies that it merged with a 

large (M* ~ 2.5 x 10^10 M_sun) galaxy ~2 Gyr ago. The simulated properties of the merger debris 

help to interpret a broader set of observations of M31's stellar halo and satellites 

than previously considered: its compact and metal-rich satellite M32 is the tidally-stripped core of 

the disrupted galaxy, M31's rotating and flattened inner stellar halo contains most of the merger 

debris, and the giant stellar stream is likely to have been thrown out during the merger. This accreted 

galaxy was the third largest member of the Local Group. This merger may explain the global burst of star 

formation ~2 Gyr ago in the disk of M31 in which ~1/5 of its stars were formed. Moreover, M31's disk and 

bulge were already in place before its most important merger, suggesting that mergers of this magnitude 

do not dramatically affect galaxy structure.

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