Within our own galaxy, the thick disc is a distinct population of stars that resides above and below the main (thin) disc. Its stars have a larger range of velocities, are generally older and more metal poor. While astronomers aren’t entirely sure how it formed (remnants of accretion of small galaxies or ejection from the thin disc), it’s certainly there and analogues have been observed in other galaxies, more than 10 megaparsecs away. If these thick discs are truly a product of mergers, then galaxies showing evidence of mergers in other regards should show the presence of this second population as well. Yet in the case of M31, the Andromeda galaxy, the closest major galaxy to our own, which is thought to have a rich merger history, traces of the thick disc have proved elusive. So where is it?

(…) Read the rest of Where’s M31?s Thick Disc? (382 words)

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