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We present an analysis of the proper motion of the Andromeda galaxy (M31),based on the Early Third Data Release of the Gaia mission. We use the Gaiaphotometry to select young blue main sequence stars, and apply several qualitycuts to obtain clean samples of these tracers. After correcting the propermotion measurements for the internal rotation of the M31 disk motion, we derivean apparent motion of 52.5 +/- 5.8 muas/yr with respect to the Gaia referenceframe, or 61.9 +/- 9.7 muas/yr after applying a zero-point correctiondetermined from quasars within 20 degrees from M31 and a correction fromsystemic biases. Accounting for the Solar reflex motion we deduce a relativevelocity between Andromeda and the Milky way (in a non-rotating frame at thecurrent location of the Sun) of 42.2 +/- 39.3 km/s along right ascension (40.0+/- 39.3 km/s along galactic longitude) and -59.4 +/- 30.3 km/s alongdeclination (-60.9 +/- 30.3 km/s along galactic latitude), with a totaltransverse velocity of V_trans = 82.4 +/- 31.2 km/s. These values areconsistent with (but more accurate than) earlier Hubble Space Telescopemeasurements that predict a future merger between the two galaxies. We alsonote a surprisingly large difference in the derived proper motion between theblue stars in M31 and samples of red stars that appear to lie in that galaxy.We propose several hypotheses to explain the discrepancy but found no clearevidence with the current data to privilege any one of them.

The proper motion of Andromeda from Gaia eDR3: confirming a nearly radial orbit