Implications of the Milky Way declining rotation curve

Almost all spiral galaxies have been observed to have flattening rotation curves. The new Gaia Data Release 3 (DR3) shows a Milky Way with a sharply Keplerian declining rotation curve, starting at [Formula: see text]16[Formula: see text]kpc and ending at 26.5[Formula: see text]kpc. The data reduce the total Milky Way mass by an order of magnitude, [Formula: see text], compared to the standard required dark matter halo mass, [Formula: see text]. Newtonian and MOdified Gravity (MOG) fits are applied to the Gaia DR3 rotation curve data. The fit obtained using MOG has a total mass of [Formula: see text], while the Newtonian fit predicts a mass of [Formula: see text]. These are in excess of the estimated visible baryon mass of the Milky Way, [Formula: see text]. It is possible that if the mass of the circumgalactic medium (CGM) plasma-gas continues to be confirmed experimentally, then the additional baryon mass required to account for the estimated total Milky Way mass could be attributed to the CGM hot plasma-gas halo.