Accretion of satellite galaxies and the density of the Universe

Recent work suggests that the disc mass of a galaxy like our own cannot have grown by more than 10 per cent through the accretion of satellites over the last 5 Gyr; greater accretion would have led to excessive thickening of the stellar disc. This limit has been interpreted as requiring a low-density universe, since current merger rates are then relatively small. We show that, while almost all galaxy haloes in an Ω = 1 universe have indeed increased their mass by more than 10 per cent over the last 5 Gyr, many of the associated satellites have not yet merged with the central galaxy. For a standard CDM cosmogony, our simulations suggest that fewer than 30 per cent of spiral discs have grown by over 10 per cent during the last 5 Gyr. Thus the observed thinness of spiral discs may not exclude a high-density universe.