LAUSR

It is pointed out that MOND defines a fiducial specific angular momentum (SAM) for a galaxy of total (baryonic) mass M: jM(M) ≡ M(G/a0) ≈ 383(M/10M ) kpc km s−1. It plays important roles in disc-galaxy dynamics and evolution: It underlies scaling relations in virialized galaxies that involve their angular-momentum. I show that the disc SAM should be jD ≈ [〈r〉/rM(M)]jM(M) = [ΣM/〈Σ〉]jM(M), with 〈r〉 the mean radius of the disc, 〈Σ〉 =M/2π〈r〉 some mean surface density of the galaxy, rM = (MG/a0) is the MOND radius of the galaxy, and ΣM = a0/2πG is the (universal) MOND surface density. So, e.g., for a fixed 〈Σ〉, jD ∝M, while for a fixed 〈r〉, jD ∝M. Furthermore, jM(M) is a reference predictor of the type of galaxy a protogalaxy will settle into, if it evolves in isolation: A protogalaxy of massM, and SAM j jM(M) should settle into a low-surface-density disc – with mean acceleration 〈a〉/a0 ≈ jM/j 1. While a protogalaxy with j . jM(M) should end up with a disc of massMD ≈ jM/jM(M), having a SAM jD ≈ jM(M), which is tantamount to 〈a〉 ≈ a0 (i.e., at the “Freeman limit”); it should also develop a low-SAM component (e.g., a bulge or a halo), taking up the rest of the mass MB ≈M−MD.