Angular Momentum of Early- and Late-type Galaxies: Nature or Nurture?

We investigate the origin, shape, scatter, and cosmic evolution of the observed relationship between specific angular momentum j _⋆ and the stellar mass M _⋆ in early-type galaxies (ETGs) and late-type galaxies (LTGs). Specifically, we exploit the observed star formation efficiency and chemical abundance to infer the fraction f _inf of baryons that infall toward the central regions of galaxies where star formation can occur. We find f _inf ≈ 1 for LTGs and ≈0.4 for ETGs with an uncertainty of about 0.25 dex, consistent with a biased collapse. By comparing with the locally observed j _⋆ versus M _⋆ relations for LTGs and ETGs, we estimate the fraction f _j of the initial specific angular momentum associated with the infalling gas that is retained in the stellar component. For LTGs, we find $f_j\approx {1.11}_{-0.44}^{+0.75}$, in line with the classic disk formation picture; for ETGs, we infer $f_j\approx {0.64}_{-0.16}^{+0.20}$, which can be traced back to a z ≲ 1 evolution via dry mergers. We also show that the observed scatter in the j _⋆ versus M _⋆ relation for both galaxy types is mainly contributed by the intrinsic dispersion in the spin parameters of the host dark matter halo. The biased-collapse plus merger scenario implies that the specific angular momentum in the stellar components of ETG progenitors at z ∼ 2 is already close to the local values, in good agreement with observations. All in all, we argue that such a behavior is imprinted by nature and not nurtured substantially by the environment.