Galaxy spin as a formation probe: the stellar-to-halo specific angular momentum relation

TL;DR

This paper models the relation between stellar and halo angular momentum in galaxies, revealing how this ratio varies with galaxy type and mass, and linking it to galaxy formation processes.

Contribution

It introduces a model for the stellar-to-halo specific angular momentum relation using tidal torque theory and observational data, highlighting the mass-dependent behavior and differences between galaxy types.

Findings

The ratio of stellar to halo angular momentum varies with mass as a double power-law.

The peak of this ratio coincides with the maximum of the stellar-to-halo mass ratio.

Spirals have 3-5 times higher angular momentum ratios than ellipticals.

Abstract

We derive the stellar-to-halo specific angular momentum relation (SHSAMR) of galaxies at $z=0$ by combining i) the standard $\Lambda$CDM tidal torque theory ii) the observed relation between stellar mass and specific angular momentum (Fall relation) and iii) various determinations of the stellar-to-halo mass relation (SHMR). We find that the ratio $f_j = j_\ast/j_{\rm h}$ of the specific angular momentum of stars to that of the dark matter i) varies with mass as a double power-law, ii) it always has a peak in the mass range explored and iii) it is $3-5$ times larger for spirals than for ellipticals. The results have some dependence on the adopted SHMR and we provide fitting formulae in each case. For any choice of the SHMR, the peak of $f_j$ occurs at the same mass where the stellar-to-halo mass ratio $f_\ast = M_\ast/M_{\rm h}$ has a maximum. This is mostly driven by the straightness and tightness of the Fall relation, which requires $f_j$ and $f_\ast$ to be correlated with each other roughly as $f_j\propto f_\ast^{2/3}$, as expected if the outer and more angular momentum rich parts of a halo failed to accrete onto the central galaxy and form stars (biased collapse). We also confirm that the difference in the angular momentum of spirals and ellipticals at a given mass is too large to be ascribed only to different spins of the parent dark-matter haloes (spin bias).