Oral Presentation

Radial Dependence of Halo Spin Alignment with the Cosmic Web

Author(s): Jun-Sung Moon (SNU); Jounghun Lee (SNU)

Presenter: Jun-Sung Moon (Seoul National University)

It is well known that the spins of dark matter (DM) halos are aligned with the cosmic web, but the spin alignment of galaxies at the innermost is still an open question. Here, we report a detection of radius-dependent spin transition of DM halos found in the IllustrisTNG simulations. We measure the inner spin vectors within apertures smaller than the virial radius and investigate their alignments with the tidal and velocity shear fields (i.e., Tweb and Vweb). We find that, for both the DM-only and baryonic runs, the preferred directions of massive halos transit from the intermediate to the major (maximum compression) principal axis as the aperture radius decreases. At the stellar half-mass radius, both DM and stellar spins eventually align with the major principal axis of the Tweb. The alignment with the Tweb major axis at a fixed radius strengthens as the smoothing scale decreases, while the alignment with the Vweb hardly appears on smoothing scales less than 1Mpc/h. We provide some clues for the possible origins of this peculiar spin transition as follows. (a) The same trend is observed even in the DM-only runs, indicating its non-baryonic origins. (b) The direction of the vorticity vectors always prefer to be perpendicular to the major principal axes, challenging the idea that the local vortical flow primarily affects halo spins in the non-linear regime. (c) We develop an analytic model whose key tenet is that a competition between the internal tension and the tidal compression determines the principal directions in which the galaxy spins are aligned.