Power Spectrum of Intrinsic Alignments of Galaxies in IllustrisTNG
  We present the 3-{\it dimensional} intrinsic alignment power spectra between
the projected 2d galaxy shape/spin and the 3d tidal field across $0.1<k/h{\rm
Mpc}^{-1}<60$ using cosmological hydrodynamical simulation, Illustris-TNG300,
at redshifts ranging from $0.3$ to $2$. The shape-tidal field alignment
increases with galaxy mass and the linear alignment coefficient $A_{\rm IA}$,
defined with respect to the primordial tidal field, is found to have weak
redshift dependence. We also show a promising detection of the shape/spin-tidal
field alignments for stellar mass limited samples and a weak or almost null
signal for star-forming galaxies for the TNG300 volume, $\sim 0.01~(h^{-1}{\rm
Gpc})^3$. We further study the morphology and environmental dependence of the
intrinsic alignment power spectra. The shape of massive disk- and
spheroid-galaxies tend to align with the tidal field. The spin of low mass
disks (and spheroids at low redshifts) tend to be parallel with the tidal
field, while the spin of massive spheroids and disks tend to be perpendicular
to tidal field. The shape and spin of massive centrals align with the tidal
field at both small and large scales. Satellites show a radial alignment within
the one-halo term region, and low mass satellites have an intriguing alignment
signal in the two-halo term region. We also forecast a feasibility to measure
the intrinsic alignment power spectrum for spectroscopic and imaging surveys
such as Subaru HSC/PFS and DESI. Our results thus suggest that galaxy intrinsic
alignment can be used as a promising tool for constraining the galaxy formation
models.
