UNIONS: Red and blue alignments of DESI DR1 galaxies

Intrinsic alignments (IA) of galaxies are a dominant source of systematics in weak-lensing cosmological analyses, and direct measurements on spectroscopic samples are the primary source of priors on their amplitude. We measure the IA of galaxies from the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI) as a function of colour, stellar mass, and halo mass, by cross-correlating the DESI spectroscopic catalogues with the weak-lensing shape catalogue of the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) over up to 2400 deg$^2$. We fit the three-dimensional galaxy-galaxy and galaxy-shape multipoles of the DESI BGS, LRG, and ELG samples, split by colour and stellar mass, with the non-linear linear alignment (NLA) and the tidal alignment and tidal torquing (TATT) models. Halo masses are measured for the BGS red sample from galaxy-galaxy lensing and extended to the LRGs through the resulting stellar-to-halo mass relation. Red galaxies are strongly and radially aligned, but their alignment amplitude is not a single power law of halo mass between $6\times10^{11}$ and $3\times10^{13}\,h^{-1}M_\odot$: it is nearly flat at low mass and rises steeply above a break at $1.3\times10^{13}\,h^{-1}M_\odot$. Massive blue BGS galaxies instead show a tangential alignment, detected at $3.3σ$, to our knowledge the most significant detection of a non-zero alignment of blue galaxies to date, while the low-mass blue BGS galaxies and the ELGs are consistent with zero. At our current precision, the data do not favour TATT over NLA.