Rosella: A mock catalogue of galaxy luminosities, colours and positions for cosmology

The scientific exploitation of the Dark Energy Spectroscopic Instrument Bright Galaxy Survey (DESI BGS) data requires the construction of mocks with galaxy population properties closely mimicking those of the actual DESI BGS targets. We create a high fidelity mock galaxy catalogue that can be used to interpret the DESI BGS data, as well as meeting the precision required for the tuning of thousands of approximate DESI BGS mocks needed for cosmological analyses with DESI BGS data. The mock catalogue uses subhalo abundance matching (SHAM) with scatter to populate the P-Millennium N-body simulation with galaxies at the median DESI BGS redshift of ~0.2, using formation redshift information to assign ^0.1(g−r) rest-frame colours. The mock provides information about r-band absolute magnitudes, ^0.1(g − r) rest-frame colours, 3D positions and velocities of a complete sample of DESI BGS galaxies in a volume of (542 Mpc/h)^3. This P-Millennium DESI BGS mock catalogue is ideally suited to the tuning of approximate mocks unable to resolve subhalos that DESI BGS galaxies reside in, to test for systematics in analysis pipelines and to interpret (non-cosmological focused) DESI BGS analysis.