  Despite the evidence that supermassive black holes (SMBHs) co-evolve with
their host galaxy, and that most of the growth of these SMBHs occurs via
merger-free processes, the underlying mechanisms which drive this secular
co-evolution are poorly understood. We investigate the role that both strong
and weak large-scale galactic bars play in mediating this relationship. Using
72,940 disc galaxies in a volume-limited sample from Galaxy Zoo DESI, we
analyse the active galactic nucleus (AGN) fraction in strongly barred, weakly
barred, and unbarred galaxies up to z = 0.1 over a range of stellar masses and
colours. After controlling for stellar mass and colour, we find that the
optically selected AGN fraction is 31.6 +/- 0.9 per cent in strongly barred
galaxies, 23.3 +/- 0.8 per cent in weakly barred galaxies, and 14.2 +/- 0.6 per
cent in unbarred disc galaxies. These are highly statistically robust results,
strengthening the tantalising results in earlier works. Strongly barred
galaxies have a higher fraction of AGNs than weakly barred galaxies, which in
turn have a higher fraction than unbarred galaxies. Thus, while bars are not
required in order to grow a SMBH in a disc galaxy, large-scale galactic bars
appear to facilitate AGN fuelling, and the presence of a strong bar makes a
disc galaxy more than twice as likely to host an AGN than an unbarred galaxy at
all galaxy stellar masses and colours.
