NGC 55: a disc galaxy with flat abundance gradients
  We present new spectroscopic observations obtained with GMOS@Gemini-S of a
sample of 25 hii regions located in NGC 55, a late-type galaxy in the nearby
Sculptor group. We derive physical conditions and chemical composition through
the te-method for 18 hii regions, and strong-line abundances for 22 hii
regions. We provide abundances of He, O, N, Ne, S, Ar, finding a substantially
homogenous composition in the ionised gas of the disc of NGC 55, with no trace
of radial gradients. The oxygen abundances, both derived with \te- and
strong-line methods, have similar mean values and similarly small dispersion:
12+$\log$(O/H)=8.13$\pm$0.18~dex with the former and
12+$\log$(O/H)=8.17$\pm$0.13~dex with the latter. The average metallicities and
the flat gradients agree with previous studies of smaller samples of \hii\
regions and there is a qualitative agreement with the blue supergiant radial
gradient as well. We investigate the origin of such flat gradients comparing
NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in
terms of mass and luminosity. We suggest that the differences in the metal
distributions in the two galaxies might be related to the differences in their
K-band surface density profile. The flatter profile of NGC 55 probably causes
in this galaxy higher infall/outflow rates than in similar galaxies. This
likely provokes a strong mixing of gas and a re-distribution of metals.
