Scaling relations and baryonic cycling in local star-forming galaxies:
  I. The sample
  Metallicity and gas content are intimately related in the baryonic exchange
cycle of galaxies, and galaxy evolution scenarios can be constrained by
quantifying this relation. To this end, we have compiled a sample of ~400
galaxies in the Local Universe, dubbed "MAGMA" (Metallicity And Gas for Mass
Assembly), which covers an unprecedented range in parameter space, spanning
more than 5 orders of magnitude in stellar mass (Mstar), star-formation rate
(SFR), and gas mass (Mgas), and a factor of ~60 in metallicity [Z,
12+log(O/H)]. Stellar masses and SFRs have been recalculated for all the
galaxies using IRAC, WISE and GALEX photometry, and 12+log(O/H) has been
transformed, where necessary, to a common metallicity calibration. To assess
the true dimensionality of the data, we have applied multi-dimensional
principal component analyses (PCAs) to our sample. In confirmation of previous
work, we find that even with the vast parameter space covered by MAGMA, the
relations between Mstar, SFR, Z and Mgas (MHI+MH2) require only two dimensions
to describe the hypersurface. To accommodate the curvature in the Mstar-Z
relation, we have applied a piecewise 3D PCA that successfully predicts
observed 12+log(O/H) to an accuracy of ~0.1dex. MAGMA is a representative
sample of isolated star-forming galaxies in the Local Universe, and can be used
as a benchmark for cosmological simulations and to calibrate evolutionary
trends with redshift.
