2012MNRAS.427..906H

Most galaxies follow well-defined scaling relations of metallicity (O/H), star formation rate (SFR) and stellar mass (M_star). However, low-metallicity starbursts, rare in the Local Universe but more common at high redshift, deviate significantly from these scaling relations. On the `main sequence' of star formation, these galaxies have high SFR for a given M<sub>star</sub>; and on the mass–metallicity relation, they have excess M<sub>star</sub> for their low metallicity. In this paper, we characterize O/H, M<sub>star</sub> and SFR for these deviant `low-metallicity starbursts', selected from a sample of ∼ 1100 galaxies, spanning almost two orders of magnitude in metal abundance, a factor of ∼ 10⁶ in SFR, and of ∼ 10⁵ in stellar mass. Our sample includes quiescent star-forming galaxies and blue compact dwarfs at redshift 0, luminous compact galaxies at redshift 0.3, and Lyman break galaxies at redshifts 1–3.4. Applying a principal component analysis (PCA) to the galaxies in our sample with M_star ≤ 3 x 10¹⁰M_☉ gives a Fundamental Plane (FP) of scaling relations; SFR and stellar mass define the plane itself, and O/H its thickness. The dispersion for our sample in the edge-on view of the plane is 0.17dex, independently of redshift and including the metal-poor starbursts. The same FP is followed by 55100 galaxies selected from the Sloan Digital Sky Survey, with a dispersion of 0.06dex. In a companion paper, we develop multi-phase chemical evolution models that successfully predict the observed scaling relations and the FP; the deviations from the main scaling relations are caused by a different (starburst or `active') mode of star formation. These scaling relations do not truly evolve, but rather are defined by the different galaxy populations dominant at different cosmological epochs.