Direct-method metallicity gradients derived from spectral stacking with SDSS-IV MaNGA

Chemical abundances are key tracers of the cycle of baryons driving the evolution of galaxies. Most measurements of interstellar medium (ISM) abundance and metallicity gradients in galaxies are based, however, on model-dependent strong-line methods. Direct chemical abundances can be obtained via the detection of weak auroral lines, but such lines are too faint to be detected across large spectroscopic surveys of the local Universe. In this work we overcome this limitation and obtain metallicity gradients from direct method abundances by stacking spectra from the MaNGA integral field spectroscopy survey. In particular we stack 4140 star-forming galaxies across the star formation rate-stellar mass (SFR-M$_\star$) plane and across six radial bins. We calculate electron temperatures for [OII], [SII], [NII], [SIII] and [OIII] across the majority of stacks. We find that the T[OII] $\sim$ T[SII] $\sim$ T[OII], as expected since these ions all trace the low-ionization zone of nebulae. The [OIII] temperatures become substantially larger than those of other ions at high metallicity, indicating potentially unaccounted for spectral contamination or additional physics. In light of this uncertainty we base our abundance calculation on the temperatures of [SIII] and the low-ionization ions. We recover a mass-metallicity relation (MZR) similar to that obtained with different empirical calibrations. We do not find evidence, however, for a secondary dependence on SFR using direct metallicities. Finally, we derive metallicity gradients that becomes steeper with stellar mass for $\log(M_\star/M_\odot) < 10.5$. At higher masses, the gradients flatten again, confirming with auroral line determinations the trends previously defined with strong-line calibrators.