Validity of abundances derived from spaxel spectra of the MaNGA survey

摘要

We measured the emission lines in the spaxel spectra of MaNGA galaxies inorder to determine the abundance distributions therein. It has been suggestedthat the strength of the low-ionization lines, R_2, N_2, and S_2 may beincreased (relative to Balmer lines) in (some) spaxel spectra of the MaNGAsurvey due to a contribution of the radiation of the diffuse ionized gas.Consequently, the abundances derived from the spaxel spectra throughstrong-line methods may suffer from large errors. We examined this expectationby comparing the behaviour of the line intensities and the abundances estimatedthrough different calibrations for slit spectra of HII regions in nearbygalaxies, for fibre spectra from the SDSS, and for spaxel spectra of the MaNGAsurvey. We found that the S_2 strength is increased significantly in the fibreand spaxel spectra. The mean enhancement changes with metallicity and can be aslarge as a factor of 2. The mean distortion of R_2 and N_2 is less than afactor of 1.3. This suggests that Kaufmann et al.'s demarcation line betweenAGNs and HII regions in the BPT diagram is a useful criterion to reject spectrawith significantly distorted strengths of the N_2 and R_2 lines. We find thatthe three-dimensional R calibration, which uses the N_2 and R_2 lines, producesreliable abundances in the MaNGA galaxies. The one-dimensional N2 calibrationproduces either reliable or wrong abundances depending on whether excitationand N/O abundance ratio in the target region (spaxel) are close to or differfrom those parameters in the calibrating points located close to thecalibration relation. We then determined abundance distributions within theoptical radii in the discs of 47 MaNGA galaxies. The optical radii of thegalaxies were estimated from the surface brightness profiles constructed basedon the MaNGA observations.