Validity of abundances derived from spaxel spectra of the MaNGA survey

Abstract

We measured the emission lines in the spaxel spectra of MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) galaxies in order to determine the abundance distributions therein. It has been suggested that the strength of the low-ionization lines, R2, N2, and S2, may be increased (relative to Balmer lines) in (some) spaxel spectra of the MaNGA survey due to a contribution of the radiation of the diffuse ionized gas. Consequently, the abundances derived from the spaxel spectra through strong-line methods may suffer from large errors. We examined this expectation by comparing the behaviour of the line intensities and the abundances estimated through different calibrations for slit spectra of H II regions in nearby galaxies, for fibre spectra from the Sloan Digital Sky Survey (SDSS), and for spaxel spectra of the MaNGA survey. We found that the S2 strength is increased significantly in the fibre and spaxel spectra. The mean enhancement changes with metallicity and can be as large as a factor of ∼ 2. The mean distortion of R2 and N2 is less than a factor of ∼ 1.3. This suggests that Kaufmann et al.'s demarcation line between active galactic nuclei (AGNs) and H II regions in the Baldwin, Phillips, & Terlevich (BPT) diagram is a useful criterion to reject spectra with significantly distorted strengths of the N2 and R2 lines. We find that the three-dimensional R calibration, which uses the N2 and R2 lines, produces reliable abundances in the MaNGA galaxies. The one-dimensional N2 calibration produces either reliable or wrong abundances depending on whether excitation and N/O abundance ratio in the target region (spaxel) are close to or differ from those parameters in the calibrating points located close to the calibration relation. We then determined abundance distributions within the optical radii in the discs of 47 MaNGA galaxies. The optical radii of the galaxies were estimated from the surface brightness profiles constructed based on the MaNGA observations.

Relations between abundance characteristics and rotation velocity for star-forming MaNGA galaxies

“…We found in a previous study (Pilyugin et al 2018) that the three-dimensional R calibration, which uses the N 2 , R 2 and R 3 lines, produces reliable abundances for regions (spaxels) with H II region-like spectra in the MaNGA galaxies. One should distinguish clearly two questions: the nature of the line-emitting gas (the sources of the ionizing radiation) and the diagnostic of the emitting gas (the abundance determination).…”

“…Therefore, we obtained the position angle of the major axis of the galaxy, the galaxy's inclination angle, and the rotation velocity from the analysis of the Hα velocity field. The normalized histogram of the differences V Hβ -V Hα between the line-of-sight velocities measured for Hβ and Hα emission lines in 46,350 regions (spaxels) in MaNGA galaxies (Pilyugin et al 2018).…”

Properties of galaxies with an offset between the position angles of the major kinematic and photometric axes

“…It has been found that the three-dimensional R calibration from produces reliable abundances for MaNGA galaxies, that is, the R-calibration produces abundances compatible to the T e -based abundance scale and is workable over the whole metallicity scale of H II regions (Pilyugin et al 2018(Pilyugin et al , 2019Zinchenko et al 2019a,b). We use those relations for abundance determinations also here.…”

“…We examine the images of galaxies as represented by their surface brightness distribution maps in Fig. 1 The surface brightness distribution within a galaxy is fitted by a broken exponential profile for the disc and by a general Sérsic profile for the bulge following to our previous paper (Pilyugin et al 2018). We use the surface-brightness profile in solar units for the bulge-disc decomposition.…”

Peculiar motions of the gas at the centre of the barred galaxy UGC 4056

“…Panel a1 of Fig. 2 shows the surface brightness distribution in the B band across the image of UGC 4056 in sky coordinates (pixels).The geometric parameters of UGC 4056 (the coordinates of the centre X 0 = 38.5 pixels and Y 0 = 37.5 pixels, the inclination angle i = 64.2 • , and the position angle of the major axis P A = 208.4 • ) were obtained in our current study from the analysis of the surface brightness map in the way described in Pilyugin et al (2018). The formal error is 0.5 pixels for X 0 , Y 0 and 1 • for the inclination and PA.…”