Hardness of ionizing radiation fields in MaNGA star-forming galaxies

Abstract

We investigate radiation hardness within a representative sample of 67 nearby (0.02 ≤ z ≤ 0.06) star-forming (SF) galaxies using the integral field spectroscopic data from the MaNGA survey. The softness parameter η = O+/O2+S+/S2+ is sensitive to the spectral energy distribution of the ionizing radiation. We study η via the observable quantity η' (=[O II]/[O III] [S II][S III] ). We analyze the relation between radiation hardness (traced by η and η') and diagnostics sensitive to gas-phase metallicity, electron temperature, density, ionization parameter, effective temperature, and age of ionizing populations. It is evident that low metallicity is accompanied by low log η', i.e. hard radiation field. No direct relation is found between radiation hardness and other nebular parameters though such relations cannot be ruled out. We provide empirical relations between log η and strong emission line ratios N2, O3N2, and Ar3O3, which will allow future studies of radiation hardness in SF galaxies where weak auroral lines are undetected. We compare the variation of [OIII]/[O II] and [S III]/[S II] for MaNGA data with SF galaxies and HII regions within spiral galaxies from literature and find that the similarity and differences between different data set are mainly due to the metallicity. We find that predictions from photoionization models considering young and evolved stellar populations as ionizing sources in good agreement with the MaNGA data. This comparison also suggests that hard radiation fields from hot and old low-mass stars within or around SF regions might significantly contribute to the observed η values. © 2021 The Author(s). ; NK acknowledges financial support from the Schlumberger Foundation, which facilitated her stay at the KICC during which a majority of work was carried out. RA acknowledges support from FONDECYT (Fondo Nacional de Desarrollo Científico y Tecnológico) regular grant 1202007. RM acknowledges ERC (European Research Council) advanced grant 695671 'QUENCH' and support by the Science and Technology Facilities Council (STFC). This project makes use of the MaNGA-Pipe3D data products. We thank the IA-UNAM MaNGA team for creating this catalogue, and the ConaCyt-180125 project for supporting them (Sánchez et al. 2016b). This research made use of Marvin, a core PYTHON package and web framework for MaNGA data, developed by Brian Cherinka, José Sánchez-Gallego, Brett Andrews, and Joel Brownstein (Cherinka et al. 2019); SAOImage DS9, developed by Smithsonian Astrophysical Observatory; and Astropy, a community-developed core PYTHON package for astronomy (Astropy Collaboration 2013). ; With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709. ; Peer reviewed