Hardness of ionizing radiation fields in MaNGA star-forming galaxies

Abstract

We investigate radiation hardness within a representative sample of 67 nearby (0.02 $\lesssim$z$\lesssim$0.06) star-forming (SF) galaxies using the integral field spectroscopic data from the MaNGA survey. The softness parameter $\eta$ = $\frac{O^{+}/O^{2+}}{S^{+}/S^{2+}}$ is sensitive to the spectral energy distribution of the ionizing radiation. We study $\eta$ via the observable quantity $\eta \prime$ (=$\frac{[OII]/[OIII]}{[SII][SIII]}$) We analyse the relation between radiation hardness (traced by $\eta$ and $\eta \prime$) 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 $\eta \prime$, i.e. hard radiation field. No direct relation is found between radiation hardness and other nebular parameters though such relations can not be ruled out. We provide empirical relations between log $\eta$ and strong emission line ratios N${}_2$, O${}_3$N${}_2$ and Ar${}_3$O${}_3$ which will allow future studies of radiation hardness in SF galaxies where weak auroral lines are undetected. We compare the variation of [O III]/[O II] and [S III]/[S II] for MaNGA data with SF galaxies and H II regions within spiral galaxies from literature, and find that the similarity and differences between different data set is mainly due to the metallicity. We find that predictions from photoionizaion 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 $\eta$ values.

31 pages, 38 figures, 2 tables, accepted for publication in MNRAS

References in corpus (27)

• The 2.5 m Telescope of the Sloan Digital Sky Survey
• The Host Galaxies and Classification of Active Galactic Nuclei
• Overview of the SDSS-IV MaNGA Survey: Mapping Nearby Galaxies at Apache Point Observatory
• Evolutionary Stellar Population Synthesis with MILES. Part I: The Base Models and a New Line Index System
• Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
• Understanding Galaxy Evolution through Emission Lines
• The Data Reduction Pipeline for the SDSS-IV MaNGA IFU Galaxy Survey
• SDSS-IV MaNGA IFS Galaxy Survey --- Survey Design, Execution, and Initial Data Quality
• New fully empirical calibrations of strong-line metallicity indicators in star forming galaxies
• Extragalactic chemical abundances: do HII regions and young stars tell the same story? The case of the spiral galaxy NGC 300
• Deriving model-based T${}_e$-consistent chemical abundances in ionised gaseous nebulae
• SDSS-IV MaNGA: Spatially resolved star formation histories in galaxies as a function of galaxy mass and type
• CHAOS I: Direct Chemical Abundances for HII Regions in NGC 628
• Precision abundance analysis of bright HII galaxies
• Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons
• The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources
• The temperature and ionization structure of the emitting gas in HII galaxies: Implications for the accuracy of abundance determinations
• Biases in metallicity measurements from global galaxy spectra: the effects of flux-weighting and diffuse ionized gas contamination
• SDSS IV MaNGA -- Metallicity and ionisation parameter in local star-forming galaxies from Bayesian fitting to photoionisation models
• [ArIII]/[OIII] and [SIII]/[OIII]: well-behaved oxygen abundance indicators for HII regions and star forming galaxies
• Metallicity calibrations for diffuse ionised gas and low ionisation emission regions
• Chemical distribution of HII regions towards the Galactic anticentre
• Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies
• A Spectroscopic Study of the Near-IR [SIII] Lines in a Sample of HII Galaxies: chemical abundances
• PNe and H II regions in the starburst irregular galaxy NGC 4449 from LBT MODS data
• Photon leaking or very hard ionizing radiation? Unveiling the nature of heII-emitters using the softness diagram
• Revisiting the hardening of the stellar ionizing radiation in galaxy disks

Cited by in corpus (6)

• PHANGS-JWST First Results: Destruction of the PAH molecules in HII regions probed by JWST and MUSE
• Chemical abundances in Seyfert galaxies -IX. Helium abundance estimates
• The resolved chemical composition of the starburst dwarf galaxy CGCG007-025: Direct method versus photoionization model fitting
• The resolved chemical abundance properties within the interstellar medium of star-forming galaxies at $\text{z}\approx 1.5$
• The softness diagram for MaNGA star-forming regions: diffuse ionized gas contamination or local HOLMES predominance?
• Exploring the hardness of the ionizing radiation with the infrared softness diagram. II. Bimodal distributions in both the ionizing continuum slope and the excitation in active galactic nuclei