Journal
ASTROPHYSICAL JOURNAL
Volume 930, Issue 1, Pages -Publisher
IOP Publishing Ltd
DOI: 10.3847/1538-4357/ac602d
Keywords
-
Categories
Funding
- World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan
- KAKENHI through the Japan Society for the Promotion of Science (JSPS) [20H00180, 21H04467]
- joint research program of the Institute for Cosmic Ray Research (ICRR), University of Tokyo
Ask authors/readers for more resources
Recent studies suggest the existence of nonthermal sources, but whether their ionizing spectra can explain multiple major emission lines remains unclear. This study derives the general shapes of the ionizing spectra for three extremely metal-poor galaxies and finds a diversity of ionizing spectrum shapes.
Strong high-ionization lines such as He ii of young galaxies are puzzling at high and low redshift. Although recent studies suggest the existence of nonthermal sources, whether their ionizing spectra can consistently explain multiple major emission lines remains a question. Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong He ii lambda 4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and nonthermal sources. We use photoionization models for nebulae and determine seven parameters of the ionizing spectra and nebulae by Markov Chain Monte Carlo methods, carefully avoiding systematics of abundance ratios. We obtain the general shapes of ionizing spectra explaining similar to 10 major emission lines within observational errors with smooth connections from observed X-ray and optical continua. We find that an ionizing spectrum of one EMPG has a blackbody-dominated shape, while the others have convex downward shapes at >13.6 eV, which indicate a diversity of the ionizing spectrum shapes. We confirm that the convex downward shapes are fundamentally different from ordinary stellar spectrum shapes, and that the spectrum shapes of these galaxies are generally explained by the combination of the stellar and ultraluminous X-ray sources. Comparisons with stellar synthesis models suggest that the diversity of the spectrum shapes arises from differences in the stellar age. If galaxies at z greater than or similar to 6 are similar to the EMPGs, high-energy (>54.4 eV) photons of the nonstellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available