In:
The Astrophysical Journal, American Astronomical Society, Vol. 913, No. 1 ( 2021-05-01), p. 22-
Abstract:
We present element abundance ratios and ionizing radiation of local young low-mass (∼10 6 M ⊙ ) extremely metal-poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H) ⊙ and a high specific star formation rate (sSFR ∼ 300 Gyr −1 ) and other (extremely) metal-poor galaxies, which are compiled from Extremely Metal-Poor Representatives Explored by the Subaru Survey (EMPRESS) and the literature. Weak emission lines such as [Fe iii ] λ 4658 and He ii λ 4686 are detected in very deep optical spectra of the EMPGs taken with 8 m class telescopes, including Keck and Subaru, enabling us to derive element abundance ratios with photoionization models. We find that neon-to-oxygen and argon-to-oxygen ratios are comparable to those of known local dwarf galaxies and that the nitrogen-to-oxygen abundance ratios (N/O) are lower than 20% (N/O) ⊙ , consistent with the low oxygen abundance. However, the iron-to-oxygen abundance ratios (Fe/O) of the EMPGs are generally high; the EMPGs with the 2%-solar oxygen abundance show high Fe/O ratios of ∼90%–140% (Fe/O) ⊙ , which are unlikely to be explained by suggested scenarios of Type Ia supernova iron productions, iron’s dust depletion, and metal-poor gas inflow onto previously metal-riched galaxies with solar abundances. Moreover, the EMPGs with the 2%-solar oxygen abundance have very high He ii λ 4686/H β ratios of ∼1/40, which are not reproduced by existing models of high-mass X-ray binaries with progenitor stellar masses 〈 120 M ⊙ . Comparing stellar-nucleosynthesis and photoionization models with a comprehensive sample of EMPGs identified by this and previous EMPG studies, we propose that both the high Fe/O ratios and the high He ii λ 4686/H β ratios are explained by the past existence of supermassive ( 〉 300 M ⊙ ) stars, which may evolve into intermediate-mass black holes (≳100 M ⊙ ).
Type of Medium:
Online Resource
ISSN:
0004-637X
,
1538-4357
DOI:
10.3847/1538-4357/abec3d
Language:
Unknown
Publisher:
American Astronomical Society
Publication Date:
2021
detail.hit.zdb_id:
2207648-7
detail.hit.zdb_id:
1473835-1
SSG:
16,12
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