TY - JOUR

T1 - EMPRESS. II. Highly Fe-enriched Metal-poor Galaxies with similar to 1.0 (Fe/O)(circle dot) and 0.02 (O/H)(circle dot)

T2 - Possible Traces of Supermassive (>300 M (circle dot)) Stars in Early Galaxies* dagger double dagger

AU - Kojima, Takashi

AU - Ouchi, Masami

AU - Rauch, Michael

AU - Ono, Yoshiaki

AU - Nakajima, Kimihiko

AU - Isobe, Yuki

AU - Fujimoto, Seiji

AU - Harikane, Yuichi

AU - Hashimoto, Takuya

AU - Hayashi, Masao

AU - Komiyama, Yutaka

AU - Kusakabe, Haruka

AU - Kim, Ji Hoon

AU - Lee, Chien-Hsiu

AU - Mukae, Shiro

AU - Nagao, Tohru

AU - Onodera, Masato

AU - Shibuya, Takatoshi

AU - Sugahara, Yuma

AU - Umemura, Masayuki

AU - Yabe, Kiyoto

PY - 2021/5/19

Y1 - 2021/5/19

N2 - We present element abundance ratios and ionizing radiation of local young low-mass (similar to 10(6) M (circle dot)) extremely metal-poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H)(circle dot) and a high specific star formation rate (sSFR similar to 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] lambda 4658 and He ii lambda 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)(circle dot), 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 similar to 90%-140% (Fe/O)(circle dot), 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 lambda 4686/H beta ratios of similar to 1/40, which are not reproduced by existing models of high-mass X-ray binaries with progenitor stellar masses M (circle dot). 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 lambda 4686/H beta ratios are explained by the past existence of supermassive (>300 M (circle dot)) stars, which may evolve into intermediate-mass black holes (greater than or similar to 100 M (circle dot)).

AB - We present element abundance ratios and ionizing radiation of local young low-mass (similar to 10(6) M (circle dot)) extremely metal-poor galaxies (EMPGs) with a 2% solar oxygen abundance (O/H)(circle dot) and a high specific star formation rate (sSFR similar to 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] lambda 4658 and He ii lambda 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)(circle dot), 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 similar to 90%-140% (Fe/O)(circle dot), 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 lambda 4686/H beta ratios of similar to 1/40, which are not reproduced by existing models of high-mass X-ray binaries with progenitor stellar masses M (circle dot). 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 lambda 4686/H beta ratios are explained by the past existence of supermassive (>300 M (circle dot)) stars, which may evolve into intermediate-mass black holes (greater than or similar to 100 M (circle dot)).

KW - ELECTRON-IMPACT EXCITATION

KW - EMISSION-LINE GALAXIES

KW - DIGITAL SKY SURVEY

KW - X-RAY BINARIES

KW - PRESSURE-DOMINATED PHOTOIONIZATION

KW - HARD IONIZING SPECTRUM

KW - LY-ALPHA EMITTERS

KW - FORMING GALAXIES

KW - TRANSITION-PROBABILITIES

KW - COLLISION STRENGTHS

U2 - 10.3847/1538-4357/abec3d

DO - 10.3847/1538-4357/abec3d

M3 - Journal article

VL - 913

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1

M1 - 22

ER -