Subaru/HSC machine learning survey for extremely metal-poor galaxies. I. A new successful approach for identifying a galaxy with M < 106M☉ AND 0.021Z☉

Takashi Kojima, Masami Ouchi, Michael Rauch, Yoshiaki Ono, Yuki Isobe, Seiji Fujimoto, Yuichi Harikane, Takuya Hashimoto, Masao Hayashi, Yutaka Komiyama, Haruka Kusakabe, Ji Hoon Kim, Chien Hsiu Lee, Shiro Mukae, Tohru Nagao, Masato Onodera, Takatoshi Shibuya, Yuma Sugahara, Masayuki Umemura, Kiyoto Yabe

Research output: Contribution to journalArticlepeer-review


We have initiated a new survey for local extremely metal-poor galaxies (EMPGs) with Subaru/Hyper Suprime-Cam (HSC) large-area (∼ 500 deg2) optical images reaching a 5σ limit of ∼ 26 magnitude, about 100 times deeper than the one of Sloan Digital Sky Survey (SDSS). To select Z/Z☉ < 0.1 EMPGs from ∼ 40 million sources detected in the Subaru images, we first develop a machine-learning (ML) classifier based on a deep neural network algorithm with a training data set consisting of optical photometry of galaxy, star, and QSO models. We test our ML classifier with SDSS objects having spectroscopic metallicity measurements, and confirm that our ML classifier accomplishes 86%-completeness and 46%-purity EMPG classifications with photometric data. Applying our ML classifier to the photometric data of the Subaru sources as well as faint SDSS objects with no spectroscopic data, we obtain 27 and 86 EMPG candidates from the Subaru and SDSS photometric data, respectively. We conduct optical follow-up spectroscopy for 10 out of our EMPG candidates with Magellan/LDSS-3+MagE, Keck/DEIMOS, and Subaru/FOCAS, and find that the 10 EMPG candidates are star-forming galaxies at z = 0.007 − 0.03 with large Hβ equivalent widths of 104-265 Å, stellar masses of log(M*/M☉)=5.0-7.1, and high specific star-formation rates of ∼300 Gyr−1, which are similar to those of early galaxies at z & 6 reported recently. Our metal-poor galaxies have small velocity dispersions of nebular gas (27.8-32.5 km s−1) and are significantly located in the relatively isolated environment compared to typical, local galaxies. We spectroscopically confirm that 3 out of 10 candidates are truly EMPGs with Z/Z☉ < 0.1, one of which is HSC J1631+4426, the most metal-poor galaxy with Z/Z☉ = 0.021 so far identified among star-forming galaxies in the low-mass regime of log(M*/M☉)<6.0.

Original languageEnglish
JournalUnknown Journal
Publication statusPublished - 2019 Oct 18
Externally publishedYes


  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: ISM
  • Galaxies: star formation
  • Subject headings: galaxies: dwarf

ASJC Scopus subject areas

  • General

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