Kavli Affiliate: Masahiro Kawasaki
| First 5 Authors: Kimihiko Nakajima, Masami Ouchi, Yuki Isobe, Yi Xu, Shinobu Ozaki
| Summary:
Using the Subaru/FOCAS IFU capability, we examine the spatially resolved
relationships between gas-phase metallicity, stellar mass, and star-formation
rate surface densities (Sigma_* and Sigma_SFR, respectively) in extremely
metal-poor galaxies (EMPGs) in the local universe. Our analysis includes 24
EMPGs, comprising 9,177 spaxels, which span a unique parameter space of local
metallicity (12+log(O/H) = 6.9 to 7.9) and stellar mass surface density
(Sigma_* ~ 10^5 to 10^7 Msun/kpc^2), extending beyond the range of existing
large integral-field spectroscopic surveys. Through spatially resolved emission
line diagnostics based on the [NII] BPT-diagram, we verify the absence of
evolved active galactic nuclei in these EMPGs. Our findings reveal that, while
the resolved mass-metallicity relation exhibits significant scatter in the
low-mass regime, this scatter is closely correlated with local star-formation
surface density. Specifically, metallicity decreases as Sigma_SFR increases for
a given Sigma_*. Notably, half of the EMPGs show a distinct metal-poor
horizontal branch on the resolved mass-metallicity relation. This feature
typically appears at the peak clump with the highest Sigma_* and Sigma_SFR and
is surrounded by a relatively metal-enriched ambient region. These findings
support a scenario in which metal-poor gas infall fuels episodic star formation
in EMPGs, consistent with the kinematic properties observed in these systems.
In addition, we identify four EMPGs with exceptionally low central
metallicities (12+log(O/H) <~ 7.2), which display only a metal-poor clump
without a surrounding metal-rich region. This suggests that such ultra-low
metallicity EMPGs, at less than a few percent of the solar metallicity, may
serve as valuable analogs for galaxies in the early stages of galaxy evolution.
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