To understand galaxy evolution, astronomers need to study galaxies in
various stages of formation and evolution. Most of the galaxies in the
modern Universe are mature galaxies, but standard cosmology predicts
that there may still be a few galaxies in the early formation stage in
the modern Universe. Because these early-stage galaxies are rare, an
international research team searched for them in wide-field imaging data
taken with the Subaru Telescope. "To find the very faint, rare
galaxies, deep, wide-field data taken with the Subaru Telescope was
indispensable," emphasizes Dr. Takashi Kojima, the leader of the team.
However,
it was difficult to find galaxies in the early stage of galaxy
formation from the data because the wide-field data includes as many as
40 million objects. So the research team developed a new machine
learning method to find such galaxies from the vast amount of data. They
had a computer repeatedly learn the galaxy colors expected from
theoretical models, and then let the computer select only galaxies in
the early stage of galaxy formation.
The research team then
performed follow-up observations to determine the elemental abundance
ratios of 4 of the 27 candidates selected by the computer. They have
found that one galaxy (HSC J1631+4426; Figure 1), located 430 million
light-years away in the constellation Hercules, has an oxygen abundance
only 1.6 percent of that of the Sun. This is the lowest values ever
reported for a galaxy. The measured oxygen abundance suggests that most
of the stars in this galaxy formed very recently. In other words, this
galaxy is undergoing an early stage of the galaxy evolution.
"What
is surprising is that the stellar mass of the HSC J1631+4426 galaxy is
very small, 0.8 million solar masses. This stellar mass is only about
1/100,000 of our Milky Way galaxy, and comparable to the mass of a star
cluster in our Milky Way," said Prof. Ouchi of the National Astronomical
Observatory of Japan and the University of Tokyo. This small mass also
supports the primordial nature of the HSC J1631+4426 galaxy.
The
research team thinks that there are two interesting indications from
this discovery. First, this is the evidence about a galaxy at such an
early stage of galaxy evolution existing today.
In the framework of the standard cosmology, new galaxies are thought to
be born in the present universe. The discovery of the HSC J1631+4426
galaxy backs up the picture of the standard cosmology. Second, we may
witness a new-born galaxy at the latest epoch of the cosmic history. The
standard cosmology suggests that the matter density of the universe
rapidly drops in our universe whose expansion accelerates. In the future
universe with the rapid expansion, matter does not assemble by gravity,
and new galaxies won’t be born. The HSC J1631+4426 galaxy may be the
last generation galaxy in the long cosmic history.
This research will be published in the Astrophysical Journal on August 3, 2020 (Kojima et al. 2020, "Extremely
Metal-Poor Representatives Explored by the Subaru Survey (EMPRESS). I. A
Successful Machine Learning Selection of Metal-Poor Galaxies and the
Discovery of a Galaxy with M*<106 Msun and 0.016 Zsun"). A preprint is available here.
This research was supported by the Ministry of Education, Culture,
Sports, Science and Technology's World Premier International Research
Center Program and Grants-in-Aid for Scientific Research (JP15H02064,
JP17H01110, JP17H01114, JP17K14257, JP18J12840, JP18J12727, and
JP18K13578).
