A group of researchers using the Suprime-Cam instrument on the Subaru
Telescope has discovered about 80 young galaxies that existed in the
early universe about 1.2 billion years after the Big Bang. The team,
with members from Ehime University, Nagoya University, Tohoku
University, Space Telescope Science Institute (STScI) in the U.S., and
California Institute of Technology, then made detailed analyses of
imaging data of these galaxies taken by the Advanced Camera for Surveys
(ACS) on the Hubble Space Telescope. At least 54 of the galaxies
are spatially resolved in the ACS images. Among them, 8 galaxies show
double-component structures (Figure 1) and the remaining 46 seem to have elongated structures (Figure 2).
Through a further investigations using a computer simulation, the group
found that the observed elongated structures can be reproduced if two
or more galaxies reside in close proximity to each other.
These results strongly suggest that 1.2 billion years after the Big
Bang, galactic clumps in the young universe grow to become large
galaxies through mergers, which then causes active star formation to
take place. This research was conducted as part of the treasury program
of Hubble Space Telescope (HST), "Cosmic Evolution Survey
(COSMOS)". The powerful survey capability of the Subaru Telescope
provided the essential database of the candidate objects in the early
universe for this research project.
Figure 1: An example of galaxy that shows a
double-component structure in the HST/ACS image. North is up and east is
left. Each panel has a size of 4" x 4", which corresponds to 85 kly x
85 kly at the distance of 12.6 billion light-years. The thumbnails of
the HST/ACS I band (effective wavelength = 814 nm), the Subaru
Telescope/Suprime-Cam NB711 and i'- and z'-bands are shown from left to
right. Note that the NB711 image, which captures the Lyman-alpha line
emitted by neutral hydrogen, shows spatially extended gas which is
ionized by ultraviolet (UV) radiation from many massive stars. On the
other hand, other band images prove the UV radiation from massive stars
themselves. (Credit: Ehime University)
Figure 2: Same as Figure 1 but for an example of galaxy that shows a single-component structure in the HST/ACS image.
(Credit: Ehime University)
Exploring the Young Universe Using Subaru Telescope and Hubble Space Telescope
While the wide field of view of the Subaru Telescope has played an
important role in finding such young galaxies, the high spatial
resolution of the Hubble Space Telescope (HST) is required to
investigate the details of their shapes and internal structures. The
research team looked back to a point 12.6 billion years ago using a
two-pronged approach. The first step was to use the Subaru Telescope in a
deep survey to search out the early galaxies, and then follow that up
to investigate their shapes using the Advanced Camera for Surveys (ACS)
on board the HST. The ACS revealed 8 out of 54 galaxies to have
double-component structures, where two galaxies seem to be merging with
each other (Note 1), as shown in Figure 1.
Then, a question arose as to whether the remaining 46 galaxies are
really single galaxies. Here, the research team questioned why many of
these galaxies show elongated shapes in the HST/ACS images (Figure 2). This is because such elongated shapes, together with the positive correlation between ellipticity (Note 2) and size (Figure 3),
strongly suggest a possibility that two small galaxies reside so close
to each other that they cannot be resolved into two distinct galaxies,
even using ACS.
To examine whether the idea of closely crowded galaxies is viable,
the researchers conducted so-called Monte Carlo computer simulations.
First, the group put two identical artificial sources at random
locations with various angular separations onto the real observed ACS
image. Then, the group tried to extract the images with the same method
used for the actual observed ACS image and measured their ellipticities
and sizes. The results are shown in Figure 3.
Figure 3: The relation between ellipticity and size.
Red data points are the observed data; most of them have elongated
shapes and larger galaxies tend to have larger ellipticities.
Gray-colored regions represent the probability distributions calculated
with the computer simulations, in which two galaxies are located at so
close distance that they are blended as an elongated galaxy, as shown in
the right pictures schematically. (Credit: Ehime University)
As shown in Figure 3,
the simulated distribution reproduces the observed results very well.
That is, most of the galaxies that were observed as single sources in
the HST/ACS images are actually two merging galaxies. However, the
distances between two merging galaxies are so small they cannot be
spatially resolved, even by HST's high resolution!
If this idea is valid for the galaxies that appear to be single, then
it's possible to assume that the galaxies with the highest rate of
activities have the smallest sizes. This is expected because the
smallest sizes imply the smallest separation between two merging
galaxies. If this is the case, such galaxies would experience intense
star formation activity triggered by their mergers.
On the other hand, some galaxies with the smallest sizes are
moderately separated pairs, but are observed along the line of sight, or
are just single, isolated star-forming galaxies. These are basically
the same as large-size galaxies.
The research team has confirmed that the observed relation between
star formation activity and size is consistently explained by the team's
idea (Figure 4).
Figure 4: The relation between star formation activity
and size. Red circles and blue double-circles represent the galaxies
with single- and double-component ACS sources, respectively. The
research team's idea that most of the galaxies consist of two (or more)
interacting small galaxies with small angular separation is also
illustrated, which explains the observed trend very well. (Credit: Ehime
University)
To date, the shapes and structures of small young galaxies have been investigated by using ACS on HST.
If a source was detected as a single ACS source, it was treated as a
single galaxy and its morphological parameters were evaluated. This
research suggests that such a small galaxy can consist of two (or
perhaps, more) interacting/merging galaxies located so close together
that they cannot be resolved by even the high angular resolution of the
ACS.
Looking into the Future of Studying the Past
Current galaxy formation theories predict that small galaxies in the
young universe evolve into large galaxies via successive mergers. The
question remains: what is the next step in observational studies for
galaxy formation in the young universe? This is one of the frontier
fields that requires future "super telescopes," e.g. Thirty Meter
Telescope (TMT) and the James Webb Space Telescope (JWST). They will
enable the next breakthroughs in the study of early galaxy formation and
evolution.
This research will be published in the Astrophysical Journal titled
"Morphological Properties of Lyman Alpha Emitters at Redshift 4.86 in
the COSMOS Field: Clumpy Star Formation or Merger?" by Masakazu A. R.
Kobayashi, Katsuhiro L. Murata, Anton M. Koekemoer, Takashi Murayama,
Yoshiaki Taniguchi, Masaru Kajisawa, Yasuhiro Shioya, Nick Z. Scoville,
Tohru Nagao, and Peter L. Capak. Online version was posted February 24, 2016 and print version was on March 1, 2016 (Volume 819, article id. 25).
Note:
- A mean size (that is, a mean diameter of the circle which encloses half of the total light of galaxy) of individual small galaxies is about 5.5 thousand light-years (kly). A mean distance between the two small galaxies, which is projected distance on the sky, is 13 kly (13,000 light-years).
- Ellipticity is defined as 1 - b/a, where a and b represent the major and minor radii of an ellipse. In the case of a circle, ellipticity is equal to zero because a equals to b. A more elongated shape results in a larger ellipticity.
Authors:
- Masakazu A. R. Kobayashi: Research Center for Space and Cosmic Evolution, Ehime University, Japan
- Katsuhiro L. Murata: Department of Physics, Nagoya University, Japan
- Anton M. Koekemoer: Space Telescope Science Institute, USA
- Takashi Murayama: Astronomical Institute, Graduate School of Science, Tohoku University, Japan
- Yoshiaki Taniguchi: Research Center for Space and Cosmic Evolution, Ehime University, Japan
- Masaru Kajisawa: Research Center for Space and Cosmic Evolution and Graduate School of Science and Engineering, Ehime University, Japan
- Yasuhiro Shioya: Research Center for Space and Cosmic Evolution, Ehime University, Japan
- Nick Z. Scoville: Department of Astronomy, California Institute of Technology, USA
- Tohru Nagao: Research Center for Space and Cosmic Evolution, Ehime University, Japan
- Peter L. Capak: Department of Astronomy and Spitzer Science Center, California Institute of Technology, USA
