Deciphering compact galaxies in the young universe
The Importance of Studying Early Galaxies
In the present universe, at a point 13.8 billion years after the Big Bang, there are many giant galaxies like our Milky Way, which contains about 200 billion stars in a disk a hundred thousand light years across. However, there were definitely no galaxies like it in the epoch just after the Big Bang.
Much effort has been made through deep surveys to detect actively star-forming galaxies in the young universe. As a result, the distances of the earliest galaxies are now known to be at more than 13 billion light-years. We see them at a time when the age of the universe was only 800 million years (or about 6% of the present age). However, since most of the galaxies in the young universe were quite small, their detailed structures have not yet been observed.
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 .
 and size, 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 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.
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.
 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.
Source: National Institutes of Natural Sciences [March 08, 2016]